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 A253821 #7 Jun 13 2015 00:55:22 %S A253821 1,181,589,208489,679321,240595741,783935461,277647276241, %T A253821 904660842289,320404716185989,1043977828065661,369746764831354681, %U A253821 1204749508926930121,426687446210667115501,1390279889323849293589,492396943180345019933089,1604381787530213157871201 %N A253821 Indices of octagonal numbers (A000567) which are also centered triangular numbers (A005448). %C A253821 Also positive integers x in the solutions to 6*x^2 - 3*y^2 - 4*x + 3*y - 2 = 0, the corresponding values of y being A253822. %H A253821 Colin Barker, <a href="/A253821/b253821.txt">Table of n, a(n) for n = 1..653</a> %H A253821 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,1154,-1154,-1,1). %F A253821 a(n) = a(n-1)+1154*a(n-2)-1154*a(n-3)-a(n-4)+a(n-5). %F A253821 G.f.: -x*(x^4+180*x^3-746*x^2+180*x+1) / ((x-1)*(x^2-34*x+1)*(x^2+34*x+1)). %e A253821 181 is in the sequence because the 181st octagonal number is 97921, which is also the 256th centered triangular number. %o A253821 (PARI) Vec(-x*(x^4+180*x^3-746*x^2+180*x+1)/((x-1)*(x^2-34*x+1)*(x^2+34*x+1)) + O(x^100)) %Y A253821 Cf. A000567, A005448, A253822, A253823. %K A253821 nonn,easy %O A253821 1,2 %A A253821 _Colin Barker_, Jan 14 2015