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 A048921 #20 Feb 16 2025 08:32:40
%S A048921 1,26884,542041975,10928650279834,220343446399977901,
%T A048921 4442564555387704166896,89570986345383445012986019,
%U A048921 1805930222253056462964119954950,36411165051495138060899141518722649,734121907962314751330792028336366100956
%N A048921 9-gonal heptagonal numbers (A000566).
%C A048921 As n increases, this sequence is approximately geometric with common ratio r = lim(n->Infinity,a(n)/a(n-1)) = (6+sqrt(35))^4 = 10081+1704*sqrt(35). - _Ant King_, Dec 31 2011
%H A048921 Colin Barker, <a href="/A048921/b048921.txt">Table of n, a(n) for n = 1..233</a>
%H A048921 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NonagonalHeptagonalNumber.html">Nonagonal Heptagonal Number.</a>
%H A048921 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (20163,-20163,1).
%F A048921 From _Ant King_, Dec 31 2011: (Start)
%F A048921 a(n) = 20163*a(n-1)-20163*a(n-2)+a(n-3).
%F A048921 a(n) = 20162*a(n-1)-a(n-2)+6768.
%F A048921 a(n) = 1/560*((39+4*sqrt(35))*(6+sqrt(35))^(4*n-3)+(39-4*sqrt(35))*(6-sqrt(35))^(4*n-3)-188).
%F A048921 a(n) = floor(1/560*(39+4*sqrt(35))*(6+sqrt(35))^(4*n-3)).
%F A048921 G.f.: x(1+6721*x+46*x^2) / ((1-x)(1-20162*x+x^2)).
%F A048921 (End)
%t A048921 LinearRecurrence[{20163, -20163, 1}, {1, 26884, 542041975}, 9]; (* _Ant King_, Dec 31 2011 *)
%o A048921 (PARI) Vec(x*(1+6721*x+46*x^2)/((1-x)*(1-20162*x+x^2)) + O(x^20)) \\ _Colin Barker_, Jun 22 2015
%Y A048921 Cf. A048919, A048920.
%K A048921 nonn,easy
%O A048921 1,2
%A A048921 _Eric W. Weisstein_