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 A063867 #31 Nov 02 2023 12:32:15 %S A063867 1,2,2,2,2,6,10,8,14,46,80,70,124,442,794,722,1314,4820,8882,8220, %T A063867 15272,56920,106444,99820,187692,707486,1336546,1265204,2399784, %U A063867 9119656,17358560,16547220,31592878,120801376,231266520,221653776 %N A063867 Number of solutions to +- 1 +- 2 +- 3 +- ... +- n = 0 or +- 1. %H A063867 Ray Chandler, <a href="/A063867/b063867.txt">Table of n, a(n) for n = 0..3339</a> (terms < 10^1000; first 101 terms from T. D. Noe) %H A063867 Steven R. Finch, <a href="/A000980/a000980.pdf">Signum equations and extremal coefficients</a>, February 7, 2009. [Cached copy, with permission of the author] %F A063867 a(n) = A063865(n) + 2*A063866(n). %t A063867 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, 0]+2f[n, 1] %Y A063867 Cf. A025591, A063865, A063866. %K A063867 nonn,easy,nice %O A063867 0,2 %A A063867 _N. J. A. Sloane_, following a suggestion by _J. H. Conway_, Aug 27 2001 %E A063867 More terms from _Dean Hickerson_ and _Vladeta Jovovic_, Aug 28 2001