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 A259078 #11 May 17 2018 10:56:19
%S A259078 6426,9875715583923,15176968126834688342280,
%T A259078 23323916081375479417207129139097,
%U A259078 35844119643974208514403771705730853759974,55085128439364942192092971212729781713850848998511,84654649223375294035879228202426981713600292140166013136308
%N A259078 Heptagonal numbers (A000566) that are other heptagonal numbers divided by 2.
%H A259078 Colin Barker, <a href="/A259078/b259078.txt">Table of n, a(n) for n = 1..109</a>
%H A259078 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1536796803,-1536796803,1).
%F A259078 G.f.: -189*x*(x^2+1372105*x+34) / ((x-1)*(x^2-1536796802*x+1)).
%e A259078 6426 is in the sequence because 6426 is the 51st heptagonal number, and 2*6426 is the 72nd heptagonal number.
%t A259078 LinearRecurrence[{1536796803,-1536796803,1},{6426,9875715583923,15176968126834688342280},20] (* _Harvey P. Dale_, May 17 2018 *)
%o A259078 (PARI) Vec(-189*x*(x^2+1372105*x+34)/((x-1)*(x^2-1536796802*x+1)) + O(x^20))
%Y A259078 Cf. A000566, A075528, A107736, A259079.
%K A259078 nonn,easy
%O A259078 1,1
%A A259078 _Colin Barker_, Jun 18 2015