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A041011 Denominators of continued fraction convergents to sqrt(8). 6
1, 1, 5, 6, 29, 35, 169, 204, 985, 1189, 5741, 6930, 33461, 40391, 195025, 235416, 1136689, 1372105, 6625109, 7997214, 38613965, 46611179, 225058681, 271669860, 1311738121, 1583407981, 7645370045, 9228778026, 44560482149 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Sqrt(8) = 2 + continued fraction [0; 1, 4, 1, 4, 1, 4,...] = 4/2 + 4/5 + 4/(5*29) + 4/(29*169) + 4/(169*985)... - Gary W. Adamson, Dec 21 2007

This is the sequence of Lehmer numbers U_n(sqrt(R),Q) with the parameters R = 4 and Q = -1. It is a strong divisibility sequence, that is, GCD(a(n),a(m)) = a(GCD(n,m)) for all natural numbers n and m. - Peter Bala, May 12 2014

Apparently the same as A152118(n). Georg Fischer, Jul 01 2019

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..199 [shifted by Georg Fischer, Jul 01 2019]

P. Barry, Symmetric Third-Order Recurring Sequences, Chebyshev Polynomials, and Riordan Arrays, JIS 12 (2009) 09.8.6

Eric Weisstein's World of Mathematics, Lehmer Number

Index to divisibility sequences

Index entries for linear recurrences with constant coefficients, signature (0,6,0,-1).

FORMULA

a(2n) = A000129(2n+1), a(2n+1) = A000129(2n+2)/2.

a(n) = 6*a(n-2) - a(n-4). Also:

a(2n) = a(2n-1)+a(2n-2), a(2n+1)=4*a(2n)+a(2n-1).

G.f.: (1+x-x^2)/(1-6*x^2+x^4).

From Peter Bala, May 12 2014: (Start)

For n even, a(n) = (alpha^n - beta^n)/(alpha - beta), and for n odd, a(n) = (alpha^n - beta^n)/(alpha^2 - beta^2), where alpha = 1 + sqrt(2) and beta = 1 - sqrt(2).

a(n) = product {k = 1..floor((n-1)/2)} ( 4 + 4*cos^2(k*Pi/n) ) for n >= 1. (End)

From Gerry Martens, Jul 11 2015: (Start)

Interspersion of 2 sequences [a0(n),a1(n)] for n>0:

a0(n) = ((3-2*sqrt(2))^n*(2+sqrt(2))-(-2+sqrt(2))*(3+2*sqrt(2))^n)/4.

a1(n) = (-(3-2*sqrt(2))^n+(3+2*sqrt(2))^n)/(4*sqrt(2)). (End)

a(n) = ((-(-1 - sqrt(2))^n - 3*(1-sqrt(2))^n + (-1+sqrt(2))^n + 3*(1+sqrt(2))^n))/(8*sqrt(2)). - Colin Barker, Mar 27 2016

MATHEMATICA

Denominator[NestList[(4/(4 + #))&, 0, 60]] (* Vladimir Joseph Stephan Orlovsky, Apr 13 2010 *)

CoefficientList[Series[(x + x^2 - x^3)/(1 - 6 x^2 + x^4), {x, 0, 40}], x] (* Vincenzo Librandi, Dec 10 2013 *)

a0[n_] := ((3-2*Sqrt[2])^n*(2+Sqrt[2])-(-2+Sqrt[2])*(3+2*Sqrt[2])^n)/4 // Simplify

a1[n_] := (-(3-2*Sqrt[2])^n+(3+2*Sqrt[2])^n)/(4*Sqrt[2]) // Simplify

Flatten[MapIndexed[{a0[#], a1[#]} &, Range[20]]] (* Gerry Martens, Jul 11 2015 *)

PROG

(MAGMA) I:=[1, 1, 5, 6]; [n le 4 select I[n] else 6*Self(n-2)-Self(n-4): n in [1..30]]; // Vincenzo Librandi, Dec 10 2013

(PARI) a(n)=([0, 1, 0, 0; 0, 0, 1, 0; 0, 0, 0, 1; -1, 0, 6, 0]^n*[1; 1; 5; 6])[1, 1] \\ Charles R Greathouse IV, Nov 13 2015

(PARI) x='x+O('x^99); concat(0, Vec((1+x-x^2)/(1-6*x^2+x^4))) \\ Altug Alkan, Mar 27 2016

CROSSREFS

Cf. A000129, A010466, A041010. A089499, A136211, A152118.

Sequence in context: A320047 A249221 A127040 * A152118 A041056 A042643

Adjacent sequences:  A041008 A041009 A041010 * A041012 A041013 A041014

KEYWORD

nonn,cofr,easy

AUTHOR

N. J. A. Sloane

EXTENSIONS

Entry improved by Michael Somos

First term 0 in b-file, formulas and programs removed by Georg Fischer, Jul 01 2019

STATUS

approved

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Last modified September 17 22:53 EDT 2019. Contains 327147 sequences. (Running on oeis4.)