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Tuesday, June 17, 2025

Entry 177

From Entry 176, we gave the fundamental unit Ud U163=64080026+5019135163=(8005+6271632)2 and from eπ1636403203+744, observed that 640320=80(80051) This may be just coincidence, but what is not is when U3d for d=7,11,19,43,67,163. This was also observed by H. H. Chan. Given the fundamental units

U21=(3+72)2U33=(23+11)2U57=(53+219)2U129=(533+1443)2U201=(2933+6267)2U489=(355733+4826163)2

Define the function

Fd=33(U3d1/U3d)+6

then we get the rather familiarF7=15F11=6(1+33)F19=96F43=960F67=5280F163=640320

which (except for d=11) are the cube roots of the j-function (negated). 

P.S. I don't know why d=11 does not obey the pattern, but it does yield the integer 42 if the positive sign of the second square root ±1/U3d is used, though the correct value should be 32.

Entry 176

Ramanujan found the exact value of the Ramanujan G-function G69=(5+232)1/12(33+232)1/8(2+334+6+334)1/2Note the fundamental units Un U23=24+523=(5+232)2U69=25+3692=(33+232)2 and how he uses the squared version. As a second example

G77=(8+37)1/8(7+112)1/8(2+114+6+114)1/2

and fundamental units U7=8+37=(3+72)2U77=9+772=(7+112)2

Ramanujan mostly uses the squared version of the Un to get "simpler" expressions with smaller integers. For prime p3mod4, one can always do since 

x2py2=2x2py2=+2 are solvable by p3mod8 and p7mod8, respectively. Checking  U67 and U163, yields the reductions U67=48842+596767=(221+27672)2U163=64080026+5019135163=(8005+6271632)2And from eπ6752803+744 and eπ1636403203+744, we find the relations 5280=24(2211)640320=80(80051) though it may be just coincidence.

Monday, June 16, 2025

Entry 175

For discriminant d=4m with class number 8 and semiprime m

If m=5p, then distinguish between p1mod8 and p5mod8

If m=7p, then distinguish between p3mod8 and p7mod8

The semiprime m=5p was in the previous entry. For m=7p, there are only four and Ramanujan found the radicals below. 

For the 1st case, p=11,43, thus m=7p=77,301  

G77=(8+37)1/8(7+112)1/8(2+114+6+114)1/2G301=(8+37)1/8(577+23432)1/8(42+7434+46+7434)1/2

For the 2nd case, p=31,79, thus m=7p=217,553 

G217=(x112+x1+12)1/2(y112+y1+12)1/2G553=(x212+x2+12)1/2(y212+y2+12)1/2 where x1=10+472,y1=14+574 x2=142+16792,y2=98+11794

But it seems not noticed that a fundamental unit is imbedded in these radicals as

x1+2y1=32(8+37)=32(3+72)2=32U7

x2+2y2=32(80+979)=32(9+792)2=32U79

Entry 174

Continuing from the previous entry, for d=4m with class number 8, the semiprime m=5p with p5mod8 is also well-behaved. And it involves the golden ratio. There are only four, namely p=13,29,53,101, thus m=5p=65,145,265,505. Ramanujan found the radicals below and the G-function have a common form

G5p=ϕkU1/4px1/2p

with powers of the golden ratio ϕ, fundamental unit Un, and x2p a root of a unit quartic

G65ϕ=(3+132)1/4(1+658+9+658)1/2G145ϕ3=(5+292)1/4(9+1458+17+1458)1/2G265ϕ3=(7+532)1/4(81+52658+89+52658)1/2G505ϕ7=(10+101)1/4(105+55058+113+55058)1/2

The case p1mod8 or p=41,89, thus m=5p=205,445 behaves slightly differently though

G205ϕ=(43+32052)1/8(1+418+7+418)G445ϕ3/2=(21+4452)1/4(5+898+13+898)

How Ramanujan found these is a mystery.

Sunday, June 15, 2025

Entry 173

There are many in the set d=4m with class number 8. When m is a prime or a semiprime (a product of two primes) like m=3p, then it may be well-behaved. For this set, there are only three, namely p=23,47,71, thus m=3p=69,141,213. Ramanujan found the radicals below and the G-function have a common form

G3p=U1/24pU1/163px1/2p

with fundamental unit Un and where x2p is a root of a unit quartic

G69=(5+232)1/12(33+232)1/8(2+334+6+334)1/2G141=(7+472)1/12(43+47)1/8(14+934+18+934)1/2G213=(59+7712)1/12(53+712)1/8(19+1232+21+1232)1/2 How Ramanujan found these is unknown as it is uncertain if he was aware of class field theory. Note also that without the factor 3, then d=23,47,71 are the smallest d with class number h(d)=3,5,7, respectively, while h(3d)=8.

P.S. Checking h(3d)=16, one finds the only primes are d=167,191,239,383,311 which are the smallest d with class number h(d)=11,13,15,17,19, respectively. Makes me wonder if their G3d would be analogous.

Entry 172

There are many fundamental d=4m with class number 8, though only seven are prime namely p=41,113,137,313,337,457,577. Their Ramanujan G-functions are

G41=(x+x242)1/2=x24+x+24G113=(y+y242)1/2=y24+y+24 where x=(5+414)(1+5+418)y=(9+1134)(1+7+1132) and similarly for the other p, though the quartic roots get more complicated.

Saturday, June 14, 2025

Entry 171

Continuing from Entry 170, there is another way to express the Ramanujan Gn-function where d=4n has class number 6 using fundamental units Un. Borrowing a trick from Ramanujan, he found

G169=13(2+13+3U13(v+339)+3U13(v339))

where U13=3+132,v=13+11132

Using this form, we propose that

U29=131/4(9+292+3U29(x+243)+3U29(x243))1/4U53=131/4(23+3532+3U53(y+1203)+3U53(y1203))1/4U61=131/4(15+261+3U61(z+723)+3U61(z723))1/4 whereU29=5+292,x=185+19292 U53=7+532,y=1721+217532 U61=39+5612,z=601+93612

and similarly for all d=4p with class number 6. All their seven fundamental units have form Up=a+bp2, hence have odd solutions to the Pell equation x2py2=4.

Entry 170

There are only seven fundamental d=4p with class number 6, namely p=29,53,61,109,157,277,397. Hence their Ramanujan G-functions are

G29=(y21+y41+42)1/4G53=(y22+y42+42)1/4G61=(y23+y43+366)1/4 wherey3y24y4=0y37y2+13y11=0y36y227y54=0 and similarly for the other p

Entry 169

Recall Ramanujan's G-function21/4Gn=η2(τ)η(τ2)η(τ) where τ=n. There are only three fundamental d=4p with class number 2 for prime p, namely p=5,13,37. Hence G5=(1+52)1/4G13=(3+132)1/4G37=(6+37)1/4Going higher, there are only four fundamental d=4p with class number 4 for prime p, namely p=17,73,97,193.

G17=3+178+5+178G73=1+738+9+738G97=5+978+13+978G193=22+21938+30+21938 all of which were already known to Ramanujan. But as was shown in Entry 161 and Entry 162, it turns out these also appear in the closed-form of the complete elliptic integral of the first kind K(kn). The next entry will be for class number 6 where one had to extract 4th roots again.

Entry 168

For class number 5, there are only four d of the kind d7mod8, namely d=47,79,103,127. We illustrate only the first two and propose, K(k47)=2π247x2/3(47m=1[Γ(m47)](47m))1/10K(k79)=2π279y2/3(79m=1[Γ(m79)](79m))1/10 where (x,y) are the real roots of the quintics solvable in radicals x52x410x313x26x1=0y511y4+17y32y25y1=0 And similarly for d=103,127. But for the next discriminant or d=131 which is of kind d3mod8, one now has to deal with an algebraic number of degree 3×5=15. And it seems difficult to find the eta quotients responsible for (x,y).

Entry 167

For odd class number h(d), the kind that is d7mod8 seems more well-behaved than d3mod8. For class number 3, there are only two of the first kind: d=23,31. Hence, K(k23)=2π223x4/3(23m=1[Γ(m23)](23m))1/6K(k31)=2π231y4/3(31m=1[Γ(m31)](31m))1/6 where (x,y) are the real roots of the cubics x3x1=0y3y21=0 or the plastic ratio and supergolden ratio, respectively. But for d=59 which is of the second kind, then the radical involved will be an algebraic number of degree 3×3=9.

Entry 166

The case d=8 is special since it is even but has odd class number h(d)=1. Given the Kronecker symbol (dm), we propose a symmetrical closed-form 

K(k8)=2π161U2(U212+U2+12)2(8m=1[Γ(m8)](8m))1/2=2π161U2(U212+U2+12)2(Γ(18)Γ(38)Γ(58)Γ(78))1/2 with fundamental unit U2=1+2 and form reminiscent of the odd d with class number 1.

Friday, June 13, 2025

Entry 165

The previous entries dealt with even class numbers. For odd class number h(d)=1, there are the nine Heegner numbers d=1,2,3,7,11,19,43,67,163. Given the Kronecker symbol (dm), we propose 

Conjecture. Let d>3 with class number h(d)=1. Then x below is an algebraic number 1x2d=1K(kd)2π4d(dm=1[Γ(md)](dm))1/2specificallyxd=21/4Gn=η2(τ)η(τ2)η(τ) with Ramanujan's G-function. This function xd has been discussed in Entry 159. For d=7, then x7=2, but for the five d11, then xd are the real roots of the following five simple cubics, 

x32x2+2x2=0x32x2=0x32x22=0x32x22x2=0x36x2+4x2=0 also discussed in Entry 160

Entry 164

To summarize, given fundamental discriminant d, class number h(d)=n, complete elliptic integral of the first kind K(kp), and Kronecker symbol (dm)

Conjecture 1. Let even d=4p for prime p1mod4 with even class number h(d)=n and x=1K(kd/4)2π2d(dm=1[Γ(md)](dm))1/(2n) Conjecture 2. Let odd d=p for prime p3mod4 with odd class number h(d)=n and y=1K(kd)2π2d(dm=1[Γ(md)](dm))1/(2n) then (x,y) are algebraic numbers as seen in entries 160163 and 165168. They seem to have a closed-form in term of eta quotients but I haven't found it yet.

Entry 163

There are only seven fundamental d=4p with class number 6, namely p=29,53,61,109,157,277,397.  To prevent clutter, only the first three will be stated. Given the Kronecker symbol (dm), we propose

K(k29)=2π21161(x1)1/3(y21+y41+42)3/4(116m=1[Γ(m116)](116m))1/12K(k53)=2π22121(x2)1/3(y22+y42+42)3/4(212m=1[Γ(m212)](212m))1/12K(k61)=2π22441(x3)1/3(y23+y43+366)3/4(244m=1[Γ(m244)](244m))1/12 where the xn are the real roots of the following cubics x35x23x1=0x315x2x1=0x327x25x1=0 while the yn are also the real roots of cubics y3y24y4=0y37y2+13y11=0y36y227y54=0 and similarly for the other p.

Entry 162

There are only four fundamental d=4p with class number 4, namely p=17,73,97,193.  Given the Kronecker symbol (dm), we propose

K(k17)=2π2681(U17)1/8(3+178+5+178)3(68m=1[Γ(m68)](68m))1/8K(k73)=2π22921(U73)1/8(1+738+9+738)3(292m=1[Γ(m292)](292m))1/8K(k97)=2π23881(U97)1/8(5+978+13+978)3(388m=1[Γ(m388)](388m))1/8K(k193)=2π27721(U193)1/8(22+21938+30+21938)3(772m=1[Γ(m772)](772m))1/8 where the Un are fundamental units U17=4+17 U73=1068+12573 U97=5604+56997 U193=1764132+126985193 Going to class number 6, the red exponent will now be 1/12.

Entry 161

Entry 160 dealt with discriminants d with class number 1. Going higher, there are only three fundamental d=4p with class number 2, namely p=5,13,17.  Given the Kronecker symbol (dm), we propose

K(k5)=2π220(1+52)3/4(20m=1[Γ(m20)](20m))1/4K(k13)=2π252(3+132)3/4(52m=1[Γ(m52)](52m))1/4K(k37)=2π2148(6+37)3/4(148m=1[Γ(m148)](148m))1/4Going to class number 4, the red exponent will now be 1/8.

Wednesday, June 11, 2025

Entry 160

We continue with closed-forms for the Dedekind eta function η2(d) for d=11,19,43,67,163. As discussed in Entry 159, the closed-form for the complete elliptic integral of the first kind K(kd) then necessarily follows. 

