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 A061309 #4 Dec 05 2013 19:54:50 %S A061309 1,0,0,0,1,0,0,-1,0,0,4,0,-1,1,0,0,2,-1,0,1,0,0,-1,0,1,1,0,-1,1,0,0,1, %T A061309 -1,0,2,0,0,-1,2,0,1,0,-1,4,0,1,1,-1,0,1,0,0,-1,0,0,2,0,-1,1,3,3,1,-1, %U A061309 0,1,0,4,-1,0,0,2,2,-1,5,0,0,1,-1,0,4,2,0,-1,0,3,1,0,-1,4,0,0,13 %N A061309 a(n) is the smallest number required to make the concatenation of a(n) and 2n+1 a prime, or -1 if no such number exists. %C A061309 a(0) = 0 for a prime. %e A061309 a(9) = 0 as 19 itself is a prime; a(10) = 4 as 421 is a prime but 121, 221, 321 are not. %K A061309 sign,base %O A061309 0,11 %A A061309 _Amarnath Murthy_, Apr 26 2001 %E A061309 More terms from _Frank Ellermann_, Jun 15 2001