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 A278996 #25 Dec 19 2016 12:32:31
%S A278996 0,3,5,6,8,9,12,14,15,18,21,23,24,27,30,32,33,35,36,39,41,42,45,48,50,
%T A278996 51,53,54,57,59,60,62,63,66,68,69,72,75,77,78,80,81,84,86,87,89,90,93,
%U A278996 95,96,99,102,104,105,108,111,113,114,116,117,120,122,123,126,129,131
%N A278996 Numbers of the form (3h+1)*3^(2k)-1 or (3h+2)*3^(2k+1)-1 for h,k in N.
%C A278996 n is in the sequence if and only if either n == 0, 3, 5, or 6 (mod 9) or n == 8 (mod 9) and (n-8)/9 is in the sequence. - _Robert Israel_, Dec 15 2016
%H A278996 Robert Israel, <a href="/A278996/b278996.txt">Table of n, a(n) for n = 1..10000</a>
%H A278996 Hao Fu, G.-N. Han, <a href="https://arxiv.org/abs/1601.04370">Computer assisted proof for Apwenian sequences related to Hankel determinants</a>, arXiv preprint arXiv:1601.04370 [math.NT], 2016. See sequence "J" in Section 2.1.
%p A278996 filter:= proc(n) local m;
%p A278996 m:= padic:-ordp(n+1,3);
%p A278996 (n+1)/3^m mod 3 = 1 + (m mod 2)
%p A278996 end proc:
%p A278996 select(filter, [$0..100]); # _Robert Israel_, Dec 15 2016
%t A278996 isok[n_]:=Module[{ord=IntegerExponent[n+1,3]},Mod[(n+1)/3^ord,3]==Mod[ord,2]+1];Select[Range[0,131],isok](* _Ray Chandler_, Dec 17 2016 *)
%Y A278996 Complement of A278997.
%K A278996 nonn
%O A278996 1,2
%A A278996 _N. J. A. Sloane_, Dec 07 2016
%E A278996 More terms from _Robert Israel_, Dec 15 2016