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A041311 Denominators of continued fraction convergents to sqrt(168). 2
1, 1, 25, 26, 649, 675, 16849, 17524, 437425, 454949, 11356201, 11811150, 294823801, 306634951, 7654062625, 7960697576, 198710804449, 206671502025, 5158826853049, 5365498355074, 133930787374825, 139296285729899, 3477041644892401, 3616337930622300 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

The following remarks assume an offset of 1. This is the sequence of Lehmer numbers U_n(sqrt(R),Q) for the parameters R = 24 and Q = -1; it is a strong divisibility sequence, that is, gcd(a(n),a(m)) = a(gcd(n,m)) for all positive integers n and m. Consequently, this is a divisibility sequence: if n divides m then a(n) divides a(m). - Peter Bala, May 28 2014

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..200

Eric W. Weisstein, MathWorld: Lehmer Number

Index entries for linear recurrences with constant coefficients, signature (0,26,0,-1).

FORMULA

G.f.: -(x^2-x-1) / (x^4-26*x^2+1). - Colin Barker, Nov 15 2013

a(n) = 26*a(n-2) - a(n-4). - Vincenzo Librandi, Dec 15 2013

From Peter Bala, May 28 2014: (Start)

The following remarks assume an offset of 1.

Let alpha = sqrt(6) + sqrt(7) and beta = sqrt(6) - sqrt(7) be the roots of the equation x^2 - sqrt(24)*x - 1 = 0. Then a(n) = (alpha^n - beta^n)/(alpha - beta) for n odd, while a(n) = (alpha^n - beta^n)/(alpha^2 - beta^2) for n even.

a(n) = Product_{k = 1..floor((n-1)/2)} ( 24 + 4*cos^2(k*Pi/n) ).

Recurrence equations: a(0) = 0, a(1) = 1 and for n >= 1, a(2*n) = a(2*n - 1) + a(2*n - 2) and a(2*n + 1) = 24*a(2*n) + a(2*n - 1). (End)

MATHEMATICA

Denominator[Convergents[Sqrt[168], 30]] (* Vincenzo Librandi, Dec 15 2013 *)

PROG

(MAGMA) I:=[1, 1, 25, 26]; [n le 4 select I[n] else 26*Self(n-2)-Self(n-4): n in [1..40]]; // Vincenzo Librandi, Dec 15 2013

CROSSREFS

Cf. A041310, A040155, A002530.

Sequence in context: A042290 A042292 A042294 * A042296 A042297 A042298

Adjacent sequences:  A041308 A041309 A041310 * A041312 A041313 A041314

KEYWORD

nonn,frac,easy

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Colin Barker, Nov 15 2013

STATUS

approved

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Last modified April 16 17:01 EDT 2021. Contains 343050 sequences. (Running on oeis4.)