OFFSET
0,2
COMMENTS
Convolution of A006882 with itself.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..807
Poloni, Federico; Del Corso, Gianna M. Counting Fiedler pencils with repetitions. Linear Algebra Appl. 532, 463-499 (2017), corollary 24.
Eric Weisstein's World of Mathematics, Double Factorial
FORMULA
G.f.: (Sum_{k>=0} k!!*x^k)^2.
MAPLE
a:= proc(n) option remember; `if`(n<4, n^2+1,
((3*n^2-4*n-2)*a(n-2) +(n+1)*a(n-3)
-2*a(n-1) -(n-1)^2*n*a(n-4))/(2*n-4))
end:
seq(a(n), n=0..35); # Alois P. Heinz, Jun 14 2018
MATHEMATICA
Table[Sum[k!! (n - k)!!, {k, 0, n}], {n, 0, 28}]
nmax = 28; CoefficientList[Series[Sum[k!! x^k, {k, 0, nmax}]^2, {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Jun 05 2018
STATUS
approved