η2(11)=1x211Γ(111)Γ(311)Γ(411)Γ(511)Γ(911)111/4(2π)3 η2(19)=1x219Γ(119)Γ(419)Γ(519)Γ(619)Γ(1719)191/4(2π)5

η2(43)=1x243Γ(143)Γ(443)Γ(643)Γ(943)Γ(4143)431/4(2π)11

η2(67)=1x267Γ(167)Γ(467)Γ(667)Γ(967)Γ(6567)671/4(2π)17

η2(163)=1x2163Γ(1163)Γ(4163)Γ(6163)Γ(9163)Γ(161163)1631/4(2π)41

where the xd are the real roots of the following simple cubics, respectively

x32x2+2x2=0x32x2=0x32x22=0x32x22x2=0x36x2+4x2=0

The numerators, for example, of Γ(n19) can be found using the Kronecker symbol and this Wolfram command which yields the 1912=9 numerators as n=1,4,5,6,7,9,11,16,17.

Entry 159

Given the McKay-Thompson series of Class 4A for the Monster (A097340)

j4A(τ)=(η2(2τ)η(τ)η(4τ))24=1q+24+276q+2048q2+ We take the 6th root, scale ττ2, and use the version, (xd)4=(η2(τ)η(τ2)η(2τ))4=2F1(12,12,1,λ(τ))η2(τ)=2π×K(kd)η2(τ) where K(kd) is a complete elliptic integral of the first kind. Compared also to Ramanujan's G-function, 21/4Gn=η2(τ)η(τ2)η(2τ)=q124n>0(1+q2n1) where Ramanujan used odd n for Gn and calculated a lot of n. Mathworld also has a partial list of closed-forms for K(kd). Knowing xd or Gn immediately leads to a closed-form for η2(τ) as well. Examples, 

2π×K(k7)=x27Γ(17)Γ(27)Γ(47)71/4(2π)2η2(7)=1x27Γ(17)Γ(27)Γ(47)71/4(2π)2

where x7=2. And

2π×K(k11)=x211Γ(111)Γ(311)Γ(411)Γ(511)Γ(911)111/4(2π)3η2(11)=1x211Γ(111)Γ(311)Γ(411)Γ(511)Γ(911)111/4(2π)3

where x11=1T+1 and T is the tribonacci constant, the real root of x3x2x1=0. For d=11,19,43,67,163, then xd is just the real root of a cubic.

Tuesday, June 10, 2025

Entry 158

In the previous entry, given the Jacobi theta functions ϑn(0,q) with nome q=eπiτ, modular lambda function λ(τ), and complete elliptic integral of the first kind K(k)=π22F1(12,12,1,k2) we proposed three identities, one as,

(ϑ2(0,q)2F1(12,12,1,λ))4?=λ where λ=λ(τ). Using the known definitions of ϑn(0,q) and λ(τ), the proposed identity implies 

2F1(12,12,1,λ)η2(τ)=(η2(τ)η(τ2)η(2τ))4

For appropriate complex quadratics such as τ=n, then the RHS is a radical. But if the numerator of the LHS has a closed-form (implied in this list), then this also gives a closed-form for the denominator η2(τ) such as

η2(1)=Γ2(14)(2π)3/2121/2η2(2)=Γ(18)Γ(38)(2π)3/2125/4η2(3)=Γ3(13)(2π)231/422/3

and so on. Since an eta quotient is involved, then η(d) for d with class number 1 will behave quite orderly and will be discussed in the next entry.

Entry 157

Given the Jacobi theta functions ϑn(0,q) which traditionally uses the nome q=eπiτ. Define the modular lambda function λ(τ),

λ(τ)=16(η(τ/2)η(2τ))8+16=(2η(τ/2)η2(2τ)η3(τ))8=(ϑ2(0,q)ϑ3(0,q))4

Then we propose the three identities below and for appropriate τ such as τ=d and λ=λ(τ) that the ratios below are radicals,

(ϑ2(0,q)2F1(12,12,1,λ))4?=λ(ϑ4(0,q)2F1(12,12,1,λ))4?=1α(ϑ3(0,q)2F1(12,12,1,λ))4?=1

Adding the first two implies the third. Hence, after removing the common denominator

(ϑ2(0,q))4+(ϑ4(0,q))4=(ϑ3(0,q))4

which is known to be true. As eta quotients in the same order above, 

(2η2(2τ)η(τ))4+(η2(τ2)η(τ))4=(η5(τ)η2(τ2)η2(2τ))4

Also, the equalities ϑ3(0,q)=m=qm2=η5(τ)η2(τ2)η2(2τ)which has a cubic version given in Entry 156.

Entry 156

Given the square of the nome, so q=e2πiτ and the Borwein cubic theta functions a(q),b(q),c(q). Define,

β=(3(η(τ/3)η(3τ))3+3)3=(c(q)a(q))3

Then we conjecture,

(c(q)2F1(13,23,1,β))3?=β(b(q)2F1(13,23,1,β))3?=1β(a(q)2F1(13,23,1,β))3?=1

Adding the first two implies the third,

(c(q))3+(b(q))3=(a(q))3

which is a known relationship of the Borwein cubic theta functions. As eta quotients,

(3η3(3τ)η(τ))3+(η3(τ)η(3τ))3=(η3(τ)+9η3(9τ)η(3τ))3

Like in the previous entry, the RHS is also a sum,

a(q)=m,n=qm2+mn+n2=η3(τ)+9η3(9τ)η(3τ)

Entry 155

It turns out we can use the previous entries to parameterize the equation xn+yn=zn for n=2,3,4. Given the square of the nome q=e2πiτ and assume the functions A(q),B(q),C(q). Define

γ=(8(η(τ/2)η(2τ))8+8)2=(C(q)A(q))2

Then we conjecture,

(C(q)2F1(14,34,1,γ)2)2?=γ(B(q)2F1(14,34,1,γ)2)2?=1γ(A(q)2F1(14,34,1,γ)2)2?=1

where C(q),B(q),A(q) are defined by the relation and eta quotients below,

(C(q))2+(B(q))2=(A(q))2

(8η8(2τ)η4(τ))2+(η8(τ)η4(2τ))2=(η8(τ)+32η8(4τ)η4(2τ))2 Note also that A(q)=1+24n=1nqn1+qn=(ϑ2(0,q))4+(ϑ2(0,q))4=η8(τ)+32η8(4τ)η4(2τ) where, in this particular instance, we let the Jacobi theta functions ϑn(0,q) use q=e2πiτ

Monday, June 9, 2025

Entry 154

As an overview of the previous four entries, Ramanujan's theory of elliptic functions to alternative bases uses the hypergeometric function 2F1(a,b;c;z) with a+b=c=1 for the cases a=16,14,13,12. This can be related to the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145 for n=1,2,3,4. Consider the equations, 

2F1(16,56,1,1α1)2F1(16,56,1,α1)=τ12F1(14,34,1,1α2)2F1(14,34,1,α2)=τ22F1(13,23,1,1α3)2F1(13,23,1,α3)=τ32F1(12,12,1,1α4)2F1(12,12,1,α4)=τ4 Then the αn can be solved and expressed in terms of the jn(τ) as discussed in the said entries.

Entry 153

Ramanujan's theory of elliptic functions to alternative bases can be related to the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145. Define,

α4(τ)=16(η(τ)η(4τ))8+16=(2η(τ)η2(4τ)η3(2τ))8

Let α4=α4(τ). Then we conjecture that,

2F1(12,12,1,1α4)2F1(12,12,1,α4)=τ4 as well as

j4(τ)=16α4(1α4)=((η(τ)η(4τ))4+42(η(4τ)η(τ))4)2=(η2(2τ)η(τ)η(4τ))24=(2F1(12,12,1,α4)η2(τ)×η(τ)η(4τ))24/5

Example. Let τ=3. Then α4=(32)4(21)4 solves 2F1(12,12,1,1α4)2F1(12,12,1,α4)=4×3 Alternatively and more familiar

2F1(12,12,1,1λ(r))2F1(12,12,1,λ(r))=r where λ(τ) is the modular lambda function.

Entry 152

We relate Ramanujan's theory of elliptic functions to alternative bases to the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145. Define,

α3(τ)=27(η(τ)η(3τ))12+27=(3(η(τ/3)η(3τ))3+3)3

Let α3=α3(τ). Then we conjecture that,

2F1(13,23,1,1α3)2F1(13,23,1,α3)=τ3 as well as

j3(τ)=27α3(1α3)=((η(τ)η(3τ))6+33(η(3τ)η(τ))6)2=(2F1(13,23,1,α3)η2(τ)×η(τ)η(3τ))24/4

Example. Let τ=3. Then α3=1250(18717121/3+1822/3) solves 2F1(13,23,1,1α3)2F1(13,23,1,α3)=3×3

Entry 151

Ramanujan's theory of elliptic functions to alternative bases can be related to the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145. Define,

α2(τ)=64(η(τ)η(2τ))24+64=(8(η(τ/2)η(2τ))8+8)2

Let α2=α2(τ). Then we conjecture that,

2F1(14,34,1,1α2)2F1(14,34,1,α2)=τ2 as well as

j2(τ)=64α2(1α2)=((η(τ)η(2τ))12+26(η(2τ)η(τ))12)2=(2F1(14,34,1,α2)η2(τ)×η(τ)η(2τ))24/3

Example. Let τ=3. Then α2=13(2+3)2(13+43) solves 2F1(14,34,1,1α2)2F1(14,34,1,α2)=2×3

Entry 150

Ramanujan's theory of elliptic functions to alternative bases considers the hypergeometric function 2F1(a,b;c;z) with a+b=c=1 for the cases a=16,14,13,12. We can relate this to the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145 for n=1,2,3,4. Define,

α1(τ)=12(111728j1(τ))

where j=j1(τ) is the j-function. Let α1=α1(τ). Then we conjecture that,

2F1(16,56,1,1α1)2F1(16,56,1,α1)=τ1 as well as

j1(τ)=432α1(1α1)=((η(τ)η(2τ))8+28(η(2τ)η(τ))16)3=(2F1(16,56,1,α1)η2(τ))24/2

Example. Let τ=3. Then α1=155(1+52)5 solves 2F1(16,56,1,1α1)2F1(16,56,1,α1)=3 since τ1=3i×i=3.

Entry 149

This is the octic overview. Using the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145, if τ are complex quadratics such that jn(τ) is a radical, then the following octics have a solvable Galois group, hence solvable in radicals j1=(x2+5x+1)3(x2+13x+49)xj22=j2(7x4196x31666x23860x+49)+(x2+14x+21)4xj3=(x+1)6(x2+x+7)xj42=7j4(x+1)4+(x+1)7(x7)x

It would be good if the one for j2 can be simplified. They have discriminants (with another factor of D2 suppressed),

D1=77(j11728)4j14D2=77(j2256)4j26D3=77(j3108)3j35D4=77(j464)4j412

Examples. Let τ=1+41/32 so j3(τ)=(43)6. Let τ=1+89/32 so j3(τ)=(103)6. Then (x+1)6(x2+x+7)x=(43)6(x+1)6(x2+x+7)x=(103)6 are octics both solvable in radicals. And also eπ41/3=(43)6+41.993eπ89/3=(103)6+41.99997 There are infinitely many τ but special ones such that jn(τ) are integers can be found in Entry 145.

Entry 148

This is the 7th-deg overview though only results by Klein for the j-function j=j1(τ) are known. In Klein's "On the Order-Seven Transformations of Elliptic Functions", he gave two elegant resolvents of degrees 7 and 8 in pages 306 and 313. Translated to more understandable notation, we have,

x(x2+7(172)x+7(1+72)3)3=j

y8+14y6+63y4+70y27=yj1728

If τ are complex quadratics such that j=j1(τ) is a radical, then the two resolvents have a solvable Galois group, hence solvable in radicals

Example. Let τ=1+1632, then j=6403203 and x(x2+7(172)x+7(1+72)3)3=6403203 is solvable in radicals. There are infinitely many such τ and some can be found in Entry 145. Note also that (y4+14y3+63y2+70y7)2y+1728=(y2+5y+1)3(y2+13y+49)y where the octic on the RHS will appear in the next entry.

Sunday, June 8, 2025

Entry 147

This is the sextic overview. Using the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145, if τ are complex quadratics such that jn(τ) is a radical, then the following sextics have a solvable Galois group, hence solvable in radicals j1=(x2+10x+5)3xj2=(x+1)4(x2+6x+25)xj32=j3(2x6+29x5+85x4+50x3)+54x6(2x1)j4=(x+1)5(x+5)x

with the ones in red by Joachim König. (It would be nice if the one for j3 can be simplified.) They have discriminants,

D1=55(j11728)2j14D2=55(j2256)3j23D3=55(j3108)3j311D4=55(j464)2j44

Examples. Let τ=1+1632 so j1(τ)=6403203. Let τ=582 so j2(τ)=3964. Then (x2+10x+5)3x=6403203(x+1)4(x2+6x+25)x=3964 are sextics both solvable in radicals. There are infinitely many τ but special ones such that jn(τ) are integers can be found in Entry 145.

