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 A109763 #22 Jun 14 2019 00:11:46
%S A109763 2,2,3,3,5,5,7,7,11,11,13,13,17,17,19,19,23,23,29,29,31,31,37,37,41,
%T A109763 41,43,43,47,47,53,53,59,59,61,61,67,67,71,71,73,73,79,79,83,83,89,89,
%U A109763 97,97,101,101,103,103,107,107,109,109,113,113,127,127,131,131
%N A109763 Primes repeated.
%F A109763 a(n) = A000040(ceiling(n/2)). - _Jon Perry_, Oct 29 2014
%t A109763 With[{prs=Prime[Range[40]]},Riffle[prs,prs]] (* _Harvey P. Dale_, Oct 10 2014 *)
%K A109763 easy,nonn
%O A109763 1,1
%A A109763 _Giovanni Teofilatto_, Aug 13 2005