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 A248639 #9 Oct 16 2014 04:26:27 %S A248639 0,0,0,1,0,0,1,0,0,0,1,1,1,2,0,0,0,1,0,1,1,1,2,2,2,0,0,2,0,0,0,1,1,0, %T A248639 1,1,0,1,1,1,2,2,1,2,1,2,2,4,0,0,0,2,0,0,2,0,0,0,1,1,1,2,0,1,0,0,0,1, %U A248639 1,1,2,3,1,2,1,1,1,2,2,0,2,2,2,4,4,3,0,0,0,2,0,1,2,0,4,2,1,5,0,2,1,1,2,1,0,0,2,2,0,0,0,3,0,0,1,1,1,4,1,2,3,0,1,2,1,0,3,3,4,1,1,3 %N A248639 Least nonnegative sequence which does not contain a 4-term equidistant subsequence (a(n+k*d); k=0,1,2,3) in arithmetic progression. %C A248639 See A248625 for more information, links and examples. %C A248639 See A248641 for the "positive integers" variant. %o A248639 (PARI) a=[];for(n=1,190,a=concat(a,0);while(hasAP(a,4),a[#a]++));a \\ See A248625 for hasAP(). %Y A248639 Cf. A248625, A248640, A248641, A248627, A229037, A241752. %K A248639 nonn %O A248639 0,14 %A A248639 _M. F. Hasler_, Oct 10 2014