摘要： f ( n ) = ∏ i = 1 n ∑ j = 0 + ∞ x i ⋅ j ( 1 ) a n s = [ x n ] f ( n ) f ( n ) = ∏ i = 1 n x + ∞ − 1 x i − 1 \begin{aligned} f (n) &= \prod_{i=1}^{n} \ 阅读全文

摘要： 我们构造一个生成函数 f ( x , y ) = ∏ i = 1 n ∧ [ i ≠ y ] ( 1 + x v i ) c n t ( a , b ) = [ x b ] f ( x , a ) \begin{aligned} f (x, y) = \prod_{i = 1}^{n \wedge 阅读全文