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 A304884 #13 Jul 05 2018 00:55:08
%S A304884 1,2,2,2,2,2,3,3,4,4,4,4,4,4,4,4,5,5,6,5,6,6,6,6,7,7,8,8,8,8,8,8,8,8,
%T A304884 9,8,10,8,10,9,9,9,9,9,10,10,10,10,10,10,11,12,11,11,11,11,12,11,12,
%U A304884 12,13,12,13,13,14,13,13,13,14,14,14,14,14,14,14,14,14,14,15
%N A304884 Size of the largest subset of the cyclic group of order n which does not contain a nontrivial 3-term arithmetic progression.
%C A304884 Each term is at most the corresponding term of A003002.
%C A304884 Arithmetic progressions are trivial if they are of the form x,x,x.
%H A304884 L. Halbeisen and S. Halbeisen, <a href="http://user.math.uzh.ch/halbeisen/publications/pdf/colmar39.pdf">Avoiding arithmetic progressions in cyclic groups</a>, Swiss Mathematical Society, 2005.
%e A304884 For n=10, the integers (mod 10) have sets with four elements like {1,2,4,5} which contain no arithmetic progressions with 3 elements, but no such sets with five elements. For example, {1,2,4,5,8} has the progression 2,8,4, and {1,2,4,5,9} has the progression 4,9,4. Since four is the most elements possible, a(10) = 4. - _Michael B. Porter_, May 26 2018
%Y A304884 Cf. A003002.
%K A304884 nonn
%O A304884 1,2
%A A304884 _Daniel Scheinerman_, May 20 2018
%E A304884 a(51)-a(79) from _Giovanni Resta_, May 22 2018