This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A063866 #27 Nov 02 2023 05:29:40
%S A063866 0,1,1,0,0,3,5,0,0,23,40,0,0,221,397,0,0,2410,4441,0,0,28460,53222,0,
%T A063866 0,353743,668273,0,0,4559828,8679280,0,0,60400688,115633260,0,0,
%U A063866 817175698,1571588177,0,0,11243980807,21704569869,0,0,156860869714
%N A063866 Number of solutions to +- 1 +- 2 +- 3 +- ... +- n = 1.
%H A063866 Ray Chandler, <a href="/A063866/b063866.txt">Table of n, a(n) for n = 0..3340</a> (terms < 10^1000; first 101 terms from T. D. Noe)
%F A063866 a(n) equals the coefficient of x in Product_{k=1..n} (x^k + 1/x^k). - _Paul D. Hanna_, Jul 10 2018
%t A063866 f[n_, s_] := f[n, s]=Which[n==0, If[s==0, 1, 0], Abs[s]>(n*(n+1))/2, 0, True, f[ n-1, s-n]+f[n-1, s+n]]; a[n_] := f[n, 1]
%t A063866 nmax = 50; d = {1}; a1 = {};
%t A063866 Do[
%t A063866 i = Ceiling[Length[d]/2] + 1;
%t A063866 AppendTo[a1, If[i > Length[d], 0, d[[i]]]];
%t A063866 d = PadLeft[d, Length[d] + 2 n] + PadRight[d, Length[d] + 2 n];
%t A063866 , {n, nmax}];
%t A063866 a1 (* _Ray Chandler_, Mar 14 2014 *)
%Y A063866 Cf. A025591, A063865, A063867.
%K A063866 nonn,easy,nice
%O A063866 0,6
%A A063866 _N. J. A. Sloane_, following a suggestion by _J. H. Conway_, Aug 27 2001
%E A063866 More terms from _Dean Hickerson_ and _Vladeta Jovovic_, Aug 28 2001