Entry 146

This is the quintic overview of Entries 140-144. Using the McKay-Thompson series jn=jn(τ) for the Monster defined in Entry 145, if τ are complex quadratics such that jn(τ) is a radical, then the following simple quintics have a solvable Galois group hence solvable in radicals x5+5x4+40x3=j1x(x5)4=j2x3(x5)2=j3x5+5x=64j4j4x5+5x310x2=j6 Example. Let \tau = \tfrac{1+\sqrt{-163}}2 so j_1(\tau) = -640320^3. Let \tau = \tfrac{\sqrt{-58}}2 so j_2(\tau) = 396^4. Then x^5 + 5x^4 + 40x^3 = -640320^3\\ x(x - 5)^4 = 396^4 are quintics both solvable in radicals. Of course, it is well-known that \qquad e^{\pi\sqrt{163}} = 640320^3+743.99999999999925\dots\\ e^{\pi\sqrt{58}} = 396^4-104.00000017\dotsThere are infinitely many \tau but special ones such that j_n(\tau) are integers can be found in Entry 145.

Entry 145

Summarizing the McKay-Thompson series of the Monster discussed in Entries 140-144, 

\begin{align}\quad j_1 &=\left(\left(\frac{\eta(\tau)}{\eta(2\tau)}\right)^{8}+2^8 \left(\frac{\eta(2\tau)}{\eta(\tau)}\right)^{16}\right)^3 \\ \quad j_{2} &=\left(\left(\frac{\eta(\tau)}{\eta(2\tau)}\right)^{12}+2^6 \left(\frac{\eta(2\tau)}{\eta(\tau)}\right)^{12}\right)^2 \\ \quad j_{3} &=\left(\left(\frac{\eta(\tau)}{\eta(3\tau)}\right)^{6}+3^3 \left(\frac{\eta(3\tau)}{\eta(\tau)}\right)^{6}\right)^2 \\ \quad j_{4} &=\left(\left(\frac{\eta(\tau)}{\eta(4\tau)}\right)^{4} + 4^2 \left(\frac{\eta(4\tau)}{\eta(\tau)}\right)^{4}\right)^2 = \left(\frac{\eta^2(2\tau)}{\eta(\tau)\,\eta(4\tau)} \right)^{24}\\ j_{6} &=\left( \left(\frac{\eta(\tau)\,\eta(3\tau)}{\eta(2\tau)\,\eta(6\tau)}\right)^3 + 2^3\left(\frac{\eta(2\tau)\,\eta(6\tau)}{\eta(\tau)\,\eta(3\tau)}\right)^3\right)^2  \end{align}

where j_1(\tau) is the j-function. Let \tau be complex quadratics \tau = \tfrac12\sqrt{-d} or \tau =\frac12+ \sqrt{-d} such that the j_n(\tau) are radicals. For the following special \tau, then j_n(\tau) are integers 

j_1(\tau)\; \text{where}\; \tau=\sqrt{-d}\;\text{for}\; d = 1, 2, 3, 4, 7,\; \text{and}\, \tau =\tfrac{1+\sqrt{-d}}2\,\text{for}\; d =1, 3, 7, 11, 19, 127, 43, 67, 163.

j_2(\tau)\; \text{where}\; \tau=\tfrac{\sqrt{-d}}2\;\text{for}\; d = 4, 6, 10, 18, 22, 58,\; \text{and}\, \tau =\tfrac{1+\sqrt{-d}}2\,\text{for}\; d =5, 7, 9, 13, 25, 37.

j_3(\tau)\; \text{where}\; \tau=\tfrac{\sqrt{-d/3}}2\;\text{for}\; d = 4,8,16,20,\; \text{and}\, \tau =\tfrac{1+\sqrt{-d/3}}2\,\text{for}\; d =5, 9, 17, 25, 41, 49, 89.

j_4(\tau)\; \text{where}\; \tau=\tfrac{\sqrt{-d}}2\;\text{for}\; d = 3, 7,\; \text{and}\, \tau =\tfrac{1+\sqrt{-d}}2\,\text{for}\; d =1, 2, 4.

j_6(\tau)\; \text{where}\; \tau=\tfrac{\sqrt{-d/3}}2\;\text{for}\; d = 10, 14, 26, 34,\; \text{and}\, \tau =\tfrac{1+\sqrt{-d/3}}2\,\text{for}\; d = 7, 11, 19, 31, 59.

Saturday, June 7, 2025

Entry 144

Define the McKay-Thompson series of Class 6A for the Monster j_6 = j_{6}(\tau) =\left( \left(\frac{\eta(\tau)\,\eta(3\tau)}{\eta(2\tau)\,\eta(6\tau)}\right)^3 + 2^3\left(\frac{\eta(2\tau)\,\eta(6\tau)}{\eta(\tau)\,\eta(3\tau)}\right)^3\right)^2 and the quintic x^5-5\alpha x^3+10\alpha^2x-\alpha^2=0 where \small\alpha = -\dfrac1{j_6-32}. Alternatively (y^2+15)^2(y-5) = 32\big(j_6-32\big) z^5 + 5z^3 - 10z^2 = j_6

Conjecture: "If \tau is a complex quadratic such that j_6=j_{6}(\tau) is an algebraic number with j_6\neq 32, then the quintics above have a solvable Galois group."

Example: Let j_6\big(\tfrac{1\sqrt{-59/3}}2\big)=-1060^2, then (y^2+15)^2(y-5) = 32\big({-1060^2}-32\big) z^5 + 5z^3 - 10z^2=-1060^2 are solvable in radicals.

Entry 143

Define the McKay-Thompson series of Class 4A for the Monster j_4 = j_{4}(\tau) = \left(\left(\frac{\eta(\tau)}{\eta(4\tau)}\right)^{4} + 4^2 \left(\frac{\eta(4\tau)}{\eta(\tau)}\right)^{4}\right)^2 = \left(\frac{\eta^2(2\tau)}{\eta(\tau)\,\eta(4\tau)} \right)^{24} and the Bring-Jerrard quintic y^5+5y=\left(\frac{64}{\sqrt{j_4}}-\sqrt{j_4}\right)^{1/2}

Conjecture: "If \tau is a complex quadratic such that j_4=j_{4}(\tau) is an algebraic number, then the quintic above has a solvable Galois group."

Example: Let j_4\big(\tfrac12\sqrt{-7}\big)=2^{12}, then y^5+5y=\sqrt{-63} is solvable in radicals.

Entry 142

Define the McKay-Thompson series of Class 3A for the Monster j_3 = j_{3}(\tau) =\left(\left(\frac{\eta(\tau)}{\eta(3\tau)}\right)^{6}+3^3 \left(\frac{\eta(3\tau)}{\eta(\tau)}\right)^{6}\right)^2 and the Euler-Jerrard quintic x^5+5\sqrt{\alpha}\, x^2 -\sqrt{\alpha} = 0 Alternatively y^3(y-5)^2 =j_3

Conjecture: "If \tau is a complex quadratic such that j_3=j_{3}(\tau) is an algebraic number, then the quintic above has a solvable Galois group."

Example: Let j_3\big(\tfrac{1+\sqrt{-89/3}}2\big)=-300^3, then y^3(y-5)^2 = -300^3 is solvable in radicals.

Entry 141

Define the McKay-Thompson series of Class 2A for the Monster j_2 = j_{2}(\tau) =\left(\left(\frac{\eta(\tau)}{\eta(2\tau)}\right)^{12}+2^6 \left(\frac{\eta(2\tau)}{\eta(\tau)}\right)^{12}\right)^2 and the Bring-Jerrard quintic x^5-5\alpha x -\alpha=0 Alternatively y(y-5)^4 =j_2

Conjecture: "If \tau is a complex quadratic such that j_2=j_{2}(\tau) is an algebraic number, then the quintic above has a solvable Galois group."

Example: Let j_2\big(\tfrac12\sqrt{-10}\big)=12^4, then \quad y(y-5)^4=12^4\\ z^5-5z-12=0 are solvable in radicals.

Entry 140

Given the Dedekind eta function \eta(\tau) and define the McKay-Thompson series of Class 1A for the Monster, or better known as the j-function j j=j_1(\tau) =\left(\left(\frac{\eta(\tau)}{\eta(2\tau)}\right)^{8}+2^8 \left(\frac{\eta(2\tau)}{\eta(\tau)}\right)^{16}\right)^3 and the Brioschi quintic x^5-10\alpha x^3+45\alpha^2x-\alpha^2=0 where \small\alpha = -\dfrac1{j-1728}. Alternatively (y^2+20)^2(y-5) = j-1728 z^5 + 5z^4 + 40z^3=jConjecture: "If \tau is a complex quadratic such that j is an algebraic number j\neq1728, then the quintics above have a solvable Galois group." 

Example. Let j\big(\tfrac{1+\sqrt{-163}}2\big) = -640320^3 and \alpha = \tfrac1{640320^3+1728}, then x^5-10\alpha x^3+45\alpha^2x-\alpha^2=0 (y^2+20)^2(y-5) = -640320^3-1728 z^5 + 5z^4 + 40z^3=-640320^3 are quintics solvable in radicals. (In fact, they factor into a quadratic and a cubic.)

Entry 139

The general quintic can be reduced to the following one-parameter forms

x^5-10\alpha x^3+45\alpha^2x-\alpha^2=0\tag1

x^5-5\alpha x -\alpha = 0\tag2

x^5+5\sqrt{\alpha}\, x^2 -\sqrt{\alpha} = 0\tag3

\; x^5+5x+\left(\frac1{\sqrt{\alpha}}-64\sqrt{\alpha}\right)^{1/2} = 0\tag4

x^5-5\alpha x^3+10\alpha^2x-\alpha^2=0\tag5 with the last found by yours truly. They have neat discriminants

\begin{align}D_1 &= 5^5\,(1-1728\alpha)^2\,\alpha^8\\ D_2 &= 5^5\,(1-256\alpha)\,\alpha^4\\ D_3 &= 5^5\,(1-108\alpha)\,\alpha^2\\ D_4 &= 5^5\,(1+64\alpha)^2\,\alpha^{-1}\\ D_5 &= 5^5\,(1-36\alpha)(1-32\alpha)\,\alpha^8\end{align} The integers (1728, 256, 108, 64) appear in Ramanujan's theory of elliptic functions to alternative bases and we will connect these quintics to the McKay-Thompson series of class 1A, 2A, 3A, 4A, 6A for the Monster in subsequent entries. 

Entry 138

This summarizes the last several entries. Let fundamental discriminant d = 4m with class number h(-d)=2^k and even m = 2p for prime p. Previously, p \equiv 1\,\text{mod}\,4 and p \equiv 3\,\text{mod}\,4 were distinguished, but we can have a more unified approach. Given the modular lambda function \lambda(\tau) and define the three simple functions \begin{align}\alpha(n) &= \Big(n+\sqrt{n^2-1}\Big)^2\\ \beta(n) &= \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2\\ \gamma(n) &= \Big(n+\sqrt{n^2+1}\Big)^2 \end{align} If p \equiv 3\,\text{mod}\,4, then \frac1{\lambda(\sqrt{-2p})} = \alpha(n)\,\beta(n),\quad \text{where}\, n = \frac{2\sqrt{\lambda}+1-\lambda}{2\lambda^{1/4}\sqrt{1-\lambda}} If p \equiv 1\,\text{mod}\,4, then \frac1{\lambda(\sqrt{-2p})} = \alpha(n)\,\gamma(n),\quad \text{where}\, n = \frac{\lambda+1}{2\lambda^{1/4}\sqrt{1-\lambda}} with \lambda=\lambda(\tau) for simplicity and \alpha(n),\,\beta(n),\,\gamma(n) are units. Examples. Let m = 2p with class number 4

For p = 7,23 \begin{align}\frac1{\lambda(\sqrt{-14})} &= \alpha(n)\,\beta(n),\quad n=2(1+\sqrt2)\\ \frac1{\lambda(\sqrt{-46})} &= \alpha(n)\,\beta(n),\quad n=2(13+9\sqrt2) \end{align} For p=17,41 \begin{align}\frac1{\lambda(\sqrt{-34})} &= \alpha(n)\,\gamma(n),\quad n=3(4+\sqrt{17})\\ \frac1{\lambda(\sqrt{-82})} &= \alpha(n)\,\gamma(n),\quad n=3(51+8\sqrt{41})\end{align} One can observe that n is an algebraic number of degree half that of the class number. For m = 2p with class number 8, examples for p \equiv 3\,\text{mod}\,4 were given in Entry 136. For p \equiv 1\,\text{mod}\,4 like p=89, then n^4 - 8886 n^3 + 648 n^2 - 10314 n + 5751=0 and so on for other p.

Friday, June 6, 2025

Entry 137

This continues Entry 136. Recall the function \beta(n) = \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2 and the examples of n which were quartic roots. It turns out these n have additional properties which yield fundamental units U_k though I don't know why.

For p=31, let n_{\color{red}\pm} = 2(1+\sqrt2)^2\Big(1+3\sqrt2\color{red}{\pm}2\sqrt{1+4\sqrt2}\Big) or the two real roots of the quartic. Then \frac{\beta(n_{+})}{\beta(n_{-})} = U_{31}\sqrt{U_{62}} = (1520+273\sqrt{31})\,(4\sqrt2+\sqrt{31}) For p=47, let n_{\color{red}\pm} = 2(1+\sqrt2)^3\Big(9\color{red}{\pm}2\sqrt{9+8\sqrt2}\Big) or again the two real roots. Then \frac{\beta(n_{+})}{\beta(n_{-})} = U_{47}\sqrt{U_{94}} = (48+7\sqrt{47})\,(732\sqrt2+151\sqrt{47}) and so on for p = 31, 47, 79, 191, 239, 431.

