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 A072851 #18 Jun 22 2025 16:27:17
%S A072851 0,1,0,1,1,1,0,1,2,1,2,3,1,3,4,1,3,5,6,1,7,29,14,41,82,39,58,109,119,
%T A072851 1,120,579,432,675,1320,1325,291,259,3332,3657,3724,6015,11114,6465,
%U A072851 4325,20433,28884,381,5813,91505,96956,70329,106037,260323,260690,78399
%N A072851 a(n) = s(2*n) where s(0) = 0, s(1) = s(2) = 1, s(n) = abs(Sum_{k=2..n-1} (-1)^k * s(n-k) * s(k)).
%C A072851 Derived from G. J. Chaitin's s formula.
%C A072851 Chaitin's expression is s(0)=0, s(1)=alpha, s(2)=1, s(n)=Sum_{k=2..n-1} s(n-k)*s(k), but here it is made to alternate with the introduction of (-1)^k so that the numbers do not get large fast and alternate back and forth like a boustrophedon (A072231).
%D A072851 G. J. Chaitin, Algorithmic Information Theory, Cambridge Press, 1987, page 169.
%t A072851 s[n_Integer?Positive] := s[n]=Abs[Sum[(-1)^k*s[n-k]*s[k], {k, 2, n-1}]]; s[0]=0; s[1]=1; s[2]=1; Table[ s[n], {n, 0, 200, 2}]
%K A072851 nonn
%O A072851 0,9
%A A072851 _Roger L. Bagula_, Jul 25 2002
%E A072851 Edited and extended by _Robert G. Wilson v_, Jul 29 2002
%E A072851 Name clarified by _Sean A. Irvine_, Nov 01 2024