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 A213524 #8 Jul 06 2018 09:47:56
%S A213524 4,5,6,7,11,12,13,14,15,18,19,20,25,26,27,28,31,32,33,34,35,36,38,39,
%T A213524 44,45,46,47,51,52,53,54,55,57,58,59,60,64,68,70,71,72,75,76,77,78,79,
%U A213524 83,84,85,89,90,91,92,93,95,99,100,102,103,108,109,110,111
%N A213524 Numbers not representable as the sum of three octagonal numbers.
%C A213524 There are an infinite number of numbers that are not the sum of three octagonal numbers.
%D A213524 R. K. Guy, Unsolved Problems in Number Theory, D3.
%H A213524 T. D. Noe, <a href="/A213524/b213524.txt">Table of n, a(n) for n = 1..10000</a>
%H A213524 R. K. Guy, <a href="https://www.jstor.org/stable/2324367">Every number is expressible as the sum of how many polygonal numbers?</a>, Amer. Math. Monthly 101 (1994), 169-172.
%t A213524 nn = 100; oct = Table[n*(3*n-2), {n, 0, nn}]; t = Table[0, {oct[[-1]]}]; Do[n = oct[[i]] + oct[[j]] + oct[[k]]; If[n <= oct[[-1]], t[[n]] = 1], {i, nn}, {j, i, nn}, {k, j, nn}]; Flatten[Position[t, 0]]
%Y A213524 Cf. A000567 (octagonal numbers).
%Y A213524 Cf. A118278, A118279.
%K A213524 nonn
%O A213524 1,1
%A A213524 _T. D. Noe_, Jul 16 2012