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 A258131 #12 Aug 31 2018 18:40:52
%S A258131 49665,348161,19701781,138502485,7841194625,55123576321,3120775694421,
%T A258131 21939044808725,1242060885120385,8731684710231681,494337111502154261,
%U A258131 3475188575627335765,196744928316972210945,1383116321414969338241,78303987133043437737301
%N A258131 Octagonal numbers (A000567) that are the sum of eleven consecutive octagonal numbers.
%H A258131 Colin Barker, <a href="/A258131/b258131.txt">Table of n, a(n) for n = 1..766</a>
%H A258131 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,398,-398,-1,1).
%F A258131 G.f.: -11*x*(95*x^4 -64*x^3 -37550*x^2 +27136*x +4515)/((x-1)*(x^2 -20*x +1)*(x^2 +20*x +1)).
%e A258131 49665 is in the sequence because Oct(129) = 49665 = 3400 + 3605 + 3816 + 4033 + 4256 + 4485 + 4720 + 4961 + 5208 + 5461 + 5720 = Oct(34) + ... + Oct(44).
%t A258131 CoefficientList[Series[-11*x*(95*x^4 -64*x^3 -37550*x^2 +27136*x +4515)/((x-1)*(x^2 -20*x +1)*(x^2 +20*x +1)), {x, 0, 50}], x] (* _G. C. Greubel_, Oct 18 2017 *)
%t A258131 Select[Total/@Partition[Table[n(3n-2),{n,5*10^6}],11,1],IntegerQ[(Sqrt[1+3#]+1)/3]&] (* _Harvey P. Dale_, Aug 31 2018 *)
%o A258131 (PARI) Vec(-11*x*(95*x^4 -64*x^3 -37550*x^2 +27136*x +4515)/((x-1)*(x^2 -20*x +1)*(x^2 +20*x +1)) + O('x^20))
%Y A258131 Cf. A000567, A258128, A258129, A258130, A258132.
%K A258131 nonn,easy
%O A258131 1,1
%A A258131 _Colin Barker_, May 21 2015