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 A203628 #10 Aug 01 2015 10:03:21
%S A203628 1,589,528601,474682789,426264615601,382785150126589,
%T A203628 343740638549061001,308678710631906651989,277193138406813624424801,
%U A203628 248919129610608002826818989,223529101197187579724859027001,200728883955944835984920579427589
%N A203628 Indices of 9-gonal (nonagonal) numbers which are also 10-gonal (decagonal).
%C A203628 As n increases, this sequence is approximately geometric with common ratio r = lim(n->Infinity, a(n)/a(n-1)) = (2*sqrt(2)+sqrt(7))^4 = 449+120*sqrt(14).
%H A203628 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (899, -899, 1).
%F A203628 G.f.: x*(1-310*x-11*x^2) / ((1-x)*(1-898*x+x^2)).
%F A203628 a(n) = 898*a(n-1)-a(n-2)-320.
%F A203628 a(n) = 899*a(n-1)-899*a(n-2)+a(n-3).
%F A203628 a(n) = 1/56*((sqrt(2)+2*sqrt(7))*(2*sqrt(2)+sqrt(7))^(4*n-3)+(sqrt(2)-2*sqrt(7))*(2*sqrt(2)-sqrt(7))^(4*n-3)+20).
%F A203628 a(n) = ceiling(1/56*(sqrt(2)+2*sqrt(7))*(2*sqrt(2)+sqrt(7))^(4*n-3)).
%e A203628 The second number that is both 9-gonal (nonagonal) and 10-gonal (decagonal) is A001106(589) = 1212751. Hence a(2) = 589.
%t A203628 LinearRecurrence[{899, -899, 1}, {1, 589, 528601}, 12]
%Y A203628 Cf. A203627, A203629, A001107, A001106.
%K A203628 nonn,easy
%O A203628 1,2
%A A203628 _Ant King_, Jan 06 2012