A188202 Central coefficients in (1 + 2^n*x + x^2)^n.

1, 2, 18, 560, 68614, 34210752, 69223161876, 564393502852608, 18462508476328312902, 2418515748615522678533120, 1267759405004680431879193624828, 2658525712652053771685500828954042368

Offset

0,2

Comments

Variant of the central trinomial coefficients: A002426(n) = [x^n] (1+x+x^2)^n.

Links

G. C. Greubel, Table of n, a(n) for n = 0..57

Formula

a(n) = Sum_{k=0..floor(n/2)} C(n, k)*C(n-k, n-2k) * 2^(n*(n-2k)).

Equals the logarithmic derivative of A188203 (ignoring initial term).

a(n) ~ 2^(n^2). - Vaclav Kotesovec, Feb 12 2015

Example

Ignoring the initial term, this sequence forms the logarithmic series:
L(x) = 2*x + 18*x^2/2 + 560*x^3/3 + 68614*x^4/4 + 34210752*x^5/5 + ...
where the g.f. of A188203 begins:
exp(L(x)) = 1 + 2*x + 11*x^2 + 206*x^3 + 17586*x^4 + 6878604*x^5 + ...
Illustrate definition.
The coefficients of x^k in (1 + 2^n*x + x^2)^n, k=0..2n, n>=0, begin:
n=0: [(1)];
n=1: [1, (2), 1];
n=2: [1, 8, (18), 8, 1];
n=3: [1, 24, 195, (560), 195, 24, 1];
n=4: [1, 64, 1540, 16576, (68614), 16576, 1540, 64, 1];
n=5: [1, 160, 10245, 328320, 5273610, (34210752), 5273610, 328320, ...];
n=6: [1, 384, 61446, 5244800, 251904015, 6458183424, (69223161876), ...];
n=7: [1, 896, 344071, 73405696, 9396961301, 721848120448, 30814514741283, (564393502852608), ...]; ...
where the above central coefficients in parenthesis form this sequence.

Mathematica

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

Prog

(PARI) {a(n)=polcoeff((1+2^n*x+x^2)^n, n)}
(PARI) {a(n)=sum(k=0, n\2, binomial(n, k)*binomial(n-k, n-2*k)*2^(n*(n-2*k)))}

Crossrefs

Cf. A188203 (exp); variants: A002426, A186925.

Sequence in context: A255433 A121936 A063389 * A324308 A071352 A258384

Adjacent sequences: A188199 A188200 A188201 * A188203 A188204 A188205

Keyword

nonn

Author

Paul D. Hanna, Mar 24 2011

Status

approved