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 A178931 #13 May 25 2015 16:41:19
%S A178931 2,6,8,11,16,18,23,24,26,33,34,35,38,48,49,50,52,54,56,69,71,72,73,74,
%T A178931 77,78,80,83,99,102,103,105,106,107,109,110,114,115,116,119,124,144,
%U A178931 147,148,150,152,154,156,157,158,160,162,163,164,168,170,172,173,178,185
%N A178931 This sequence S is generated by the following rules: 2 is in S, and if n is in S, then floor[(3n-1)/2] and 3n are in S.
%C A178931 This sequence results from flattening and sorting the tree at A183212. Complement of A183213, obtained from the tree at A183211.
%C A178931 Sequence A117943 is the characteristic sequence of this one. - _M. F. Hasler_, Mar 07 2015
%H A178931 Ivan Neretin, <a href="/A178931/b178931.txt">Table of n, a(n) for n = 1..10000</a>
%F A178931 (See the Mathematica code.)
%t A178931 nn=200; t={2}; t0=t; While[t=Select[Union[t,Floor[(3*t-1)/2],3*t], #<=nn &]; t0 != t, t0=t]; t
%Y A178931 Cf. A183213, A183212.
%K A178931 nonn
%O A178931 1,1
%A A178931 _Clark Kimberling_, Dec 30 2010