>>79
　α = arcsin(√a) = arccos(1-2a)/2,
とおくと
　a_n = {sin(2^(n-1)･α)}^2 = {1 - cos(2^n･α)}/2

　α = (奇数)π/{2^(n0-1)}　⇔　　a_n = 0　(n≧n0)