OFFSET
0,2
LINKS
Vaclav Kotesovec, Recurrence (of order 8)
FORMULA
a(n) = Sum_{k=0..n} Sum_{j=0..k} C(2*n-k-j,k)^2 * C(k,j)^2.
EXAMPLE
G.f.: A(x) = 1 + 2*x + 15*x^2 + 116*x^3 + 1001*x^4 + 9322*x^5 + 89363*x^6 +...
where
A(x) = 1/(1-x) + x/(1-x)^5 * (1 + 2^2*x + x^2)^2
+ x^2/(1-x)^9 * (1 + 4^2*x + 6^2*x^2 + 4^2*x^3 + x^4)^2
+ x^3/(1-x)^13 * (1 + 6^2*x + 15^2*x^2 + 20^2*x^3 + 15^2*x^4 + 6^2*x^5 + x^6)^2 +...
The square-root of the g.f. is an integer series:
A(x)^(1/2) = 1 + x + 7*x^2 + 51*x^3 + 425*x^4 + 3879*x^5 + 36527*x^6 + 355333*x^7 + 3531175*x^8 + 35673875*x^9 +...+ A246572(n)*x^n +...
MATHEMATICA
Table[Sum[Sum[Binomial[2*n-k-j, k]^2 * Binomial[k, j]^2, {j, 0, k}], {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Sep 02 2014 *)
PROG
(PARI) /* By definition: */
{a(n)=local(A=1); A=sum(m=0, n, x^m/(1-x)^(4*m+1) * sum(k=0, 2*m, binomial(2*m, k)^2 * x^k)^2 +x*O(x^n)); polcoeff(A, n)}
for(n=0, 25, print1(a(n), ", "))
(PARI) /* From a formula for a(n): */
{a(n)=sum(k=0, n, sum(j=0, min(k, 2*n-2*k), binomial(2*n-k-j, k)^2 * binomial(k, j)^2 ))}
for(n=0, 25, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Aug 30 2014
STATUS
approved