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 A093680 #15 May 24 2022 02:49:50
%S A093680 1,19,20,22,23,28,29,31,32,46,47,49,50,56,58,59,82,100,101,103,104,
%T A093680 109,110,112,113,127,128,130,131,137,139,140,244,262,263,265,266,271,
%U A093680 272,274,275,289,290,292,293,299,301,302,325,343,344,346,347,352,353
%N A093680 Sequence contains no 3-term arithmetic progression, starting with 1, 19.
%C A093680 a(1)=1, a(2)=19; a(n) is least k such that no three terms of a(1), a(2), ..., a(n-1), k form an arithmetic progression.
%H A093680 <a href="/index/No#non_averaging">Index entries related to non-averaging sequences</a>
%F A093680 a(n) = (Sum_{k=1..n-1} (3^A007814(k) + 1)/2) + f(n), with f(n) a 16-periodic function with values {1, 18, 17, 18, 14, 18, 17, 19, 5, 18, 17, 18, 14, 19, 19, 19, ...}, as proved by Lawrence Sze.
%Y A093680 Cf. A004793, A033157, A093678, A093679, A093681, A092482.
%Y A093680 Row 5 of array in A093682.
%K A093680 nonn
%O A093680 1,2
%A A093680 _Ralf Stephan_, Apr 09 2004