OFFSET
0,3
FORMULA
E.g.f.: x * exp(4*x) / (1 - x).
a(0) = 0; a(n) = n * (a(n-1) + 4^(n-1)).
a(n) ~ exp(4)*n!. - Stefano Spezia, Oct 11 2021
MATHEMATICA
Table[n! Sum[4^k/k!, {k, 0, n - 1}], {n, 0, 21}]
nmax = 21; CoefficientList[Series[x Exp[4 x]/(1 - x), {x, 0, nmax}], x] Range[0, nmax]!
PROG
(PARI) a(n) = n!*sum(k=0, n-1, 4^k/k!); \\ Michel Marcus, Oct 11 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Oct 11 2021
STATUS
approved