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 A183231 #13 Mar 30 2012 18:57:12 %S A183231 1,4,3,19,7,13,6,229,25,43,11,118,18,34,10,26794,250,376,32,1033,52, %T A183231 89,16,7258,133,208,24,664,42,76,15,359026204,27025,31876,272,71629, %U A183231 403,593,40,536128,1078,1483,62,4184,102,169,22,26357428 %N A183231 First of two complementary trees generated by the triangular numbers. The second tree is A183232. %C A183231 Begin with the main tree A183079 generated by the triangular numbers: %C A183231 ......................1 %C A183231 ......................2 %C A183231 .............3.................4 %C A183231 .........6.......5........10........7 %C A183231 .......21..9...15..8....55..14....28..11 %C A183231 Every n>2 is in the subtree from 3 or the subtree from 4. %C A183231 Therefore, on subtracting 2 from all entries of those subtrees, we obtain complementary trees: A183231 and A183232. %F A183231 See the formulas at A183079 and A183233. %e A183231 First three levels: %e A183231 ............1 %e A183231 .......4.........3 %e A183231 ....19...7.....13..6 %Y A183231 Cf. A183079, A183232 (second tree), A183233. %K A183231 nonn,tabf %O A183231 1,2 %A A183231 _Clark Kimberling_, Jan 02 2011