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 A199639 #6 Mar 30 2012 17:23:03
%S A199639 1,2,3,4,6,7,8,12,16,19,22,24,28,31,32,43,48,64,76,79,96,103,112,128,
%T A199639 139,142,163,166,184,192,199,211,223,256,262,268,271,283,304,307,316,
%U A199639 331,352,367,376,379,384,412,439,448,454,463,496,499,512,526,547
%N A199639 Indices of hexagonal numbers that are not the difference of two positive hexagonal numbers.
%t A199639 Select[Range[600], Reduce[#*(2*#-1) == x*(2*x-1) - y*(2*y-1) && x > 0 && y > 0, {x, y}, Integers] == False &]
%Y A199639 Cf. A000384 (hexagonal numbers), A135768 (pentagonal case).
%K A199639 nonn
%O A199639 1,2
%A A199639 _T. D. Noe_, Dec 06 2011