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.