OFFSET
0,2
LINKS
Indranil Ghosh, Table of n, a(n) for n = 0..631
Christian Aebi and Grant Cairns, Lattice equable quadrilaterals III: tangential and extangential cases, Integers (2023) Vol. 23, #A48.
Tanya Khovanova, Recursive Sequences
Giovanni Lucca, Integer Sequences and Circle Chains Inside a Hyperbola, Forum Geometricorum (2019) Vol. 19, 11-16.
Index entries for linear recurrences with constant coefficients, signature (38,-1).
FORMULA
G.f.: (1 + x)/(1 - 38*x + x^2).
a(n) = S(n, 38) + S(n-1, 38) = S(2*n, 2*sqrt(10)), with Chebyshev polynomials of the second kind. See A049310 for the triangle of S(n, x) = U(n, x/2) coefficients. S(-1, x) := 0 =: U(-1, x).
a(n) = (-1)^n*T(2*n + 1, 3*i)/(3*i) with the imaginary unit i and Chebyshev polynomials of the first kind. See the T-triangle A053120.
a(n) = ((3 + sqrt(10))*(19 + 6*sqrt(10))^n - ((-3 + sqrt(10))*(19 - 6*sqrt(10))^n))/6. - Gerry Martens, Jul 09 2015
a(n) = (1/3)*sinh((2*n + 1)*arcsinh(3)). - Bruno Berselli, Apr 03 2018
EXAMPLE
(x,y) = (3,1), (117,37), (4443,1405), ... give the positive integer solutions to x^2 - 10*y^2 = -1.
MATHEMATICA
LinearRecurrence[{38, -1}, {1, 39}, 20] (* Ray Chandler, Aug 11 2015 *)
PROG
(PARI) Vec((1+x)/(1-38*x+x^2) + O(x^20)) \\ Michel Marcus, Jul 10 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Aug 31 2004
EXTENSIONS
More terms from Indranil Ghosh, Feb 04 2017
STATUS
approved