OFFSET
0,2
REFERENCES
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..700
J. M. Alonso, Growth functions of amalgams, in Alperin, ed., Arboreal Group Theory, Springer, pp. 1-34, esp. p. 32.
Index entries for linear recurrences with constant coefficients, signature (26,-1).
FORMULA
From Colin Barker, Apr 16 2016: (Start)
a(n) = sqrt(7/6)*((13+2*sqrt(42))^(-n)*(-1+(13+2*sqrt(42))^(2*n))) for n>0.
a(n) = 26*a(n-1) - a(n-2) for n>2.
(End)
MATHEMATICA
CoefficientList[Series[(1+2*x+x^2)/(1-26*x+x^2), {x, 0, 20}], x] (* Vincenzo Librandi, Jun 13 2012 *)
PROG
(PARI) Vec((1+2*x+x^2)/(1-26*x+x^2)+O(x^99)) \\ Charles R Greathouse IV, Sep 26 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved