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 A253675 #8 Dec 09 2017 14:42:32
%S A253675 1,361,6241,3463321,59923081,33254804881,575381414521,319312633001041,
%T A253675 5524812282304561,3066039868821187801,53049246959306977201,
%U A253675 29440114501108412261161,509378863778453312776441,282683976373603105710477121,4891055796951461749972406281
%N A253675 Centered triangular numbers (A005448) which are also centered octagonal numbers (A016754).
%H A253675 Colin Barker, <a href="/A253675/b253675.txt">Table of n, a(n) for n = 1..503</a>
%H A253675 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,9602,-9602,-1,1).
%F A253675 a(n) = a(n-1)+9602*a(n-2)-9602*a(n-3)-a(n-4)+a(n-5).
%F A253675 G.f.: -x*(x^4+360*x^3-3722*x^2+360*x+1) / ((x-1)*(x^2-98*x+1)*(x^2+98*x+1)).
%e A253675 361 is in the sequence because it is the 16th centered triangular number and the 10th centered octagonal number.
%t A253675 LinearRecurrence[{1,9602,-9602,-1,1},{1,361,6241,3463321,59923081},20] (* _Harvey P. Dale_, Dec 09 2017 *)
%o A253675 (PARI) Vec(-x*(x^4+360*x^3-3722*x^2+360*x+1)/((x-1)*(x^2-98*x+1)*(x^2+98*x+1)) + O(x^100))
%Y A253675 Cf. A005448, A016754, A253673, A253674.
%K A253675 nonn,easy
%O A253675 1,2
%A A253675 _Colin Barker_, Jan 08 2015