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 A214858 #12 Jan 31 2017 15:10:11
%S A214858 5,8,12,15,19,22,26,29,32,36,39,43,46,49,53,56,60,63,67,70,73,77,80,
%T A214858 84,87,90,94,97,101,104,108,111,114,118,121,125,128,131,135,138,142,
%U A214858 145,149,152,155,159,162,166,169,172,176,179,183,186,189,193,196,200
%N A214858 Natural numbers missing from A214857.
%F A214858 a(n+1) - a(n) = 3 or 4.
%t A214858 tri = Table[n (n + 1)/2, {n, 0, 1000}]; t = Table[Count[tri, _?(# <= n^2 &)], {n, 0, Sqrt[tri[[-1]]]}]; Complement[Range[t[[-1]]], t] (* _T. D. Noe_, Mar 11 2013 *)
%Y A214858 Cf. A063957, A214857.
%K A214858 nonn,easy
%O A214858 1,1
%A A214858 _Philippe Deléham_, Mar 09 2013
%E A214858 Extended by _T. D. Noe_, Mar 11 2013