Entry 136

Given fundamental discriminants d = 4m with class number h(-d)=8. Then there are exactly ten m = 2p for prime p, namely p \equiv 1\,\text{mod}\,4 = 89, 113, 233, 281 and p \equiv 3\,\text{mod}\,4 = 31, 47, 79, 191, 239, 431. The modular lambda function \lambda(\sqrt{-2p}) for both is a root of a deg-16 equation, but the latter is easier to factor into two deg-8 equations. Define the two simple functions \begin{align}\alpha(n) &= \Big(n+\sqrt{n^2-1}\Big)^2\\ \beta(n) &= \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2 \end{align} It seems for the p \equiv 3\,\text{mod}\,4, then \frac1{\lambda(\sqrt{-2p})} = \alpha(n)\,\beta(n) where \alpha(n),\beta(n) are octic units and n is just a quartic root given by n = \frac{2\sqrt{\lambda}+1-\lambda}{2\lambda^{1/4}\sqrt{1-\lambda}} with \lambda=\lambda(\tau) for simplicity. Examples,

Let p=31 and n= 2(1+\sqrt2)^2\Big(1+3\sqrt2+2\sqrt{1+4\sqrt2}\Big) then \frac1{\lambda(\sqrt{-62})} = \alpha(n)\,\beta(n) = \Big(n+\sqrt{n^2-1}\Big)^2 \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2 Let p=47 and  n= 2(1+\sqrt2)^3\Big(9+2\sqrt{9+8\sqrt2}\Big) then \frac1{\lambda(\sqrt{-94})} = \alpha(n)\,\beta(n) = \Big(n+\sqrt{n^2-1}\Big)^2 \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2 and so on for p = 31, 47, 79, 191, 239, 431. P.S. Note that solutions to the Pell-like equation x^2-2y^2 = -p appear in the nested radicals \sqrt{x+y\sqrt2\,} above, namely1^2-2\cdot4^2=-31\\ 9^2-2\cdot8^2=-47

Entry 135

Mathworld has a list of the modular lambda function \lambda(\tau) with the particular case \tau=\sqrt{-14} as the rather complicated \sqrt{\lambda(\sqrt{-14})}=-11-8\sqrt2-2(2+\sqrt2)\sqrt{5+4\sqrt2}\qquad \\\ \qquad+\sqrt{11+8\sqrt2}\,\Big(2+2\sqrt2+\sqrt2\sqrt{5+4\sqrt2}\Big)\qquad which is approximately 0.011208. It can be calculated in Mathematica or WolframAlpha as ModularLambda[tau]. However, we can simplify and factor that into two quartic units as \begin{align}\frac1{\lambda(\sqrt{-14})} & =\frac1{2^8}(8+3\sqrt7)\,(\sqrt7+\sqrt8)\,\Big(2^{1/4}+\sqrt{4+\sqrt2}\Big)^8\\ &=7960.423255\dots\end{align} It is then just a matter of getting the reciprocal and square roots. One can do so similarly for discriminants d=4m with class number h(-d)=4 and even m=14,62,142 as described in Entry 134.

Entry 134

Given fundamental discriminants d = 4m with class number h(-d)=4, there are exactly five m = 2p for prime p, namely p=7,17,23,41,71. The cases p = 17, 41 were in Entry 115 while p = 7,23,71 which are p \equiv 3\,\text{mod}\,4 will be tackled here. Define the two simple functions\begin{align}\alpha(n) &= \Big(n+\sqrt{n^2-1}\Big)^2\\ \beta(n) &= \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2 \end{align}and let\begin{align}n_1 &=2+2\sqrt2 \\ n_2 &= 26+18\sqrt2 \\ n_3 &=1450+1026\sqrt2\end{align}Then the first quartic unit can be an eighth power 

\begin{align}\alpha(n_1) &=\frac1{2^8}\Big(\sqrt{0+\sqrt2}+\sqrt{4+\sqrt2}\Big)^8\\ \alpha(n_2)  &=\frac1{2^8}\Big(\sqrt{4+3\sqrt2}+\sqrt{8+3\sqrt2}\Big)^8\\ \alpha(n_3)  &=\frac1{2^8}\Big(\sqrt{36+27\sqrt2}+\sqrt{40+27\sqrt2}\Big)^8 \end{align} while the second is a product of quadratic units 

\begin{align}\beta(n_1) &= U_{7}\,\sqrt{U_{14}}=\big(8+3\sqrt{2}\big) \big(2\sqrt2+\sqrt{7}\big) \\ \beta(n_2)  &= U_{23}\,\sqrt{U_{46}}=\big(25+5\sqrt{23}\big) \big(78\sqrt2+23\sqrt{23}\big) \\ \beta(n_3)  &= U_{71}\,\sqrt{U_{142}}=\big(3480+413\sqrt{71}\big) \big(1710\sqrt2+287\sqrt{71}\big) \\ \end{align} which gives the radical expressions of  \begin{align}\frac1{\lambda(\sqrt{-14})} &= \alpha(n_1)\,\beta(n_1),\quad n_1=2+2\sqrt2\\ \frac1{\lambda(\sqrt{-46})} &= \alpha(n_2)\,\beta(n_2),\quad n_2=26+18\sqrt2 \\ \frac1{\lambda(\sqrt{-142})} &= \alpha(n_3)\,\beta(n_3),\quad n_3=1450+1026\sqrt2 \end{align} And since \beta(n) = \Big(\sqrt{n^2-2}+\sqrt{n^2-1}\Big)^2, then they are also nested radicals that use only \sqrt2

Entry 133

Given fundamental discriminants d=4m with class number h(-d)=2, there are exactly four even m = 6, 10, 22, 58. The most well-known is m=58 because of the near-integer \quad e^{\pi\sqrt{58}} = 396^4-104.00000017\dots and the appearance of 396^4 in the denominator of Ramanujan's famous 1/\pi formula. These m=2p for prime p have other interesting properties. Recall the modular lambda function \lambda(\tau) also discussed in Entry 112 \lambda(\tau) = \left(\frac{\sqrt2\,\eta\big(\tfrac12\tau\big)\,\eta^2\big(2\tau\big)}{\eta^3(\tau)}\right)^8 We focus on m=2p for prime p = 3\,\text{mod}\,4 hence \begin{align}\frac1{\sqrt{\lambda(\sqrt{-6})}} &= U_3\sqrt{U_6} = (2+\sqrt3)(\sqrt2+\sqrt3)\\ \frac1{\sqrt{\lambda(\sqrt{-22})}} &= U_{11}\sqrt{U_{22}}= (10+3\sqrt{11})(7\sqrt2+3\sqrt{11}) \end{align} with fundamental units U_n. However, special d with class number h(-d)=2^k surprisingly can be expressed by nested radicals using only the square root of 2. So,\begin{align}\frac1{\sqrt{\lambda(\sqrt{-6})}} &= (1+\sqrt2)^2+\sqrt{1+ (1+\sqrt2)^4}\\ \frac1{\sqrt{\lambda(\sqrt{-22})}} &=  (1+\sqrt2)^6+\sqrt{1+ (1+\sqrt2)^{12}} \end{align}\quad Similar behavior can also be observed for 2p for p=7,23,71 which now have class number 4.

Tuesday, June 3, 2025

Entry 132

Given _2F_1(a,b;c;z) and Dedekind eta function \eta(\tau) where \tau = \frac{1+n\sqrt{-3}}2 for positive integer n. Then for type a+b=c=\color{blue}{\tfrac56} \begin{align}&\,_2F_1\big(\tfrac12,\tfrac13;\tfrac56;(1-2\delta_1)^2\big),\quad\;\delta_1 = \delta_2\\ &\,_2F_1\big(\tfrac13,\tfrac12;\tfrac56;(1-2\delta_2)^2\big),\quad\;\frac1{\delta_2}-1=\sqrt{\frac1{27}\left(\tfrac{\eta\big(\frac{\tau+1}{3}\big)}{\eta(\tau)}\right)^{12}}\\ &\,_2F_1\big(\tfrac14,\tfrac7{12};\tfrac56;(1-2\delta_3)^2\big),\quad\color{red}{\delta_3 =\,?} \\ &\,_2F_1\big(\tfrac16,\tfrac23;\tfrac56;(1-2\delta_4)^2\big),\quad\;\frac1{\delta_4}-1\,=\,\frac1{27}\left(\tfrac{\eta\big(\frac{\tau+1}{3}\big)}{\eta(\tau)}\right)^{12}\end{align} For this type, there are infinitely many hypergeometrics such that both (z_1, z_2) in _2F_1(a,b;c;z_1) = z_2 are algebraic numbers when n is a positive integer. Note that  _2F_1\big(\tfrac12,\tfrac13;\tfrac56;z\big) =\,_2F_1\big(\tfrac13,\tfrac12;\tfrac56;z\big) so the first form is superfluous. Examples: Let \tau = \frac{1+5\sqrt{-3}}2, _2F_1\Big(\frac13,\frac12;\frac56;\frac45\Big)=\;\frac35\sqrt5 \quad _2F_1\Big(\frac16,\frac23;\frac56;\frac{80}{81}\Big)=\frac35\,(9\sqrt5)^{1/3}

Entry 131

Given _2F_1(a,b;c;z) and Dedekind eta function \eta(\tau) where \tau = \frac{1+n\sqrt{-1}}2 for positive integer n. Then for type a+b=c=\color{blue}{\tfrac34} \begin{align}&\,_2F_1\big(\tfrac14,\tfrac12;\tfrac34;(1-2\gamma_1)^2\big), \quad\frac1{\gamma_1}-1=\sqrt{-\frac1{64}\Big(\tfrac{\sqrt2\,\eta(2\tau)}{\eta(\tau)}\Big)^{24}}\\ &\,_2F_1\big(\tfrac16,\tfrac7{12};\tfrac34;(1-2\gamma_2)^2\big),\quad\color{red}{\gamma_2 =\,?} \\ &\,_2F_1\big(\tfrac18,\tfrac58;\tfrac34;(1-2\gamma_3)^2\big),\quad\frac1{\gamma_3}-1\,=\, -\frac1{64}\Big(\tfrac{\sqrt2\,\eta(2\tau)}{\eta(\tau)}\Big)^{24}\\ &\,_2F_1\big(\tfrac1{12},\tfrac23;\tfrac34;(1-2\gamma_4)^2\big),\quad\color{red}{\gamma_4 =\,?}\end{align} For this type, there are infinitely many hypergeometrics such that both (z_1, z_2) in _2F_1(a,b;c;z_1) = z_2 are algebraic numbers when n is a positive integer. Examples: Let \tau = \frac{1+5\sqrt{-1}}2

_2F_1\Big(\frac14,\frac12;\frac34;\frac{80}{81}\Big)=\frac95

\quad _2F_1\Big(\frac18,\frac58;\frac34;\frac{25920}{25921}\Big)=\frac35\,161^{1/4}

Sunday, June 1, 2025

Entry 130

Given _2F_1(a,b;c;z) and j-function j = j(\tau) where \tau = \frac{1+n\sqrt{-3}}2 for positive integer n. Then for type a+b=c=\color{blue}{\tfrac23} \begin{align}&\,_2F_1\big(\tfrac14,\tfrac5{12};\tfrac23;(1-2\beta_1)^2\big),\qquad \color{red}{\beta_1 =\,?} \\ &\,_2F_1\big(\tfrac16,\tfrac12;\tfrac23;(1-2\beta_2)^2\big),\qquad \frac{1}{\beta_2}-1=\sqrt{\frac{-2j+1728-2\sqrt{j(j-1728)}}{1728}}\\ &\,_2F_1\big(\tfrac18,\tfrac{13}{24};\tfrac23;(1-2\beta_3)^2\big),\qquad \color{red}{\beta_3 =\,?} \\ &\,_2F_1\big(\tfrac1{12},\tfrac7{12};\tfrac23;(1-2\beta_4)^2\big),\qquad \frac{1}{\beta_4}-1=\frac{-2j+1728-2\sqrt{j(j-1728)}}{1728}\\\end{align} For this type, there are infinitely many hypergeometrics such that both (z_1, z_2) in _2F_1(a,b;c;z_1) = z_2 are algebraic numbers when n is a positive integer. Examples: Let \tau = \frac{1+3\sqrt{-3}}2, _2F_1\Big(\frac16,\frac12;\frac23;\frac{125}{128}\Big) =\frac43\times2^{1/6} _2F_1\Big(\frac1{12},\frac7{12};\frac23;\frac{64000}{64009}\Big) =\frac23\times253^{1/6} Let \tau = \frac{1+5\sqrt{-3}}2, _2F_1\left(\frac16,\frac12;\frac23;\Big(\frac45\Big)^2\Big(\frac{15-\sqrt5}{11}\Big)^3\right) =\frac35(5+4\sqrt5)^{1/6}

