OFFSET
0,2
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..200
FORMULA
a(n) = 3^n*A000110(n).
a(n) = (1/e)*Sum_{k>=0} (3*k)^n/k!. (This is a Dobinski-type formula.)
O.g.f.: (1/e)*Sum_{k>=0} 1/(k!*(1-3*k*z)).
E.g.f.: exp(exp(3*z)-1).
a(n) is the n-th moment of a discrete, positive weight function w(x) consisting of an infinite comb of Dirac delta functions situated at x=3*k, with k = 0, 1, ..., defined as w(x) = (1/e)*Sum_{k>=0} Dirac(x-3*k)/k!.
G.f.: 1/(1-3x/(1-3x/(1-3x/(1-6x/(1-3x/(1-9x/(1-...)...) (continued fraction). - Philippe Deléham, Sep 18 2014
a(0) = 1; a(n) = Sum_{k=1..n} binomial(n-1,k-1) * 3^k * a(n-k). - Ilya Gutkovskiy, Jan 16 2020
MATHEMATICA
Table[3^n BellB[n], {n, 0, 20}] (* Vincenzo Librandi, Sep 19 2014 *)
PROG
(Python)
# Python 3.2 or above required.
from itertools import accumulate
A247452_list, blist, b, n3 = [1, 3], [1], 1, 9
for _ in range(2, 201):
blist = list(accumulate([b]+blist))
b = blist[-1]
A247452_list.append(b*n3)
n3 *= 3 # Chai Wah Wu, Sep 19 2014
(Magma) [3^n*Bell(n): n in [0..20]]; // Vincenzo Librandi, Sep 19 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Karol A. Penson, Sep 17 2014
STATUS
approved