OFFSET
0,2
COMMENTS
Compare to a g.f. of Pell numbers (A000129):
Sum_{n>=0} x^n * Product_{k=1..n} (2*k + x)/(1 + 2*k*x) = 1/(1-2*x-x^2).
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..282
FORMULA
Self-convolution square-root yields A231277.
a(n) == (n+1) (mod 4).
a(n) ~ 2^(n-1) * n! / (log(2))^(n+1). - Vaclav Kotesovec, Nov 02 2014
EXAMPLE
G.f.: A(x) = 1 + 2*x + 11*x^2 + 96*x^3 + 1117*x^4 + 16226*x^5 + 282503*x^6 +...
where
A(x) = 1 + x*(2-x)/(1-2*x) + x^2*(2-x)*(4-x)/((1-2*x)*(1-4*x)) + x^3*(2-x)*(4-x)*(6-x)/((1-2*x)*(1-4*x)*(1-6*x)) + x^4*(2-x)*(4-x)*(6-x)*(8-x)/((1-2*x)*(1-4*x)*(1-6*x)*(1-8*x)) +...
The g.f. equals the square of an integer series (A231277):
A(x)^(1/2) = 1 + x + 5*x^2 + 43*x^3 + 503*x^4 + 7395*x^5 + 130417*x^6 + 2677347*x^7 + 62652163*x^8 + 1645424927*x^9 + 47918249503*x^10 +...
because the g.f. A(x) is congruent to 1/(1-x)^2 modulo 4.
PROG
(PARI) {a(n)=polcoeff(sum(m=0, n, x^m*prod(k=1, m, (2*k-x)/(1-2*k*x +x*O(x^n)))), n)}
for(n=0, 30, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Nov 05 2013
STATUS
approved