Entry 129

Given _2F_1(a,b;c;z) where a+b=c. The previous four (Entries 125-128) discuss closed-forms and can be neatly summarized as type a+b=c=\color{blue}{\tfrac12}  \begin{align}&\,_2F_1\big(\tfrac14,\tfrac14;\tfrac12;(1-2\alpha_1)^2\big),\qquad \frac1{\alpha_1}-1=\frac1{16}\Big(\tfrac{\eta(\tau/4)}{\eta(\tau)}\Big)^8,\qquad \tau_1=n\sqrt{-4}\\ &\,_2F_1\big(\tfrac16,\tfrac13;\tfrac12;(1-2\alpha_2)^2\big),\qquad \frac1{\alpha_2}-1=\frac1{27}\Big(\tfrac{\eta(\tau/3)}{\eta(\tau)}\Big)^{12},\qquad \tau_2=n\sqrt{-3}\\ &\,_2F_1\big(\tfrac18,\tfrac38;\tfrac12;(1-2\alpha_3)^2\big),\qquad \frac1{\alpha_3}-1=\frac1{64}\Big(\tfrac{\eta(\tau/2)}{\eta(\tau)}\Big)^{24},\qquad \tau_3=n\sqrt{-2}\\ &\,_2F_1\big(\tfrac1{12},\tfrac5{12};\tfrac12;(1-2\alpha_4)^2\big),\quad\; \frac{1}{\alpha_4(1-\alpha_4)}=\frac1{432}\,j(\tau),\qquad\tau_4=n\sqrt{-1}\end{align} For this type, there are infinitely many hypergeometrics such that both (z_1, z_2) in _2F_1(a,b;c;z_1) = z_2 are algebraic numbers when n is a positive integer. However, for the parameters c=\frac12, \frac23,\frac34. \frac56, it seems only the first is easy. For type a+b=c=\color{blue}{\tfrac23}, it's more challenging and will be discussed in the next entry.

Entry 128

Assume \tau=n\sqrt{-\color{blue}{4}} for some positive integer n. Given _2F_1(a,b;c;z) where a+b=c=\frac12 for the case a=\tfrac1{4}. Let z_1 = (1-2w)^2 where w is w=\frac{16}{16+\Big(\tfrac{\eta(\tau/4)}{\eta(\tau)}\Big)^8} Then (z_1, z_2) are algebraic numbers in

_2F_1\left(\tfrac14,\tfrac14;\tfrac12;z_1\right) = z_2

Examples: 

If n=2 so \tau=2\sqrt{-4}, then,

_2F_1\left(\frac14,\frac14;\frac12;\,\frac{9\,(3+\sqrt2)^2}{(1+\sqrt2)^6}\right)=\frac38\big(2+\sqrt2\big) 

If n=3 so \tau=3\sqrt{-4}, then,

\quad _2F_1\left(\frac14,\frac14;\frac12;\,\frac{8\sqrt3}{(2+\sqrt3)^2}\right)=\frac23\big(3+2\sqrt3\big)^{1/2}

Saturday, May 31, 2025

Entry 127

Assume \tau=n\sqrt{-\color{blue}{3}} for some positive integer n. Given _2F_1(a,b;c;z) where a+b=c=\frac12 for the case a=\tfrac1{6}. Let z_1 = (1-2w)^2 where w is w=\frac{27}{27+\Big(\tfrac{\eta(\tau/3)}{\eta(\tau)}\Big)^{12}} Then (z_1, z_2) are algebraic numbers in

_2F_1\left(\tfrac16,\tfrac13;\tfrac12;z_1\right) = z_2

Example: 

If n=2 so \tau=2\sqrt{-3}, then,

_2F_1\left(\frac16,\frac13;\frac12;\,\frac{25}{27}\right)=\frac{3\sqrt3}{4} 

If n=4 so \tau=4\sqrt{-3}, then,

\qquad _2F_1\left(\frac16,\frac13;\frac12;\,\frac{45^2\,(\sqrt2+\sqrt3)^2}{(1+\sqrt6)^8}\right)=\frac58\big(1+\sqrt6\big)

Entry 126

Assume \tau=n\sqrt{-\color{blue}{2}} for some positive integer n. Given _2F_1(a,b;c;z) where a+b=c=\frac12 for the case a=\tfrac1{8}. Let z_1 = (1-2w)^2 where w is w=\frac{64}{64+\Big(\tfrac{\eta(\tau/2)}{\eta(\tau)}\Big)^{24}} Then (z_1, z_2) are algebraic numbers in

_2F_1\left(\tfrac18,\tfrac38;\tfrac12;z_1\right) = z_2

Examples: 

If n=2 so \tau=2\sqrt{-2}, then,

_2F_1\left(\frac18,\frac38;\frac12;\frac{(70\sqrt{2})^2\,(1+\sqrt2)^2}{(2+3\sqrt2)^6}\right)=\frac34\left(\frac{2+3\sqrt2}2\right)^{1/2}

If n=3 so \tau=3\sqrt{-2}, then,

_2F_1\left(\frac18,\frac38;\frac12;\frac{2400}{2401}\right)=\tfrac23\cdot7^{1/2}

Entry 125

Assume \tau=n\sqrt{-\color{blue}{1}} for some positive integer n. Given _2F_1(a,b;c;z) where a+b=c=\frac12 for the case a=\tfrac1{12}. Let j(\tau) be the j-function and z_1 = (1-2w)^2 where w is a root of \frac{12^3}{4w(1-w)} =j(\tau) Or more simply z_1=1-\frac{1728}{j(\tau)}, then (z_1, z_2) are algebraic numbers in

\,_2F_1\left(\tfrac1{12},\tfrac5{12};\tfrac12;z_1\right) = z_2

Examples: 

If n=2 so \tau=2\sqrt{-1} and j(\tau)=66^3, then,

_2F_1\left(\frac1{12},\frac5{12};\frac12;\frac{1323}{1331}\right)=\frac34\cdot11^{1/4} 

If n=3 so \tau=3\sqrt{-1}, then,

\qquad _2F_1\left(\frac1{12},\frac5{12};\frac12;\frac{(14\sqrt6)^2\,(72+43\sqrt3)}{(21+20\sqrt3)^3}\right)=\frac23(21+20\sqrt3)^{1/4}

Entry 124

In Entry 123, the 24th power of the golden ratio \phi and Gauss' constant G was discussed. We will use it again and connect it to the Dedekind eta function \eta(\tau) and Watson's triple integral \begin{align}I_1 &= \frac{1}{\pi^3}\int_0^\pi \int_0^\pi \int_0^\pi \frac{dx\, dy\, dz}{1-\cos x\cos y\cos z}\\ &=\frac{\Gamma^4(\frac{1}{4})}{4\pi^3}\\ &= 2\,G^2 = 4\,\eta(i)^4 = 1.393203\dots\end{align} where I_1 =\frac{\,25\phi^6}{\sqrt{\phi^{24}-4}}\sum_{n=0}^\infty \frac{(6n)!}{(3n)!\,n!^3} \left(\frac{-\phi^{16}}{4(\phi^{24}-4)}\right)^{3n}=1.393203\dots Notice that the 24th power of the golden ratio is off by 4.

Entry 123

In the previous entry, we had the unusual hypergeometric function, call it \beta \beta =\,_2F_1\Big(\tfrac12,\tfrac12;1;\lambda\big(1+2i/5\big)\Big) = 2^{1/4}\left(\Big(\tfrac{1+\sqrt5}2\Big)^{12}-\sqrt{\Big(\tfrac{1+\sqrt5}2\Big)^{24}-1}\right)^{1/4}G with Gauss's constant G. Ramanujan found the unusual equality \sqrt[8]{1+\sqrt{1-\left(\tfrac{-1+\sqrt{5}}{2}\right)^{24}}} = \tfrac{-1+\sqrt{5}}{2}\,\left(\tfrac{1+\sqrt[4]{5}}{\sqrt{2}}\right) and one can notice the 24th powers in both cases. After some manipulation, we find the similar \sqrt[8]{\left(\tfrac{1+\sqrt{5}}2\right)^{12}-\sqrt{\left(\tfrac{1+\sqrt{5}}2\right)^{24}-1}} = \sqrt{\tfrac{1+\sqrt{5}}2}\left(\tfrac{-1+\sqrt[4]{5}}{\sqrt{2}}\right)\quad therefore \beta =\,_2F_1\Big(\tfrac12,\tfrac12;1;\lambda\big(1+2i/5\big)\Big) = 2^{1/4}\left(\tfrac{1+\sqrt5}2\right) \left(\tfrac{-1+\sqrt[4]{5}}{\sqrt{2}}\right)^2 G

Entry 122

Given Gauss' constant G, the elliptic integral singular value K(k_r), \begin{align}G &=\frac{\sqrt2}{\pi} K(k_1) = \frac2{\pi}\int_0^{\pi/2}\frac{d\,\theta}{\sqrt{1+\sin^2\theta}}\\ &= \tfrac1{\sqrt2}\,_2F_1\big(\tfrac12,\tfrac12;1;\tfrac12\big) = \,_2F_1\big(\tfrac12,\tfrac12;1;-1\big)\\ &=(2\pi)^{-3/2}\,\Gamma^2\big(\tfrac14\big) = 0.834626\dots\end{align} and modular lambda function \lambda(\tau) calculated by Mathematica as ModularLambda[tau]. The two hypergeometrics can be expressed as \tfrac1{\sqrt2}\,_2F_1\big(\tfrac12,\tfrac12;1;\lambda(i)\big) = \,_2F_1\big(\tfrac12,\tfrac12;1;\lambda(1+i)\big) = G while more complicated ones are \begin{align}\,_2F_1\Big(\tfrac12,\tfrac12;1;\lambda\big(1+2i/1\big)\Big) &= 2^{1/4}G\\ \,_2F_1\Big(\tfrac12,\tfrac12;1;\lambda\big(1+2i/3\big)\Big) &= 6^{1/4}\left((1+\sqrt{3})^2-\sqrt{(2+\sqrt3)^4-1}\right)^{1/4}G\\ \quad\,_2F_1\Big(\tfrac12,\tfrac12;1;\lambda\big(1+2i/5\big)\Big) &= 2^{1/4}\left(\Big(\tfrac{1+\sqrt5}2\Big)^{12}-\sqrt{\Big(\tfrac{1+\sqrt5}2\Big)^{24}-1}\right)^{1/4}G\end{align} and so on, with fundamental units U_3 = 2+\sqrt3 and U_5= \frac{1+\sqrt5}2, and where the \lambda(\tau) are actually radicals. Curiously, \lambda(i)\ = \lambda\big(\sqrt{3+4i}\big) = \frac12.

Entry 121

This is the case a=\frac13 of {_2F_1\left(a ,a ;a +\tfrac12;-u\right)}=2^{a}\frac{\Gamma\big(a+\tfrac12\big)}{\sqrt\pi\,\Gamma(a)}\int_0^\infty\frac{dx}{(1+2u+\cosh x)^a} we have \frac{1}{48^{1/4}\,K(k_3)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[3]{x^2+\color{blue}{4}x^3}}=\,_2F_1\big(\tfrac{1}{3},\tfrac{1}{3};\tfrac{5}{6};-\color{blue}{4}\big)= \frac3{5^{5/6}}

\frac{1}{48^{1/4}\,K(k_3)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[3]{x^2+\color{blue}{27}x^3}}=\,_2F_1\big(\tfrac{1}{3},\tfrac{1}{3};\tfrac{5}{6};-\color{blue}{27}\big)=\frac{4}{7} (To be continued.) 

Friday, May 30, 2025

Entry 120

This is the case a=\frac14 of {_2F_1\left(a ,a ;a +\tfrac12;-u\right)}=2^{a}\frac{\Gamma\big(a+\tfrac12\big)}{\sqrt\pi\,\Gamma(a)}\int_0^\infty\frac{dx}{(1+2u+\cosh x)^a} we have \frac{1}{2\sqrt2\,K(k_1)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[4]{x^3+\color{blue}{3}x^4}}=\,_2F_1\big(\tfrac{1}{4},\tfrac{1}{4};\tfrac{3}{4};-\color{blue}3\big) = \frac{2}{3^{3/4}}

\frac{1}{2\sqrt2\,K(k_1)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[4]{x^3+\color{blue}{80}x^4}}=\,_2F_1\big(\tfrac{1}{4},\tfrac{1}{4};\tfrac{3}{4};-\color{blue}{80}\big) = \frac35 (To be continued.) 

