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A041419 Denominators of continued fraction convergents to sqrt(224). 2
1, 1, 29, 30, 869, 899, 26041, 26940, 780361, 807301, 23384789, 24192090, 700763309, 724955399, 20999514481, 21724469880, 629284671121, 651009141001, 18857540619149, 19508549760150, 565096933903349, 584605483663499, 16934050476481321, 17518655960144820 (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 = 28 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 (0,30,0,-1).

FORMULA

G.f.: -(x^2-x-1) / (x^4-30*x^2+1). - Colin Barker, Nov 17 2013

a(n) = 30*a(n-2) - a(n-4) for n>3. - Vincenzo Librandi, Dec 17 2013

From Peter Bala, May 28 2014: (Start)

The following remarks assume an offset of 1.

Let alpha = sqrt(7) + 2*sqrt(2) and beta = sqrt(7) - 2*sqrt(2) be the roots of the equation x^2 - sqrt(28)*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)} ( 28 + 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) = 28*a(2*n) + a(2*n - 1). (End)

MATHEMATICA

Denominator[Convergents[Sqrt[224], 30]] (* Harvey P. Dale, May 07 2012 *)

CoefficientList[Series[(1 + x - x^2)/(x^4 - 30 x^2 + 1), {x, 0, 30}], x] (* Vincenzo Librandi, Dec 17 2013 *)

PROG

(MAGMA) I:=[1, 1, 29, 30]; [n le 4 select I[n] else 30*Self(n-2)-Self(n-4): n in [1..100]]; // Vincenzo Librandi, Dec 17 2013

CROSSREFS

Cf. A041418, A040209, A002530.

Sequence in context: A042730 A042732 A042734 * A042736 A042737 A042738

Adjacent sequences:  A041416 A041417 A041418 * A041420 A041421 A041422

KEYWORD

nonn,frac,easy

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Colin Barker, Nov 17 2013

STATUS

approved

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Last modified February 19 21:59 EST 2019. Contains 320328 sequences. (Running on oeis4.)