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 A258128 #12 Feb 19 2018 17:11:26
%S A258128 5461,813281,7272157205,1083057360705,9684433559760981,
%T A258128 1442322650052752161,12896895753596262561301,
%U A258128 1920761265591267733640321,17174976631595008767000306005,2557904668044167195987033355105,22872156829955018609383449248248341
%N A258128 Octagonal numbers (A000567) that are the sum of two consecutive octagonal numbers.
%H A258128 Colin Barker, <a href="/A258128/b258128.txt">Table of n, a(n) for n = 1..326</a>
%H A258128 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,1331714,-1331714,-1,1).
%F A258128 G.f.: -x*(x^4+20*x^3-1146230*x^2+807820*x+5461) / ((x-1)*(x^2-1154*x+1)*(x^2+1154*x+1)).
%e A258128 5461 is in the sequence because Oct(43) = 5461 = 2640 + 2821 = Oct(30) + Oct(31).
%t A258128 CoefficientList[Series[-x*(x^4 +20*x^3 -1146230*x^2 +807820*x +5461)/((x-1)*(x^2 -1154*x +1)*(x^2 +1154*x +1)), {x, 0, 50}], x] (* _G. C. Greubel_, Oct 18 2017 *)
%t A258128 LinearRecurrence[{1,1331714,-1331714,-1,1},{5461,813281,7272157205,1083057360705,9684433559760981},20] (* _Harvey P. Dale_, Feb 19 2018 *)
%o A258128 (PARI) Vec(-x*(x^4 +20*x^3 -1146230*x^2 +807820*x +5461)/((x-1)*(x^2 -1154*x +1)*(x^2 +1154*x +1)) + O('x^20))
%Y A258128 Cf. A000567, A258129, A258130, A258131, A258132.
%K A258128 nonn,easy
%O A258128 1,1
%A A258128 _Colin Barker_, May 21 2015