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 A131521 #28 Jan 30 2020 21:29:16
%S A131521 1,10,190,4420,113950,3128140,89608780,2647358920,80065458910,
%T A131521 2466432898300,77115832253380,2440820453410360,78053018025315340,
%U A131521 2517915855707814520,81839894422876183000,2677554649095487584400
%N A131521 Expansion of 9/(4 + 5*sqrt(1-36*x)).
%C A131521 Number of walks of length 2n on the 10-regular tree beginning and ending at some fixed vertex. Hankel transform is A135321. - _Philippe Deléham_, Feb 25 2009
%H A131521 G. C. Greubel, <a href="/A131521/b131521.txt">Table of n, a(n) for n = 0..640</a>
%F A131521 G.f.: 9/(4 + 5*sqrt(1-36*x)).
%F A131521 a(n) = Sum_{k=0..n} A039599(n,k)*9^(n-k). - _Philippe Deléham_, Aug 25 2007
%F A131521 a(n) ~ 45*36^n/(32*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Jun 29 2013
%F A131521 D-finite with recurrence: n*a(n) +2*(-68*n+27)*a(n-1) +1800*(2*n-3)*a(n-2)=0. - _R. J. Mathar_, Jan 20 2020
%t A131521 CoefficientList[Series[9/(4+5*Sqrt[1-36*x]),{x,0,30}],x] (* _Harvey P. Dale_, Aug 21 2012 *)
%o A131521 (PARI) Vec(9/(4 + 5*sqrt(1-36*x)) + O(x^50)) \\ _G. C. Greubel_, Jan 28 2017
%Y A131521 Column k=10 of A183135.
%Y A131521 Cf. A039599, A135321.
%K A131521 nonn
%O A131521 0,2
%A A131521 _Philippe Deléham_, Aug 23 2007
%E A131521 More terms from _Olivier Gérard_, Sep 22 2007