产品{k=1..n}(1-x^k)
n=1 1-x
n=2 1-x-x ^2+x ^3
n=31-x-x^2+x^4+x^5-x^6
积分积_{k=1..n}(1-x^k)dx
n=1 x-x ^2/2
n=2 x-x ^2/2-x ^3/3+x ^4/4
n=3 x-x ^2/2-x ^3/3+x ^5/5+x ^6/6-x ^7/7
对于积分{x=0..1}集x=1
n=1 1-1/2=1/2,a(1)=2
n=2 1-1/2-1/3+1/4=5/12,a(2)=12
n=3 1-1/2-1/3+1/5+1/6-1/7=41/105,a(3)=105