数学において、ウォリス積 とは無限積 ∏ n = 1 ∞ = 2 ⋅ 2 1 ⋅ 3 ⋅ 4 ⋅ 4 3 ⋅ 5 ⋅ 6 ⋅ 6 5 ⋅ 7 ⋅ 8 ⋅ 8 7 ⋅ 9 ⋯ {\displaystyle \prod _{n=1}^{\infty }\left={\frac {2\cdot 2}{1\cdot 3}}\cdot {\frac {4\cdot 4}{3\cdot 5}}\cdot {\frac {6\cdot 6}{5\cdot 7}}\cdot {\frac {8\cdot 8}{7\cdot 9}}\cdots } のことであり、この値は π/2 に等しい。 Wikipedia