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 A049629 a(n) = (F(6*n+5) - F(6*n+1))/4 = (F(6*n+4) + F(6*n+2))/4, where F = A000045. 26
 1, 19, 341, 6119, 109801, 1970299, 35355581, 634430159, 11384387281, 204284540899, 3665737348901, 65778987739319, 1180356041958841, 21180629767519819, 380070979773397901, 6820097006153642399, 122381675130992165281, 2196050055351705332659, 39406519321199703822581 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS x(n) := 2*a(n) and y(n) := A007805(n), n >= 0, give all the positive solutions of the Pell equation x^2 - 5*y^2 = -1. The Gregory V. Richardson formula follows from this. - Wolfdieter Lang, Jun 20 2013 From Peter Bala, Mar 23 2018: (Start) Define a binary operation o on the real numbers by x o y = x*sqrt(1 + y^2) + y*sqrt(1 + x^2). The operation o is commutative and associative with identity 0. Then we have 2*a(n) = 2 o 2 o ... o 2 (2*n+1 terms). For example, 2 o 2 = 4*sqrt(5) and 2 o 2 o 2 = 2 o 4*sqrt(5) = 38 = 2*a(1). Cf. A084068. a(n) = U(2*n+1) where U(n) is the Lehmer sequence [Lehmer, 1930] defined by the recurrence U(n) = sqrt(20)*U(n-1) - U(n-2) with U(0) = 0 and U(1) = 1. The solution to the recurrence is U(n) = (1/4)*( (sqrt(5) + 2)^n - (sqrt(5) - 2)^n ). (End) LINKS G. C. Greubel, Table of n, a(n) for n = 0..795 Andersen, K., Carbone, L. and Penta, D., Kac-Moody Fibonacci sequences, hyperbolic golden ratios, and real quadratic fields, Journal of Number Theory and Combinatorics, Vol 2, No. 3 pp 245-278, 2011. See Section 9. Tanya Khovanova, Recursive Sequences D. H. Lehmer, An extended theory of Lucas' functions, Annals of Mathematics, Second Series, Vol. 31, No. 3 (Jul., 1930), pp. 419-448. Giovanni Lucca, Integer Sequences and Circle Chains Inside a Hyperbola, Forum Geometricorum (2019) Vol. 19, 11-16. E. W. Weisstein, MathWorld: Lehmer Number H. C. Williams and R. K. Guy, Some fourth-order linear divisibility sequences, Intl. J. Number Theory 7 (5) (2011) 1255-1277. H. C. Williams and R. K. Guy, Some Monoapparitic Fourth Order Linear Divisibility Sequences Integers, Volume 12A (2012) The John Selfridge Memorial Volume. Index entries for linear recurrences with constant coefficients, signature (18,-1). FORMULA a(n) ~ (1/4)*(sqrt(5) + 2)^(2*n+1). - Joe Keane (jgk(AT)jgk.org), May 15 2002 For all members x of the sequence, 20*x^2 + 5 is a square. Lim_{n -> inf} a(n)/a(n-1) = 9 + 2*sqrt(20) = 9 + 4*sqrt(5). The 20 can be seen to derive from the statement "20*x^2 + 5 is a square". - Gregory V. Richardson, Oct 12 2002 a(n) = (((9 + 4*sqrt(5))^n - (9 - 4*sqrt(5))^n) + ((9 + 4*sqrt(5))^(n-1) - (9 - 4*sqrt(5))^(n-1)) / (8*sqrt(5)). - Gregory V. Richardson, Oct 12 2002 From R. J. Mathar, Nov 04 2008: (Start) G.f.: (1+x)/(1 - 18x + x^2). a(n) = A049660(n) + A049660(n+1). (End) a(n) = 18*a(n-1) - a(n-2) for n>1; a(0)=1, a(1)=19. - Philippe Deléham, Nov 17 2008 a(n) = S(n,18) + S(n-1,18) with the Chebyshev S-polynomials (A049310). - Wolfdieter Lang, Jun 20 2013 From Peter Bala, Mar 23 2015: (Start) a(n) = ( Fibonacci(6*n + 6 - 2*k) + Fibonacci(6*n + 2*k) )/( Fibonacci(6 - 2*k) + Fibonacci(2*k) ), for k an arbitrary integer. a(n) = ( Fibonacci(6*n + 6 - 2*k - 1) - Fibonacci(6*n + 2*k + 1) )/( Fibonacci(6 - 2*k - 1) - Fibonacci(2*k + 1) ), for k an arbitrary integer, k != 1. The aerated sequence (b(n))n>=1 = [1, 0, 19, 0, 341, 0, 6119, 0, ...] is a fourth-order linear divisibility sequence; that is, if n | m then b(n) | b(m). It is the case P1 = 0, P2 = -16, Q = -1 of the 3-parameter family of divisibility sequences found by Williams and Guy. See A100047 for the connection with Chebyshev polynomials. (End) a(n) = (A188378(n)^3 + (A188378(n)-2)^3) / 8. - Altug Alkan, Jan 24 2016 a(n) = sqrt(5 * Fibonacci(3 + 6*n)^2 - 4)/4. - Gerry Martens, Jul 25 2016 a(n) = Lucas(6*n + 3)/4. - Ehren Metcalfe, Feb 18 2017 From Peter Bala, Mar 23 2018: (Start) a(n) = 1/4*( (sqrt(5) + 2)^(2*n+1) - (sqrt(5) - 2)^(2*n+1) ). a(n) = 9*a(n-1) + 2*sqrt(5 + 20*a(n-1)^2). a(n) = (1/2)*sinh((2*n + 1)*arcsinh(2)). (End) EXAMPLE Pell, n=1: (2*19)^2 - 5*17^2 = -1. MAPLE with(numtheory): with(combinat): seq((fibonacci(6*n+5)-fibonacci(6*n+1))/4, n=0..20); # Muniru A Asiru, Mar 25 2018 MATHEMATICA a[n_] := Simplify[(2 + Sqrt@5)^(2 n - 1) + (2 - Sqrt@5)^(2 n - 1)]/4; Array[a, 16] (* Robert G. Wilson v, Oct 28 2010 *) PROG (PARI) x='x+O('x^30); Vec((1+x)/(1 - 18x + x^2)) \\ G. C. Greubel, Dec 15 2017 (Magma) [(Fibonacci(6*n+5) - Fibonacci(6*n+1))/4: n in [0..30]]; // G. C. Greubel, Dec 15 2017 CROSSREFS Bisection of A001077 divided by 2. Cf. A000045, A007805, A049310, A049660, A100047, A188378. Cf. A005013, A033890, A049685, A084608. Cf. similar sequences of the type (1/k)*sinh((2*n+1)*arcsinh(k)) listed in A097775. Sequence in context: A345963 A202043 A142549 * A162805 A049664 A163110 Adjacent sequences: A049626 A049627 A049628 * A049630 A049631 A049632 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified March 28 03:48 EDT 2023. Contains 361577 sequences. (Running on oeis4.)