Entry 119

This is the case a=\frac16 of {_2F_1\left(a ,a ;a +\tfrac12;-u\right)}=2^{a}\frac{\Gamma\big(a+\tfrac12\big)}{\sqrt\pi\,\Gamma(a)}\int_0^\infty\frac{dx}{(1+2u+\cosh x)^a} we have \frac{1}{\color{red}{432}^{1/4}\,K(k_3)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[6]{x^5+\color{blue}{\tfrac{125}3}x^6}}=\,_2F_1\big(\tfrac16,\tfrac16;\tfrac23;-\color{blue}{\tfrac{125}{3}})=\frac{2}{3^{5/6}}

\frac{1}{\color{red}{432}^{1/4}\,K(k_3)}\,\int_0^1 \frac{dx}{\sqrt{1-x}\,\sqrt[6]{x^5+\color{blue}{2^7\phi^9}\, x^6}}=\,_2F_1\big(\tfrac16,\tfrac16;\tfrac23;-\color{blue}{2^7\phi^9})=\frac{3}{5^{5/6}}\phi^{-1} with golden ratio phi. (To be continued.) 

Thursday, May 29, 2025

Entry 118

If one has a palindromic quartic of form z^4-abz^3+(a^2+b^2-2)z^2-abz+1=0 then its roots can be factored as roots (x,y) of quadratics x^2+ax+1=0\\ y^2+by+1= 0 z = xy = \left(\tfrac{-a+\sqrt{a^2-4}}2\right) \left(\tfrac{-b+\sqrt{b^2-4}}2\right) hence are products of quadratic units. (To be continued.)

Entry 117

For fundamental discriminants d=4m with class number h(-d)=16, there are exactly 60 m that are even. The largest is m = 3502 = 2\times17\times103 hence has 2^3 = 8 divisors. But this set has no m with 32 divisors so it seems one can't express their modular lambda function \lambda(\sqrt{-m}) with 16 quadratic units. (Unlike for h(-d)=8 where we can express a few \lambda(\sqrt{-m}) with 8 quadratic units.) However, using another function, we can have four quartic units. Given the nome q = e^{\pi i\tau}, \tau=\sqrt{-n}, and the Ramanujan G and g functions \begin{align}2^{1/4}G_n &= q^{-\frac{1}{24}}\prod_{k>0}(1+q^{2k-1}) = \frac{\eta^2(\tau)}{\eta\big(\tfrac{\tau}{2}\big)\eta(2\tau)}\\ 2^{1/4}g_n &= q^{-\frac{1}{24}}\prod_{k>0}(1-q^{2k-1}) = \frac{\eta\big(\tfrac{\tau}{2}\big)}{\eta(\tau)}\end{align} discussed in Entry 116. Then \color{red}u  = (g_{3502})^4 = \small\left(\frac{\;\eta\big(\tfrac{\tau}{2}\big)}{2^{1/4}\eta(\tau)}\right)^4 = \big(a+\sqrt{a^2-1}\big)^2 \big(b+\sqrt{b^2-1}\big)^2 \big(c+\sqrt{c^2-1}\big) \big(d+\sqrt{d^2-1}\big) \approx 1.43\times10^{13} where \tau = \sqrt{-3502} and (a,b,c,d) are \begin{align}a &= \tfrac{1}{2}(23+4\sqrt{34})\\ b &= \tfrac{1}{2}(19\sqrt{2}+7\sqrt{17})\\ c &= (429+304\sqrt{2})\\ d &= \tfrac{1}{2}(627+442\sqrt{2})\end{align}A version of this was first found by Daniel Shanks in 1980 (Quartic Approximations for Pi) but this one is slightly different with smaller integers since I simplified the first two expressions as squares. These radicals imply a very close approximation to pi,\pi \approx \frac{1}{\sqrt{3502}}\ln\big((2\color{red}u)^6+24\big) which differs by just 10^{-161}.

Wednesday, May 28, 2025

Entry 116

For fundamental discriminants d=4m with class number h(-d)=8, there are exactly 29 m that are even. The three m = 210, 330, 462 were discussed in Entry 114 since they have special properties. However, the four largest are m = 598, 658, 742, 862. Given the nome q = e^{\pi i\tau}, we can use the Ramanujan G and g functions \begin{align}2^{1/4}G_n &= q^{-\frac{1}{24}}\prod_{k>0}(1+q^{2k-1}) = \frac{\eta^2(\tau)}{\eta\big(\tfrac{\tau}{2}\big)\eta(2\tau)}\\ 2^{1/4}g_n &= q^{-\frac{1}{24}}\prod_{k>0}(1-q^{2k-1}) = \frac{\eta\big(\tfrac{\tau}{2}\big)}{\eta(\tau)}\end{align} with \tau=\sqrt{-n} where Ramanujan uses G_n and g_n for odd n and even n, respectively. For these four m = n, then \begin{align}(g_{598})^2 &=  \left(\frac{\;\eta\big(\tfrac{\tau}{2}\big)}{2^{1/4}\eta(\tau)}\right)^2 = \Big(6+\sqrt{26}+\sqrt{(6+\sqrt{26})^2-1}\Big)(1+\sqrt2)^2\Big(\tfrac{3+\sqrt{13}}2\Big)\\ (g_{658})^2 &=  \left(\frac{\;\eta\big(\tfrac{\tau}{2}\big)}{2^{1/4}\eta(\tau)}\right)^2 = \, ?? \\ (g_{762})^2 &= \left(\frac{\;\eta\big(\tfrac{\tau}{2}\big)}{2^{1/4}\eta(\tau)}\right)^2 = \Big(\tfrac{11+\sqrt{106}}2+\sqrt{\big(\tfrac{11+\sqrt{106}}2\big)^2-1}\Big)\,(1+\sqrt2)^2\,\Big(\tfrac{7+\sqrt{53}}2\Big)\\ (g_{862})^2 &=  \left(\frac{\;\eta\big(\tfrac{\tau}{2}\big)}{2^{1/4}\eta(\tau)}\right)^2  = \, ??\end{align} I know the octics for the other two, though I don't know how to factor them into quartic units.

Entry 115

For fundamental discriminants d=4m with class number h(-d)=4, there are exactly twelve m that are even. In Entry 113, the seven with 8 divisors were discussed. The remaining five are m = 14, 34, 46, 82, 142 which are of form m=2p for prime p=7, 17, 23, 41, 71. From experience, p\equiv 1\, (\text{mod}\, 4) are more well-behaved, hence for m=34,82 \begin{align}\frac1{\sqrt{\lambda(\sqrt{-34})}} &= (1+\sqrt2)^2\sqrt{35+6\sqrt{34}} \left(\sqrt{\frac{5+\sqrt{17}}4}+\sqrt{\frac{1+\sqrt{17}}4}\right)^4 \\ \frac1{\sqrt{\lambda(\sqrt{-82})}} &\,=\, (1+\sqrt2)^4\, (9+\sqrt{82})\,\left(\sqrt{\frac{7+\sqrt{41}}2}+\sqrt{\frac{5+\sqrt{41}}2}\right)^4 \end{align}\quad For m = 14, 46,142, presumably they may be products of two quartic units  \frac1{\sqrt{\lambda(\sqrt{-m})}} = \big(a+\sqrt{a^2\pm1}\big)\big(b+\sqrt{b^2\pm1}\big) where (a,b) are roots of quadratics, but I haven't figured out the correct values yet.

Entry 114

This continues Entry 113. Recall the modular lambda function \lambda(\tau) \lambda(\tau) = \left(\frac{\sqrt2\,\eta\big(\tfrac12\tau\big)\,\eta^2\big(2\tau\big)}{\eta^3(\tau)}\right)^8calculated in Mathematica as ModularLambda[tau]. We now chose fundamental discriminants d=4m with class number h(-d)=8 for even m with 16 divisors. There are only three, namely m = 210, 330, 462, hence \begin{align}210 &= 2\times3\times5\times7\\ 330 &= 2\times3\times5\times11\\ 462 &= 2\times3\times7\times11\end{align} Ramanujan found m=210, though I'm not sure he did for the other two, so I went ahead and found them \small\begin{align}\frac1{\sqrt{\lambda(\sqrt{-210})}} &= (1+\sqrt{2})^2 (2+\sqrt{3}) (8+3\sqrt{7}) (3+\sqrt{10})^2 (4+\sqrt{15})^2 (6+\sqrt{35}) (\sqrt{6}+\sqrt{7})^2 (\sqrt{14}+\sqrt{15}) \\ \frac1{\sqrt{\lambda(\sqrt{-330})}} &= (1+\sqrt{2})^2(2+\sqrt{3})^3(3+\sqrt{10})^2(10+3\sqrt{11})(4+\sqrt{15})(65+8\sqrt{66}) (\sqrt{44}+\sqrt{45})^2(\sqrt{54}+\sqrt{55}) \\ \frac1{\sqrt{\lambda(\sqrt{-462})}} &= (2+\sqrt{3})^2(5+2\sqrt{6})^2 (8+3\sqrt{7})^2(10+3\sqrt{11})(15+4\sqrt{14})(76+5\sqrt{231})(7\sqrt{2}+3\sqrt{11})^2(\sqrt{21}+\sqrt{22})\end{align} They are products of eight fundamental units U_n. To find products of sixteen fundamental units U_n, I checked class number h(-d)=16 for even m with 32 divisors from the class number list. Surprisingly, there are none. So it seems the pattern of \lambda(\sqrt{-m}) as a product of \color{blue}{2^k} quadratic units U_n where class number h(-4m)=\color{blue}{2^k} stops at 8. Note that \quad e^{\pi\sqrt{462}} = \frac{16}{\lambda\big(\sqrt{-462}\big)}-8.0000000000000000000000000000094\dots or for 29 zeros.

Entry 113

This continues Entry 112. Recall the modular lambda function \lambda(\tau) \lambda(\tau) = \left(\frac{\sqrt2\,\eta\big(\tfrac12\tau\big)\,\eta^2\big(2\tau\big)}{\eta^3(\tau)}\right)^8 We chose fundamental discriminants d=4m with class number h(-d)=4 for even m with eight divisors, hence only m = 30, 42, 78, 102, and m =70, 130, 190 which are evenly divisible by 3 and 5, respectively.  \begin{align}\frac1{\sqrt{\lambda(\sqrt{-30})}} &= (2+\sqrt3)(5+2\sqrt6)(4+\sqrt{15})\sqrt{11+2\sqrt{30}}\\  \frac1{\sqrt{\lambda(\sqrt{-42})}} &= (2+\sqrt3)^2(1+\sqrt2)^2(8+3\sqrt{7})\sqrt{13+2\sqrt{42}}\\ \frac1{\sqrt{\lambda(\sqrt{-78})}} &= (2+\sqrt3)^3(5+2\sqrt6)(25+4\sqrt{39})\sqrt{53+6\sqrt{78}}\\ \frac1{\sqrt{\lambda(\sqrt{-102})}} &=(2+\sqrt3)^2(5+2\sqrt6)^2(50+7\sqrt{51})\sqrt{101+10\sqrt{102}}\end{align} as well as \begin{align}\frac1{\sqrt{\lambda(\sqrt{-70})}} &= (8+3\sqrt7)(15+4\sqrt{14})(6+\sqrt{35})\sqrt{251+30\sqrt{70}}\\  \frac1{\sqrt{\lambda(\sqrt{-130})}} &= (1+\sqrt2)^4\,(3+\sqrt{10})^2\,(5+\sqrt{26})\,(57+5\sqrt{130})\\ \frac1{\sqrt{\lambda(\sqrt{-190})}} &= (170+39\sqrt{19})(37+6\sqrt{38})(39+4\sqrt{95})\sqrt{52021+3774\sqrt{190}}\end{align}

For the last, note that U_{190}=52021+3774\sqrt{190} = (51\sqrt{10}+37\sqrt{19})^2 and similarly for other U_n with composite n but I chose to retain the a+b\sqrt{n} form. Most of these do not appear in Mathworld's list. Arranged by class number, one can see some patterns like the m have eight divisors and \lambda(\tau) is a product of four U_n. Next are m that have sixteen divisors and \lambda(\tau) is a product of eight U_n.

Entry 112

Regarding the previous two entries, the more famous function with evaluations that involve fundamental units U_n is the modular lambda function \lambda(\tau). Define the McKay-Thompson series of class 4C for the Monster (A007248) j_{4C}(\tau) = \left(\frac{\eta(\tau)}{\eta(4\tau)}\right)^8+16 = \left(\frac{\eta^3(2\tau)}{\eta(\tau)\,\eta^2(4\tau)}\right)^8 compare the RHS to \lambda(\tau) = \left(\frac{\sqrt2\,\eta\big(\tfrac12\tau\big)\,\eta^2\big(2\tau\big)}{\eta^3(\tau)}\right)^8 which solves \frac{_2F_1\big(\tfrac12,\tfrac12,1,1-\lambda(\tau)\big)}{_2F_1\big(\tfrac12,\tfrac12,1,\lambda(\tau)\big)} = -\tau\,i We chose fundamental discriminants d=4m with class number h(-d)=2 for even m = 6, 10, 22, 58, hence \begin{align}\frac1{\sqrt{\lambda(\sqrt{-6})}} &= U_3\sqrt{U_6} = (2+\sqrt3)\sqrt{5+2\sqrt6}\\ \frac1{\sqrt{\lambda(\sqrt{-10})}} &= U_2^3\,U_{10} = (1+\sqrt2)^2(3+\sqrt{10})\\ \frac1{\sqrt{\lambda(\sqrt{-22})}} &= U_{11}\sqrt{U_{22}}= (10+3\sqrt{11})\sqrt{197+42\sqrt{22}}\\ \frac1{\sqrt{\lambda(\sqrt{-58})}} &=\, U_2^6\,U_{58}\; =\; (1+\sqrt2)^6(99+13\sqrt{58})\end{align} Note that these m have four divisors and \lambda(\tau) is a product of two U_n. Also \begin{align}U_6 &= 5+2\sqrt6 = (\sqrt2+\sqrt3)^2\\ U_{22} &= 197+42\sqrt{22} = (7\sqrt2+3\sqrt{11})^2\end{align} can be expressed as squares, which simplify things. For the next entry, the m have eight divisors and \lambda(\tau) is a product of four U_n.

