Thursday, September 29, 2016
Entry 28
Ramanujan gave the infinite series,1−(12)3+(1×32×4)3−(1×3×52×4×6)3+⋯=(Γ(98)Γ(54)Γ(78))2=8[K(k2)]2(1+√2)π21−5(12)3+9(1×32×4)3−13(1×3×52×4×6)3+⋯=2π1+9(14)4+17(1×54×8)4+25(1×5×94×8×12)4+⋯=23/2π1/2Γ2(34)=25/2πη2(i)with elliptic integral singular value K(k2) and Dedekind eta function η(τ). For the first and second, their apparent simplicity belies its deep connection to modular forms and there are in fact an infinite number of such formulas. The three can be succinctly expressed asS1=∞∑n=0(−1)n(Γ(n+12)n!Γ(12))3S2=∞∑n=0(−1)n(4n+1)(Γ(n+12)n!Γ(12))3S3=∞∑k=0(8k+1)(Γ(k+14)k!Γ(14))4However, sinceΓ(n+12)n!Γ(12)=(2n)!22nn!2=(2n−1)!!(2n)!!then S2 is equivalently,2π=∞∑n=0(−1)n((2n)!n!2)34n+126nAnother that belongs to the same family is16π=∞∑n=0((2n)!n!2)342n+5212nand so on. For the third, note thatΓ(n+14)n!Γ(14)=(−1)n(−1/4n)where (nk) is the binomial coefficient though I haven't yet figured out the family that this example belongs to.
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