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 A138117 #8 Nov 21 2013 12:49:19
%S A138117 2,2,3,5,2,3,5,7,11,2,3,5,7,11,13,17,2,3,5,7,11,13,17,19,23,2,3,5,7,
%T A138117 11,13,17,19,23,29,31,2,3,5,7,11,13,17,19,23,29,31,37,41,2,3,5,7,11,
%U A138117 13,17,19,23,29,31,37,41,43,47
%N A138117 Triangle read by rows: row n lists the first 2n-1 prime numbers.
%e A138117 Triangle begins:
%e A138117 ....... 2
%e A138117 ..... 2,3,5
%e A138117 ... 2,3,5,7,11
%e A138117 . 2,3,5,7,11,13,17
%t A138117 nn=10;Flatten[Table[Take[Prime[Range[2nn+1]],2n+1],{n,0,nn}]] (* _Harvey P. Dale_, Aug 16 2011 *)
%Y A138117 Cf. A000040, A037126, A138139, A138143.
%K A138117 easy,nonn,tabf
%O A138117 1,1
%A A138117 _Omar E. Pol_, Mar 14 2008, corrected Mar 15 2008