Stiring 公式：log(n!)= (n + 1/2)log(n)+ log√(2π)+ B(2)/(1*2)*1/(n^1)+ B(4)/(3*4)*1/(n^3)+ ...+ B(2k)/[(2k-1)(2k)]*1/[n^(2k-1)]+ .n!= (n/e)^n*√(2πn)*exp[B(2)/(1*3)*1/(n^1) + ...+ B(2k)/[(2k-1)(2k)]...