OFFSET
4,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 4..450
FORMULA
E.g.f.: x^4 * Product_{i=1..4} (i-1)!/(i!-x^i).
MAPLE
b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,
b(n, i-1)+`if`(i>n, 0, binomial(n, i)*b(n-i, i))))
end:
a:= n-> (k-> b(n, k) -b(n, k-1))(4):
seq(a(n), n=4..30);
MATHEMATICA
b[n_, i_] := b[n, i] = If[n == 0, 1, If[i < 1, 0, b[n, i - 1] + If[i > n, 0, Binomial[n, i]*b[n - i, i]]]];
a[n_] := b[n, 4] - b[n, 3];
a /@ Range[4, 30] (* Jean-François Alcover, Dec 11 2020, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 01 2016
STATUS
approved