Licson Lee says to YSITD f(0)(0) = sin 0 = 0 f(1)(0) = cos 0 = 1 f(2)(0) = -sin 0 = 0 f(3)(0) = -cos 0 = -1 所以 n 在 1 mod 4 和 3 mod 4 的時候才有值