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 A259158 #10 Aug 16 2015 12:04:01
%S A259158 171,355817826,739874066553765,1538466018777014327028,
%T A259158 3199027777735470191011289295,6651936797966044301302839516949686,
%U A259158 13831784604089070782667939513452521937961,28761287297921206463992005998270360082445758120
%N A259158 Positive triangular numbers (A000217) that are heptagonal numbers (A000566) divided by 2.
%H A259158 Colin Barker, <a href="/A259158/b259158.txt">Table of n, a(n) for n = 1..158</a>
%H A259158 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2079363,-2079363,1).
%F A259158 G.f.: -171*x*(1443*x+1) / ((x-1)*(x^2-2079362*x+1)).
%e A259158 171 is in the sequence because 171 is the 18th triangular number, and 2*171 is the 12th heptagonal number.
%t A259158 LinearRecurrence[{2079363, -2079363, 1}, {171, 355817826, 739874066553765}, 20] (* _Vincenzo Librandi_, Jun 20 2015 *)
%o A259158 (PARI) Vec(-171*x*(1443*x+1) / ((x-1)*(x^2-2079362*x+1)) + O(x^20))
%Y A259158 Cf. A000217, A000566, A259156, A259157, A259159-A259167.
%K A259158 nonn,easy
%O A259158 1,1
%A A259158 _Colin Barker_, Jun 19 2015