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 A253923 #11 Sep 08 2022 08:46:11
%S A253923 1,7701,219781,1789106881,51059956641,415648888795141,
%T A253923 11862351246525781,96564381140875635681,2755885166244302532001,
%U A253923 22434030154994860543881301,640252753580346501593005701,5211918753572151610134715970401,148744800214537374776845967930881
%N A253923 Octagonal numbers (A000567) which are also centered pentagonal numbers (A005891).
%H A253923 Colin Barker, <a href="/A253923/b253923.txt">Table of n, a(n) for n = 1..373</a>
%H A253923 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,232322,-232322,-1,1).
%F A253923 a(n) = a(n-1)+232322*a(n-2)-232322*a(n-3)-a(n-4)+a(n-5).
%F A253923 G.f.: -x*(x^4+7700*x^3-20242*x^2+7700*x+1) / ((x-1)*(x^2-482*x+1)*(x^2+482*x+1)).
%e A253923 7701 is in the sequence because it is the 51st octagonal number and the 56th centered pentagonal number.
%t A253923 CoefficientList[Series[(x^4 + 7700 x^3 - 20242 x^2 + 7700 x + 1) / ((1 - x) (x^2 - 482 x + 1) (x^2 + 482 x + 1)), {x, 0, 30}], x] (* _Vincenzo Librandi_, Jan 20 2015 *)
%o A253923 (PARI) Vec(-x*(x^4+7700*x^3-20242*x^2+7700*x+1)/((x-1)*(x^2-482*x+1)*(x^2+482*x+1)) + O(x^100))
%o A253923 (Magma) I:=[1,7701,219781,1789106881,51059956641]; [n le 5 select I[n] else Self(n-1)+232322*Self(n-2)-232322*Self(n-3)-Self(n-4)+Self(n-5): n in [1..25]]; // _Vincenzo Librandi_, Jan 20 2015
%Y A253923 Cf. A000567, A005891, A253921, A253922.
%K A253923 nonn,easy
%O A253923 1,2
%A A253923 _Colin Barker_, Jan 19 2015