145
1981
f n = n + 1 2! n n -1 + 1 3! n n -1 n - 2 2 3 + 1 4! n n -1 n - 2 n - 3 4
n 1 r(r +1 r + 2 r + p 1 r = 1 p! 1 n(n +1 n + 2 n + p ) ( p + 1)!