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 A091018 #8 May 05 2014 00:47:07 %S A091018 0,1,3,2,6,4,10,7,5,15,11,8,21,16,12,9,28,22,17,13,36,29,23,18,14,45, %T A091018 37,30,24,19,55,46,38,31,25,20,66,56,47,39,32,26,78,67,57,48,40,33,27, %U A091018 91,79,68,58,49,41,34,105,92,80,69,59,50,42,35,120,106,93,81,70,60,51 %N A091018 Numbers in n-th upward diagonal of triangle T : 0; 1, 2; 3, 4, 5; 6, 7, 8, 9; ... %C A091018 See also A090894 (downward diagonal). %F A091018 a(n) = A056536(n+1) - 1. %e A091018 0; 1; 3,2; 6,4; 10,7,5; 15,11,8; 21,16,12,9; 28,22,17,13; ... %o A091018 (PARI) T(n,k)=(n-k)^2+(n+k) /* _Michael Somos_, May 31 2005 */ %K A091018 easy,nonn %O A091018 0,3 %A A091018 _Philippe Deléham_, Feb 25 2004