Entry 111

Given q = e^{2\pi i \tau} and define the McKay-Thompson series of Class 10E for Monster (A138516) j_{10E}(\tau) =  \frac{\eta(2\tau)\,\eta^5(5\tau)}{\eta(\tau)\,\eta^5(10\tau)} + 1= \left(\frac{\eta^2(2\tau)\,\eta(5\tau)}{\eta(\tau)\,\eta^2(10\tau)}\right)^2 On a hunch, I decided to test this since it seems similar to Class 6E of the previous entry. It turns out j_{10E}(\tau) is also product of fundamental units U_n for appropriate \tau. (Here is a sample Wolfram calculation for U_5.) Let d=20m with class number h(-d)=4 for even m = 6,14,26,38 and odd m=17\begin{align}j_{10E}\big(\tfrac{\sqrt{-6/5}}2\big) &= U_2\, U_ {10}\\ j_{10E}\big(\tfrac{\sqrt{-14/5}}2\big) &= U_2^3\, U_{10}\\ j_{10E}\big(\tfrac{\sqrt{-26/5}}2\big) &= U_{13}^3\, U_{65}\\ j_{10E}\big(\tfrac{\sqrt{-38/5}}2\big) &= U_2^4\, U_5^9 \\ j_{10E}\big(\tfrac{1+\sqrt{-17/5}}2\big) &= -U_5^6\, U_{17}\end{align}

Entry 110

Given q = e^{2\pi i \tau}. Define the McKay-Thompson series of Class 6E for Monster (A128633) \begin{align}j_{6E}(\tau) &=\frac1{\big(\text{K}_6(q)\big)^3}+1 \\ &= \left(\frac{\eta(2\tau)\,\eta^3(3\tau)}{\eta(\tau)\,\eta^3(6\tau)}\right)^3 + 1 = \left(\frac{\eta^2(2\tau)\,\eta(3\tau)}{\eta(\tau)\,\eta^2(6\tau)}\right)^4\end{align} where \text{K}_6(q) is the cubic continued fraction. Just like the modular lambda function \lambda(\tau), it seems j_{6E}(\tau) is also a product of fundamental units U_n for appropriate \tau. (Here is a sample Wolfram calculation for U_5.) We choose d=12m with class number h(-d)=4 for even m = 10, 14, 26, 34 (also found in the previous entry)

\begin{align}j_{6E}\big(\tfrac{\sqrt{-10/3}}2\big) &= U_2^2\,U_5^6 \\ j_{6E}\big(\tfrac{\sqrt{-14/3}}2\big) &= U_6\,U_{14} \\ j_{6E}\big(\tfrac{\sqrt{-26/3}}2\big) &= U_2^4\,U_{26}^2 \\ j_{6E}\big(\tfrac{\sqrt{-34/3}}2\big) &= U_2^6\,U_{17}^2\end{align} and odd m = 7, 11, 19, 31, 59

\begin{align}j_{6E}\big(\tfrac{1+\sqrt{-7/3}}2\big) &= U_3\sqrt{U_{21}^3} \\ j_{6E}\big(\tfrac{1+\sqrt{-11/3}}2\big) &= U_6\sqrt{U_{33}} \\ j_{6E}\big(\tfrac{1+\sqrt{-19/3}}2\big) &= U_3^3\sqrt{U_{57}} \\ j_{6E}\big(\tfrac{1+\sqrt{-31/3}}2\big) &= U_3^3\sqrt{U_{93}^3} \\ j_{6E}\big(\tfrac{1+\sqrt{-59/3}}2\big) &= U_2^6\sqrt{U_{177}} \end{align} Why this factors nicely I don't know. Also, some fundamental units U_n for composite n may actually be squares. For example, \begin{align}U_{21} &= \tfrac{5+\sqrt{21}}2=\Big(\tfrac{\sqrt{3}+\sqrt{7}}2\Big)^2\\ U_{33} &= 23+4\sqrt{33}=\big(2\sqrt{3}+\sqrt{11}\big)^2\end{align} and so on, hence these \sqrt{U_n} simplifies a bit.

Tuesday, May 27, 2025

Entry 109

Let q = e^{2\pi i \tau} and Dedekind eta function \eta(\tau). Define the following \quad\text{K}_{12}(\tau) = \text{K}_{12}(q) = q\,\prod_{n=1}^\infty\frac{(1-q^{12n-1})(1-q^{12n-11})}{(1-q^{12n-5})(1-q^{12n-7})}\quad It is connected to the mod 6 version, or the cubic continued fraction \; \text{K}_{6}(\tau) = \text{K}_{6}(q) = q^{1/3}\prod_{n=1}^\infty\frac{(1-q^{6n-1})(1-q^{6n-5})}{(1-q^{6n-3})(1-q^{6n-3})}\quad

by the quadratic relation 

\frac1{\text{K}_{12}(\tau)}+\text{K}_{12}(\tau) = \frac1{\text{K}_6(\tau)\,\text{K}_6(2\tau)} = \frac{\eta^3(3\tau)\,\eta(4\tau)}{\eta(\tau)\,\eta^3(12\tau)} = \left(\frac{\eta^2(4\tau)\,\eta(6\tau)}{\eta(2\tau)\,\eta^2(12\tau)}\right)^2+1 To find exact values of \text{K}_{12}(\tau), we use discriminants d = 12m with class number h(-d) = 4 for even m=10,14,26,34 to get the orderly \begin{align}\frac1{\text{K}_{12}\Big(\tfrac{\sqrt{-10/3}}4\Big)}+\text{K}_{12}\Big(\tfrac{\sqrt{-10/3}}4\Big) &= 1+\sqrt3\left(2+\sqrt{10}+\sqrt{-1+(2+\sqrt{10})^2}\right)\\ \frac1{\text{K}_{12}\Big(\tfrac{\sqrt{-14/3}}4\Big)}+\text{K}_{12}\Big(\tfrac{\sqrt{-14/3}}4\Big) &= 1+\sqrt3\left(4+\sqrt{21}+\sqrt{1+(4+\sqrt{21})^2}\right)\\ \frac1{\text{K}_{12}\Big(\tfrac{\sqrt{-26/3}}4\Big)}+\text{K}_{12}\Big(\tfrac{\sqrt{-26/3}}4\Big) &= 1+\sqrt3\left(15+4\sqrt{13}+\sqrt{1+(15+4\sqrt{13})^2}\right)\\ \frac1{\text{K}_{12}\Big(\tfrac{\sqrt{-34/3}}4\Big)}+\text{K}_{12}\Big(\tfrac{\sqrt{-34/3}}4\Big) &= 1+\sqrt3\left(28+5\sqrt{34}+\sqrt{-1+(28+5\sqrt{34})^2}\right) \end{align} Update: It turns out all the quartic roots can be factored into fundamentals units U_n, for example 2+\sqrt{10}+\sqrt{-1+(2+\sqrt{10})^2}=(1+\sqrt2)\big(\tfrac{1+\sqrt5}2\big)^3 = U_2\,U_5^3 More on Entry 110.

Entry 108

Let q = e^{2\pi i \tau} and the Dedekind eta function \eta(\tau). Define the following \text{K}_{10}(\tau) = \text{K}_{10}(q) = q^{3/5}\prod_{n=1}^\infty\frac{(1-q^{10n-1})(1-q^{10n-9})}{(1-q^{10n-3})(1-q^{10n-7})} While no continued fraction is yet known for this, it is connected to the mod 5 version, R(\tau) =  \text{K}_{5}(q) = q^{1/5}\prod_{n=1}^\infty\frac{(1-q^{5n-1})(1-q^{5n-4})}{(1-q^{5n-2})(1-q^{5n-3})}

or the Rogers-Ramanujan continued fraction R(\tau) by the simple relation

\text{K}_{10}(q) = R(q)\,R(q^2)\\ \text{K}_{10}(\tau) = R(\tau)\,R(2\tau)

It is known that\frac1{R(\tau)}-R(\tau) = \frac{\eta(\tau/5)}{\eta(5\tau)}+1 Thus a quadratic root R(\tau) = \frac{- \frac{\eta(\tau/5)}{\eta(5\tau)}-1+\sqrt{\left( \frac{\eta(\tau/5)}{\eta(5\tau)}+1\right)^2+4}}2 which allows for easily computation of \text{K}_{10}(\tau). For example, given the golden ratio \phi = \frac{1+\sqrt5}2 and \sqrt{-1}=i, \begin{align}R\big(\tfrac12 i\big) &= 0.511428\dots = \tfrac12(\sqrt{5\phi\,}-\phi^2)(+\sqrt[4]5\sqrt{\phi}+\phi^2)\\ R\big(i\big) &= 0.284079\dots = (\sqrt[4]5\sqrt{\phi}-\phi)\\ R\big(2i\big) &=  0.081002\dots =\tfrac12(\sqrt{5\phi\,}-\phi^2)(-\sqrt[4]5\sqrt{\phi}+\phi^2) \end{align}which implies the exact values\begin{align}\text{K}_{10}\big(\tfrac12 i\big) &= R\big(\tfrac12 i\big)\,R\big(i\big) = 0.145286\dots\\ \text{K}_{10}\big(i\big) &= R\big(i\big)\,R\big(2i\big) \,= 0.203011\dots \end{align}

Entry 107

Given q = e^{2\pi i \tau} and the two q-continued fractions \begin{align}\quad\text{K}_4(q) &= \sqrt2\,q^{1/8} \prod_{n=1}^\infty\frac{(1-q^{4n-1})(1-q^{4n-3})}{(1-q^{4n-2})(1-q^{4n-2})}\\ & = \cfrac{\sqrt2\,q^{1/8}} {1+\cfrac{q} {1+q+\cfrac{q^2} {1+q^2+\cfrac{q^3} {1+q^3+\ddots }}}}\quad\end{align} \qquad \begin{align}\text{K}_8(q)\, &=\,  q^{1/2}\,\prod_{n=1}^\infty\frac{(1-q^{8n-1})(1-q^{8n-7})}{(1-q^{8n-3})(1-q^{8n-5})}\\ &=  \cfrac{q^{1/2}}{1+q+\cfrac{q^2}{1+q^3+\cfrac{q^4}{1+q^5+\cfrac{q^6}{1+q^7+\ddots}}}}\end{align}

then we can evaluate them using the modular lambda function \lambda(\tau) as \text{K}_4(\tau) = \big(\lambda(2\tau)\big)^{1/8}\quad \quad\frac1{\text{K}_8(\tau)}-\text{K}_8(\tau)  = \frac{2}{\big(\lambda(4\tau)\big)^{1/4}}Mathematica can calculate \lambda(\tau) as ModularLambda[tau] and there is a list of exact values in Mathworld. We have previously given examples for \text{K}_4(\tau) = \big(\lambda(2\tau)\big)^{1/8}. For \text{K}_8(\tau), then for d = 4m with class number h(-d)=2 and m=10,58, \begin{align}\frac1{\text{K}_8\big(\tfrac14\sqrt{-10}\big)}-\text{K}_8\big(\tfrac14\sqrt{-10}\big) &= 2(1+\sqrt2)\sqrt{3+\sqrt{10}} \;=\; 2\,U_2\sqrt{U_{10}}\\ \frac1{\text{K}_8\big(\tfrac14\sqrt{-58}\big)}-\text{K}_8\big(\tfrac14\sqrt{-58}\big) &= 2(1+\sqrt2)^3\sqrt{99+13\sqrt{58}} =2\,U_2^3\sqrt{U_{58}}\end{align} where U_n are fundamental units.

