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 A254855 #11 Aug 31 2021 13:41:32
%S A254855 1,2,16,43,407,1108,10558,28757,274093,746566,7115852,19381951,
%T A254855 184738051,503184152,4796073466,13063405993,124513172057,339145371658,
%U A254855 3232546400008,8804716257107,83921693228143,228583477313116,2178731477531702,5934365693883901
%N A254855 Indices of octagonal numbers (A000567) that are also centered heptagonal numbers (A069099).
%C A254855 Also positive integers x in the solutions to 6*x^2 - 7*y^2 - 4*x + 7*y - 2 = 0, the corresponding values of y being A254856.
%H A254855 Colin Barker, <a href="/A254855/b254855.txt">Table of n, a(n) for n = 1..1000</a>
%H A254855 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,26,-26,-1,1).
%F A254855 a(n) = a(n-1)+26*a(n-2)-26*a(n-3)-a(n-4)+a(n-5).
%F A254855 G.f.: -x*(x^4+x^3-12*x^2+x+1) / ((x-1)*(x^4-26*x^2+1)).
%e A254855 16 is in the sequence because the 16th octagonal number is 736, which is also the 15th centered heptagonal number.
%t A254855 LinearRecurrence[{1,26,-26,-1,1},{1,2,16,43,407},30] (* _Harvey P. Dale_, Aug 31 2021 *)
%o A254855 (PARI) Vec(-x*(x^4+x^3-12*x^2+x+1)/((x-1)*(x^4-26*x^2+1)) + O(x^100))
%Y A254855 Cf. A000567, A069099, A254856, A254857.
%K A254855 nonn,easy
%O A254855 1,2
%A A254855 _Colin Barker_, Feb 08 2015