ベル多項式
B n , k ( 1 ! , 2 ! , ... , ( n − k + 1 ) ! ) = ( n k ) ( n − 1 k − 1 ) ( n − k ) ! {\displaystyle B_{n,k}(1!,2!,\dots ,(n-k+1)!)={\binom {n}{k}}{\binom {n-1}{k-1}}(n-k)!}