Entry 106

Let q = e^{2\pi i \tau}. In Entry 99, we gave the cubic continued fraction \begin{align}\text{K}_6(q)  &= \frac{\eta(\tau)\,\eta^3(6\tau)}{\eta(2\tau)\,\eta^3(3\tau)}\\ & = \cfrac{q^{1/3}}{1+\cfrac{q+q^2}{1+\cfrac{q^2+q^4}{1+\cfrac{q^3+q^6}{1+\ddots}}}}\end{align} Define the McKay-Thompson series of Class 6E for Monster (A128633)\begin{align}j_{6E}(\tau) &=\frac1{\big(\text{K}_6(q)\big)^3}+1 \\ &= \left(\frac{\eta(2\tau)\,\eta^3(3\tau)}{\eta(\tau)\,\eta^3(6\tau)}\right)^3 + 1 = \left(\frac{\eta^2(2\tau)\,\eta(3\tau)}{\eta(\tau)\,\eta^2(6\tau)}\right)^4\end{align} While it is possible to evaluate K_6(q), doing it for j_{6E}(\tau) seems more "easy" since it is a 4th power. Let d=3m with class number h(-d)=2 for m=17,41,89. Then for \tau_1=\tfrac{1+\sqrt{-17/3}}2,\quad \tau_2=\tfrac{1+\sqrt{-41/3}}2,\quad \tau_3=\tfrac{1+\sqrt{-89/3}}2we get the evaluations\begin{align} -\sqrt{\frac{-3\;}{j_{6E}(\tau_1)}}+\sqrt{-\frac13\, j_{6E}(\tau_1)} &= 2^3+2(-2+2\sqrt{17})^{2/3}+2(2+2\sqrt{17})^{2/3}\\ -\sqrt{\frac{-3\;}{j_{6E}(\tau_2)}}+\sqrt{-\frac13\, j_{6E}(\tau_2)} &= 4^3+4(-2+10\sqrt{41})^{2/3}+4(2+10\sqrt{41})^{2/3}\\ -\sqrt{\frac{-3\;}{j_{6E}(\tau_3)}}+\sqrt{-\frac13\, j_{6E}(\tau_3)} &= 10^3+10(-2+106\sqrt{89})^{2/3}+10(2+106\sqrt{89})^{2/3}\end{align} where the RHS are roots of cubics and all quadratic irrationals, for example 72+8\sqrt{17} = (2+2\sqrt{17})^2, are squares.

Entry 105

Let q = e^{2\pi i\tau}. For convenience, define the Rogers-Ramanujan continued fraction in terms of \tau, hence R(q) = R(\tau). Given the Dedekind eta function \eta(\tau), it is known that \frac1{R(\tau)}-R(\tau) = \frac{\eta(\tau/5)}{\eta(5\tau)}+1Thus a quadraticR(\tau) = \frac{- \frac{\eta(\tau/5)}{\eta(5\tau)}-1+\sqrt{\left( \frac{\eta(\tau/5)}{\eta(5\tau)}+1\right)^2+4}}2 For example, let \tau = \sqrt{-1}, then the formula gives us Ramanujan's evaluation, R(\sqrt{-1}) = \sqrt[4]5\sqrt{\phi}-\phi = 0.284079\dots with golden ratio \phi = \frac{1+\sqrt5}2. The formula also works when \tau = \frac{1+\sqrt{-n}}2 though R(\tau) is now negative. Since it contains q^{1/5}, one has to be careful with 5th roots. We give some nice new evaluations for discriminants d=5m with class number h(-d)=2 for m=(3,7,23,47) also using the golden ratio \phi

\begin{align}\frac1{R^5\Big(\tfrac{1+\sqrt{-3/5}}2\Big)} -R^5\Big(\tfrac{1+\sqrt{-3/5}}2\Big)-11 &= -(\sqrt5)^3\phi\\ \frac1{R^5\Big(\tfrac{1+\sqrt{-7/5}}2\Big)} -R^5\Big(\tfrac{1+\sqrt{-7/5}}2\Big)-11 &= -(\sqrt5)^3\phi^3\\ \frac1{R^5\Big(\tfrac{1+\sqrt{-23/5}}2\Big)} -R^5\Big(\tfrac{1+\sqrt{-23/5}}2\Big)-11 &= -(\sqrt5)^3\phi^9\\ \frac1{R^5\Big(\tfrac{1+\sqrt{-47/5}}2\Big)} -R^5\Big(\tfrac{1+\sqrt{-47/5}}2\Big)-11 &= -(\sqrt5)^3\phi^{15}\\ \end{align} Note the 5th powers R^5(\tau) and how well-behaved their evaluations are.

Monday, May 26, 2025

Entry 104

Given q = e^{2\pi i \tau} and the q-continued fraction discussed in Entry 95\begin{align}\quad\text{K}_4(q) &= \frac{\sqrt2\,\eta(\tau)\,\eta^2(4\tau)}{\eta^3(2\tau)}\\ & = \cfrac{\sqrt2\,q^{1/8}} {1+\cfrac{q} {1+q+\cfrac{q^2} {1+q^2+\cfrac{q^3} {1+q^3+\ddots }}}}\end{align}

and compare it to the modular lambda function \lambda(\tau) = \left(\frac{\sqrt2\,\eta(\tfrac12\tau)\,\eta^2(2\tau)}{\eta^3(\tau)}\right)^8\quad therefore the two are related by \text{K}_4(\tau) = \big(\lambda(2\tau)\big)^{1/8} Mathematica can calculate \lambda(\tau) as ModularLambda[tau] and there is list of exact values in Mathworld which also implies exact values for \text{K}_4(\tau). But we will give a nice consistent form when d = 4m has class number h(-d)=2 for m=5,13,37 as \begin{align}\lambda\big(\sqrt{-5}\big) &= \frac12-\sqrt{\left(\tfrac{{-1}+\sqrt{5}}2\right)^3}\\ \lambda\big(\sqrt{-13}\big) &= \frac12-3\sqrt{\left(\tfrac{{-3}+\sqrt{13}}2\right)^3}\\ \lambda\big(\sqrt{-37}\big) &= \frac12-21\sqrt{\left({-6}+\sqrt{37}\right)^3}\end{align} as well as the complex values \begin{align}\frac1{\lambda\left(\frac{1+\sqrt{-5}}2\right)} &= \frac12-\sqrt{\left(\tfrac{{-1}-\sqrt{5}}2\right)^3}\\ \frac1{\lambda\left(\frac{1+\sqrt{-13}}2\right)} &= \frac12-3\sqrt{\left(\tfrac{{-3}-\sqrt{13}}2\right)^3}\\ \frac1{\lambda\left(\frac{1+\sqrt{-37}}2\right)} &= \frac12-21\sqrt{\left({-6}-\sqrt{37}\right)^3}\end{align}

Entry 103

Level 12. Given q = e^{2\pi i \tau},  the q-Pochhammer symbol (a;q)_n, and Ramanujan's functions f(a,b) and f(-q) discussed in Entry 92. We have the sum-product identities \begin{align}A(q) &= \frac{f(-q,-q^{11})}{f(-q^4)} = \sum_{n=0}^\infty \frac {q^{4n^2+4n}\,(q;q^2)_{2n+1}} {(q^4;q^4)_{2n+1}}  = \prod_{n=1}^\infty(1-q^{12n-1})(1-q^{12n-11})\,\alpha_{12}\\ B(q) &=\frac{f(-q^5,-q^{7})}{f(-q^4)} \,=\, \sum_{n=0}^\infty \frac {q^{4n^2}\,(q;q^2)_{2n}} {(q^4;q^4)_{2n}} \quad = \; \prod_{n=1}^\infty(1-q^{12n-5})(1-q^{12n-7})\,\alpha_{12}\end{align}

where \alpha_{12} = \dfrac{(1-q^{12n})}{(1-q^{4n})}

Let a = q^{7/8}A(q) and b = q^{-1/8}B(q) then (a,b) are radicals for appropriate \tau. Define their ratio a/b \text{K}_{12}(q) = q\,\prod_{n=1}^\infty\frac{(1-q^{12n-1})(1-q^{12n-11})}{(1-q^{12n-5})(1-q^{12n-7})}\quad\qquad Surprisingly, this has a continued fraction (studied by Naika) based on a general form from Ramanujan for Level 4m, though its form is not as simple as the others. It is connected to the mod 6 version, \quad\text{K}_{6}(q) = q^{1/3}\prod_{n=1}^\infty\frac{(1-q^{6n-1})(1-q^{6n-5})}{(1-q^{6n-3})(1-q^{6n-3})} = \frac{\eta(\tau)\,\eta^3(6\tau)}{\eta(2\tau)\,\eta^3(3\tau)} or the previously discussed cubic continued fraction \text{K}_{6}(q) by the quadratic relation 

j_{12I}=\frac1{\text{K}_{12}(q)}+\text{K}_{12}(q) = \frac1{\text{K}_6(q)\,\text{K}_6(q^2)} = \frac{\eta^3(3\tau)\,\eta(4\tau)}{\eta(\tau)\,\eta^3(12\tau)} = \left(\frac{\eta^2(4\tau)\,\eta(6\tau)}{\eta(2\tau)\,\eta^2(12\tau)}\right)^2+1

 where j_{12I} is the McKay-Thompson series of class 12I for the Monster (A187144).

Entry 102

Level 10. Given q = e^{2\pi i \tau},  the q-Pochhammer symbol (a;q)_n, and Ramanujan's functions f(a,b) and \varphi(q) discussed in Entry 92. We have the sum-product identities

\begin{align}A(q) &= \frac{f(-q,-q^9)}{\varphi(-q)} \;=\; \sum_{n=0}^\infty \frac {q^{n(n+3)/2}\,(-q;q)_n} {(q;q)_n (q;q^2)_{n+1}}  = \prod_{n=1}^\infty\frac{\alpha_{10}}{(1-q^{10n-3})(1-q^{10n-7})}\\ B(q) &= \frac{f(-q^3,-q^7)}{\varphi(-q)} = \sum_{n=0}^\infty \frac {q^{n(n+1)/2}\,(-q;q)_n} {(q;q)_n (q;q^2)_{n+1}}  = \prod_{n=1}^\infty\frac{\alpha_{10}}{(1-q^{10n-1})(1-q^{10n-9})}\end{align}

where \alpha_{10} = \dfrac{(1-q^{10n})^2}{(1-q^{n})(1-q^{5n})}

Let a = q^{4/5}A(q) and b = q^{1/5}B(q) then (a,b) are radicals for appropriate \tau. Define their ratio a/b, \text{K}_{10}(q) = q^{3/5}\prod_{n=1}^\infty\frac{(1-q^{10n-1})(1-q^{10n-9})}{(1-q^{10n-3})(1-q^{10n-7})} While no continued fraction is yet known for this, it is connected to the mod 5 version \text{K}_{5}(q) = q^{1/5}\prod_{n=1}^\infty\frac{(1-q^{5n-1})(1-q^{5n-4})}{(1-q^{5n-2})(1-q^{5n-3})}

simply as, \text{K}_{10}(q) = \text{K}_5(q)\,\text{K}_5(q^2)

where \text{K}_5(q)=R(q) is just the Rogers-Ramanujan continued fraction.

Entry 101

Level 8. Recall the Level 4 continued fraction \begin{align}\text{K}_4(q) &= \sqrt2\,q^{1/8} \prod_{n=1}^\infty\frac{(1-q^{4n-1})(1-q^{4n-3})}{(1-q^{4n-2})(1-q^{4n-2})} = \frac{\sqrt2\,\eta(\tau)\,\eta^2(4\tau)}{\eta^3(2\tau)}\\ &= \cfrac{\sqrt2\,q^{1/8}} {1+\cfrac{q} {1+\cfrac{q+q^2} {1+\cfrac{q^3} {1+\cfrac{q^2+q^4} {1+\ddots}}}}} = \cfrac{\sqrt2\,q^{1/8}} {1+\cfrac{q} {1+q+\cfrac{q^2} {1+q^2+\cfrac{q^3} {1+q^3+\cfrac{q^4} {1+q^4+\ddots}}}}}\end{align} Compare its similarity to the Level 8 version using the ratio of (a,b) from Entry 100 \begin{align}\text{K}_8(q) &\,=\,  q^{1/2}\,\prod_{n=1}^\infty\frac{(1-q^{8n-1})(1-q^{8n-7})}{(1-q^{8n-3})(1-q^{8n-5})}\\ &= \cfrac{q^{1/2}}{1+\cfrac{q+q^2}{1+\cfrac{q^4}{1+\cfrac{q^3+q^6}{1+\cfrac{q^8}{1+\ddots}}}}} = \cfrac{q^{1/2}}{1+q+\cfrac{q^2}{1+q^3+\cfrac{q^4}{1+q^5+\cfrac{q^6}{1+q^7+\cfrac{q^8}{1+q^9+\ddots}}}}}\end{align}

In fact, they have the quadratic relation \quad\frac1{\text{K}_8(q)}-\text{K}_8(q) =\left(\frac{\sqrt2}{\text{K}_4(q^2)}\right)^2 = \left(\frac{\eta^3(4\tau)}{\eta(2\tau)\eta^2(8\tau)}\right)^2 while \text{K}_4(q) which is an eta quotient can also be expressed another way\frac1{\big(\text{K}_4(q)\big)^2}-\big(\text{K}_4(q)\big)^2 =\frac12\left(\frac{\eta(\tau/2)}{\eta(2\tau)}\right)^4\quad