A186377 a(n) equals the least sum of the squares of the coefficients in (1 + 2*x^k + x^p + x^q)^n found at sufficiently large p and q>(n+1)p for some fixed k>0.

1, 7, 79, 1129, 18559, 333577, 6365089, 126652183, 2598628543, 54577439833, 1167481074529, 25346459683783, 557042221952881, 12368307313680871, 277027947337574911, 6251808554314780009, 142015508983550880703

Offset

0,2

Comments

Equivalently, a(n) equals the sum of the squares of the coefficients in any one of the following polynomials:

. (2 + x^k + x^p + x^q)^n, or

. (1 + x^k + 2*x^p + x^q)^n, or

. (1 + x^k + x^p + 2*x^q)^n,

for all p>(n+1)k and q>(n+1)p and fixed k>0.

Links

Table of n, a(n) for n=0..16.

Formula

(1) a(n) = Sum_{k=0..n} C(n,k)^2 *4^(n-k) *Sum_{j=0..k} C(k,j)^2*C(2j,j).

Let g.f. A(x) = Sum_{n>=0} a(n)*x^n/n!^2, then

(2) A(x) = B(x)^3 * B(2^2*x)

where B(x) = Sum_{n>=0} x^n/n!^2 = BesselI(0, 2*sqrt(x)).

Recurrence: (n-1)*n^3*(3*n - 5)*a(n) = 2*(n-1)*(54*n^4 - 174*n^3 + 192*n^2 - 99*n + 20)*a(n-1) - 2*(441*n^5 - 2604*n^4 + 6102*n^3 - 7107*n^2 + 4111*n - 940)*a(n-2) + 2*(n-2)^2*(726*n^3 - 3076*n^2 + 4188*n - 1655)*a(n-3) - 225*(n-3)^2*(n-2)^2*(3*n - 2)*a(n-4). - Vaclav Kotesovec, Feb 12 2015

a(n) ~ 5^(2*n+2) / (2^(7/2) * Pi^(3/2) * n^(3/2)). - Vaclav Kotesovec, Feb 12 2015

Example

G.f.: A(x) = 1 + 7*x + 79*x^2/2!^2 + 1129*x^3/3!^2 + 18559*x^4/4!^2 +...
The g.f. may be expressed as:
A(x) = [Sum_{n>=0} x^n/n!^2]^3 *[Sum_{n>=0} (4x)^n/n!^2] where
[Sum_{n>=0} x^n/n!^2]^3 = 1 + 3*x + 15*x^2/2!^2 + 93*x^3/3!^2 + 639*x^4/4!^2 + 4653*x^5/5!^2 +...+ A002893(n)*x^n/n!^2 +...

Mathematica

Table[Sum[Binomial[n, k]^2 * 4^(n-k) *Sum[Binomial[k, j]^2 * Binomial[2j, j], {j, 0, k}], {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Feb 11 2015 *)

Prog

(PARI) {a(n)=local(V=Vec((1+2*x+x^(n+2)+x^(n^2+2*n+3))^n)); V*V~}
(PARI) {a(n)=sum(k=0, n, binomial(n, k)^2*4^(n-k)*sum(j=0, k, binomial(k, j)^2*binomial(2*j, j)))}
(PARI) {a(n)=n!^2*polcoeff(sum(m=0, n, x^m/m!^2)^3*sum(m=0, n, (2^2*x)^m/m!^2), n)}

Crossrefs

Cf. A186375, A186376, A186378.

Sequence in context: A127859 A014293 A176792 * A112700 A365039 A365782

Adjacent sequences: A186374 A186375 A186376 * A186378 A186379 A186380

Keyword

nonn

Author

Paul D. Hanna, Feb 19 2011

Status

approved