-- this works for integer m (k is a local var) L(x,n,m,k) = (n + m)! sum(k,0,n,(-x)^k / ((n - k)! (m + k)! k!)) -- this works for real or complex a (y is a local var) L(x,n,a,y) = eval(y^(-a) exp(y) / n! d(exp(-y) y^(n + a), y, n), y, x) "Verify for n = 3" n = 3 y = L(x,n,a) y check(x d(y,x,x) + (a + 1 - x) d(y,x) + n y == 0) "ok" Run