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 A004292 Expansion of (1+x)^2/(1-18*x+x^2). 2
 1, 20, 360, 6460, 115920, 2080100, 37325880, 669785740, 12018817440, 215668928180, 3870021889800, 69444725088220, 1246135029698160, 22360985809478660, 401251609540917720, 7200167985927040300 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 REFERENCES J. M. Alonso, Growth functions of amalgams, in Alperin, ed., Arboreal Group Theory, Springer, pp. 1-34, esp. p. 32. P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 160, middle display. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..800 Hacène Belbachir, Soumeya Merwa Tebtoub, László Németh, Ellipse Chains and Associated Sequences, J. Int. Seq., Vol. 23 (2020), Article 20.8.5. Index entries for linear recurrences with constant coefficients, signature (18,-1). FORMULA a(n) = 1/2*(1-(-1)^2^n+(20+9*sqrt(5))*((9+4*sqrt(5))^(2*n)-1)/(9+4*sqrt(5))^(n+1)). - Gerry Martens, May 30 2015 MAPLE f:= gfun:-rectoproc({a(n)=18*a(n-1)-a(n-2), a(0)=1, a(1)=20, a(2)=360}, a(n), remember): map(f, [\$0..20]); # Robert Israel, Jun 01 2015 MATHEMATICA CoefficientList[Series[(1+x)^2/(1-18*x+x^2), {x, 0, 20}], x] (* Vincenzo Librandi, Jun 13 2012 *) a[n_]:=1/2(1-(-1)^2^n+(20+9 Sqrt[5])((9+4 Sqrt[5])^(2 n)-1)/(9+4 Sqrt[5])^(n+1)); Table[a[n] // FullSimplify, {n, 0, 20}] (* Gerry Martens, May 30 2015 *) PROG (PARI) Vec((1+x)^2/(1-18*x+x^2)+O(x^99)) \\ Charles R Greathouse IV, Sep 26 2012 CROSSREFS Pairwise sums of A049629. Sequence in context: A156455 A089350 A323961 * A053508 A060918 A115100 Adjacent sequences:  A004289 A004290 A004291 * A004293 A004294 A004295 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified January 17 11:57 EST 2021. Contains 340240 sequences. (Running on oeis4.)