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 A239521 #6 Apr 12 2014 16:26:13
%S A239521 25,27,2209,3481,3721,6859,6889,18769,22201,22801,157609,1079521,
%T A239521 1630729,1635841,2621161,2627641,2825761,3179089,5257849,5276209,
%U A239521 7447441,7458361,17032129,17048641,34304449,34351321,40436881,40462321,75290329,75359761,137288089,137334961
%N A239521 Prime powers (A025475) such that the distance to the nearest prime power is an oblong number (A002378).
%o A239521 (PARI) lista(nn) = {precp = 1; currp = 4; for (nextp=5, nn, if (is_A025475(nextp), if (currp - precp < nextp - currp, tt = currp - precp, tt = nextp - currp); if (!(tt % 2) && ispolygonal(tt/2, 3), print1(currp, ", ")); precp = currp; currp = nextp;););} \\ _Michel Marcus_, Apr 11 2014
%Y A239521 Cf. A002378, A025475, A239520 - A239523.
%K A239521 nonn
%O A239521 1,1
%A A239521 _Alex Ratushnyak_, Mar 20 2014