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 A097289 #2 Mar 30 2012 18:57:05 %S A097289 1,1,2,2,1,3,3,1,4,4,2,3,1,5,5,2,4,1,6,6,2,5,3,4,1,7,7,2,6,3,5,1,8,8, %T A097289 2,7,3,6,4,5,1,9,9,2,8,3,7,4,6,1,10,10,2,9,3,8,4,7,5,6,1,11,11,2,10,3, %U A097289 9,4,8,5,7,1,12,12,2,11,3,10,4,9,5,8,6,7,1,13,13,2,12,3,11,4,10,5,9,6,8,1 %N A097289 Contains exactly once every pair (i,j) satisfying 0 < i <= j. %C A097289 All pairs (i,j) having i>j occur (not necessarily uniquely) except those of the form (i,i-1) for i>=3. (Those are included at A097291.) %F A097289 Obtained from A097283 by inserting m right after the first occurrence of m, for each positive integer m. %Y A097289 Cf. A097290, A097291. %K A097289 nonn %O A097289 1,3 %A A097289 _Clark Kimberling_, Aug 05 2004