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 A133251 #8 Feb 16 2025 08:33:06
%S A133251 697,3186,3744,5221,7209,8323,12496,12852,19228,20566,21022,24850,
%T A133251 29539,35224,38254,40768,44023,44689,52345,53802,58293,62173,63760,
%U A133251 66178,67815,78057,79834,80730,82537,95746,97713,101707,115240,131905,135373
%N A133251 Heptagonal numbers A000566 which are the sum of two other heptagonal numbers > 0.
%C A133251 This is to A000566 as A136117 is to A000326.
%C A133251 The sequence contains 12852 and 19751431167846, which are the smallest heptagonal numbers equal to twice another heptagonal number. - _R. J. Mathar_, Jan 13 2008
%H A133251 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HeptagonalNumber.html">Heptagonal Number</a>.
%F A133251 {x such that x in A000566 and x = A000566(i) + A000566(j) for i, j > 0 and where A000566(k) = k*(5*k-3)/2}.
%e A133251 Where hep(k) = k-th heptagonal number = A000566(k):
%e A133251 a(1) = 697 = hep(17) = 616 + 81 = hep(16) + hep(6).
%e A133251 a(2) = 3186 = hep(36) = 1782 + 1404 = hep(27) + hep(24).
%e A133251 a(3) = 3744 = hep(39) = 2673 + 1071 = hep(33) + hep(21).
%e A133251 a(4) = 5221 = hep(46) = 4347 + 874 = hep(42) + hep(19).
%t A133251 Module[{nn=1000,heps},heps=Table[(n(5n-3))/2,{n,nn}]; Select[ Union[ Total/@ Tuples[Take[heps,nn/2],2]],MemberQ[heps,#]&]] (* _Harvey P. Dale_, Dec 18 2015 *)
%Y A133251 Cf. A000566, A136117.
%K A133251 nonn
%O A133251 1,1
%A A133251 _Jonathan Vos Post_, Dec 19 2007
%E A133251 More terms from _R. J. Mathar_, Jan 13 2008