%I #11 Mar 02 2015 16:08:50

%S 0,0,0,1,2,0,0,8,13,0,0,69,123,0,0,719,1313,0,0,8215,15260,0,0,99774,

%T 187615,0,0,1264854,2399207,0,0,16544234,31587644,0,0,221625505,

%U 425313967,0,0,3025271756,5829531261,0,0,41929052284,81066732018,0

%N Number of solutions to +- 1 +- 2 +- .. +- n = 2.

%H Ray Chandler, <a href="/A113036/b113036.txt">Table of n, a(n) for n = 0..3339</a> (terms < 10^1000)

%F a(n) is the coefficient of x^2 in product(x^(-k)+x^k, k=1..n).

%p A113036:= proc(n) local i,j,p,t; t:= NULL; for j to n do p:=1; for i to j do p:=p*(x^(-i)+x^i); od; t:=t,coeff(p,x,2); od; t; end;

%t nmax = 50; d = {1}; a1 = {};

%t Do[

%t i = Ceiling[Length[d]/2] + 2;

%t AppendTo[a1, If[i > Length[d], 0, d[[i]]]];

%t d = PadLeft[d, Length[d] + 2 n] + PadRight[d, Length[d] + 2 n];

%t , {n, nmax}];

%t a1 (* _Ray Chandler_, Mar 14 2014 *)

%Y Cf. A058377, A063865, A063866.

%K nonn

%O 0,5

%A _Floor van Lamoen_, Oct 11 2005