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%I M3244 #31 Dec 17 2017 07:20:15
%S 4,5,9,10,11,14,19,20,23,24,25,26,32,33,37,38,39,41,42,48,50,53,54,55,
%T 59,63,64,65,69,70,76,77,80,83,85,86,89,99,102,104,108,110,113,114,
%U 115,116,123,124,128,129,130,131,140,143,144,145,146,152,161,162,167
%N Numbers that are not the sum of 3 hexagonal numbers (probably finite).
%C _Jud McCranie_ remarks that a(638) = 146858 is probably the last term.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H T. D. Noe, <a href="/A007536/b007536.txt">Table of n, a(n) for n=1..638</a>
%H R. K. Guy, <a href="http://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 notSumQ[n_] := Reduce[0 <= x <= y <= z && n == x*(2x - 1) + y*(2y - 1) + z*(2z - 1), {x, y, z}, Integers] === False; A007536 = Reap[ Do[ If[notSumQ[n], Print[n]; Sow[n]], {n, 1, 135}]][[2, 1]] (* _Jean-François Alcover_, Jun 27 2012 *)
%o (MATLAB)
%o N = 10^7; % to get all terms up to N
%o M = floor((sqrt(1+8*N)+1)/4);
%o H = zeros(1,N);
%o H((1:M) .*(2*(1:M)-1)) = 1;
%o H2 = conv(H,H);
%o H2 = H2(1:N);
%o H3 = conv(H,H2);
%o HS = H(3:N) + H2(2:N-1) + H3(1:N-2);
%o find(HS==0) + 2 % _Robert Israel_, Jul 06 2016
%Y Cf. A000384 (hexagonal numbers).
%Y Cf. A118278, A118279.
%K nonn,fini,nice
%O 1,1
%A _N. J. A. Sloane_
%E Corrected by _T. D. Noe_, Feb 14 2007
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