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 A178698 #16 Mar 30 2012 18:52:41
%S A178698 0,1,0,3,4,5,4,6,7,4,0,2,7,0,5,13,15,16,13,9,11,15,20,21,18,19,25,17,
%T A178698 0,2,21,0,19,23,25,29,23,27,28,29,39,23,14,16,38,39,40,37,38,39,32,0,
%U A178698 1,29,30,31,28,29,30,23,24,25,26,35,22,23,38,39,40,50,57,43,44,54,32
%N A178698 Largest k<n such that 2*composite(n)=composite(n-k)+composite(n+k).
%e A178698 a(1)=0 because 2*composite(1)=composite(1-0)+composite(1+0)=4+4=8.
%t A178698 Composite[n_Integer] := FixedPoint[n + PrimePi@# + 1 &, n + PrimePi@ n + 1]; f[n_] := Block[{k = n - 1, m = Composite@ n}, While[k > 0 && 2 m != Composite[n + k] + Composite[n - k], k--]; k]; Array[f, 75]
%Y A178698 Cf. A002808 (composite numbers), A178609 (prime version).
%K A178698 nonn
%O A178698 1,4
%A A178698 _Juri-Stepan Gerasimov_, Dec 26 2010, Dec 31 2010