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 A135689 #16 Nov 28 2021 04:05:37
%S A135689 0,1,-1,-2,1,2,2,1,-1,1,-2,-1,-2,-5,-1,-2,1,1,-1,0,2,4,1,-1,2,5,5,4,1,
%T A135689 2,2,5,-1,-5,-1,0,1,-2,0,0,-2,0,-4,-5,-1,-3,1,-1,-2,1,-5,-3,-5,-8,-4,
%U A135689 -7,-1,-1,-2,-1,-2,1,-5,-6,1,1,5,2,1,7,0,4,-1,-5,2,1,0,-1,0,3,2,0,0,0,4,6,5,3,1,5,3,4,-1,-5,1,3,2,-2,-1,1,5
%N A135689 a(n) = a(n-2) - (a(floor(n/2)) - a(abs(floor(n/2) - 1))) if (n mod 2 = 0), otherwise a(n-1) - (a(abs(floor(n/2) - 2)) - a(abs(floor(n/2) - 3))).
%H A135689 G. C. Greubel, <a href="/A135689/b135689.txt">Table of n, a(n) for n = 0..1000</a>
%F A135689 a(n) = a(n-2) - (a(floor(n/2)) - a(abs(floor(n/2) - 1))) if (n mod 2 = 0), otherwise a(n-1) - (a(abs(floor(n/2) - 2)) - a(abs(floor(n/2) - 3))).
%t A135689 a[n_]:= a[n] = If[n<2, n, If[n<4, 1-n, If[Mod[n, 2]==0, a[n-2] - (a[Floor[n/2]] - a[Abs[Floor[n/2] -1]]), a[n-1] - (a[Abs[Floor[n/2] -2]] - a[Abs[Floor[n/2] - 3]])] ]]; Table[a[n], {n, 0, 110}]
%o A135689 (Sage)
%o A135689 def a(n): # A135689
%o A135689 if (n<4): return [0, 1, -1, -2][n]
%o A135689 elif ((n%2)==0): return a(n-2) - (a((n//2)) - a(abs((n//2) - 1)))
%o A135689 else: return a(n-1) - (a(abs((n//2) - 2)) - a(abs((n//2) - 3)))
%o A135689 [a(n) for n in (0..110)] # _G. C. Greubel_, Nov 26 2021
%Y A135689 Cf. A135690, A135692.
%K A135689 sign,easy,less
%O A135689 0,4
%A A135689 _Roger L. Bagula_, Feb 19 2008
%E A135689 Edited by _N. J. A. Sloane_, Mar 03 2008