OFFSET
0,2
COMMENTS
Compare to the o.g.f. for Euler numbers (A000364):
Sum_{n>=0} (2*n)!/2^n * x^n / Product_{k=1..n} (1 + k^2*x).
LINKS
Paul D. Hanna, Table of n, a(n) for n = 0..100
L. Naughton, G. Pfeiffer, Integer Sequences Realized by the Subgroup Pattern of the Symmetric Group, J. Int. Seq. 16 (2013) #13.5.8
FORMULA
a(n) == 0 (mod 3) for n>0;
a(n) == 3 (mod 6) for n>0;
a(2*n-1) == 3 (mod 5), a(2*n) == 2 (mod 5), for n>0;
a(2*n-1) == 3 (mod 9), a(2*n) == 6 (mod 9), for n>0;
a(2*n-1) == 3 (mod 10), a(2*n) == 7 (mod 10), for n>0.
a(n) ~ c * 2^(3/2) * Pi^(3/2) * d^n * n^(3*n+1/2) / exp(3*n), where d = 13.2458829063958687527098..., c = 0.281041890716214414121... . - Vaclav Kotesovec, Dec 05 2015
EXAMPLE
G.f.: A(x) = 1 + 3*x + 177*x^2 + 43743*x^3 + 28317777*x^4 +...
PROG
(PARI) {a(n)=polcoeff(sum(m=0, n, (3*m)!/2^m*x^m/prod(k=1, m, 1+k^3*x+x*O(x^n))), n)}
for(n=0, 21, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Sep 20 2012
STATUS
approved