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 A093681 #17 May 24 2022 08:06:36
%S A093681 1,28,29,31,32,37,38,40,41,56,58,59,64,65,67,68,82,109,110,112,113,
%T A093681 118,119,121,122,137,139,140,145,146,148,149,244,271,272,274,275,280,
%U A093681 281,283,284,299,301,302,307,308,310,311,325,352,353,355,356,361,362
%N A093681 Sequence contains no 3-term arithmetic progression, starting with 1, 28.
%C A093681 a(1)=1, a(2)=28; a(n) is least k such that no three terms of a(1), a(2), ..., a(n-1), k form an arithmetic progression.
%H A093681 <a href="/index/No#non_averaging">Index entries related to non-averaging sequences</a>
%F A093681 a(n) = (Sum_{k=1..n-1} (3^A007814(k) + 1)/2) + f(n), with f(n) a 16-periodic function with values {1, 27, 26, 27, 23, 27, 26, 27, 14, 28, 28, 28, 28, 28, 28, 28, ...}, n >= 1 (conjectured and checked up to n=1000).
%Y A093681 Cf. A004793, A033157, A093678, A093679, A093680, A092482.
%Y A093681 Row 6 of array in A093682.
%K A093681 nonn
%O A093681 1,2
%A A093681 _Ralf Stephan_, Apr 09 2004