A061685 - OEIS

%I #21 Jul 12 2020 12:17:53

%S 1,1,9,298,25097,4383626,1394519922,738298190981,608765840524809,

%T 742996254490626106,1289282092211451157634,3078466688415490018129781,

%U 9844321075186192301310239858,41209705023068976933023104392293,221473347301087557264532943397984133

%N Generalized Bell numbers.

%H Alois P. Heinz, <a href="/A061685/b061685.txt">Table of n, a(n) for n = 0..167</a>

%H J.-M. Sixdeniers, K. A. Penson and A. I. Solomon, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL4/SIXDENIERS/bell.html">Extended Bell and Stirling Numbers From Hypergeometric Exponentiation</a>, J. Integer Seqs. Vol. 4 (2001), #01.1.4.

%F a(n) = Sum_{pi} n!/(k(1)! * 1!^k(1) * k(2)! * 2!^k(2) * ... * k(n)! * n!^k(n)) * (n!/(1!^k(1) * 2!^k(2) * ... * n!^k(n)))^L, where pi runs through all partitions k(1) + 2 * k( 2) + ... + n * k(n) = n, with L = 3.

%F a(0) = 1; a(n) = (1/n) * Sum_{k=0..n-1} binomial(n,k)^4 * (n-k) * a(k). - _Ilya Gutkovskiy_, Jul 12 2020

%p b:= proc(n) option remember; `if`(n=0, 1,

%p       add(b(n-i)*binomial(n-1, i-1)/i!^3, i=1..n))

%p     end:

%p a:= n-> b(n)*n!^3:

%p seq(a(n), n=0..20);  # _Alois P. Heinz_, May 11 2016

%t b[n_] := b[n] = If[n==0, 1, Sum[b[n-i]*Binomial[n-1, i-1]/i!^3, {i, n}]];

%t a[n_] := b[n]*n!^3;

%t Table[a[n], {n, 0, 20}] (* _Jean-François Alcover_, Mar 15 2017, after _Alois P. Heinz_ *)

%Y Column k=4 of A275043.

%K nonn

%O 0,3

%A _N. J. A. Sloane_, Jun 18 2001

%E Formula and more terms from _Vladeta Jovovic_, Dec 09 2001