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A060385 Largest prime factor of n-th Fibonacci number. 8
2, 3, 5, 2, 13, 7, 17, 11, 89, 3, 233, 29, 61, 47, 1597, 19, 113, 41, 421, 199, 28657, 23, 3001, 521, 109, 281, 514229, 61, 2417, 2207, 19801, 3571, 141961, 107, 2221, 9349, 135721, 2161, 59369, 421, 433494437, 307, 109441, 28657, 2971215073, 1103 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,1

COMMENTS

For n > 12, Fibonacci(n) is divisible by a primitive prime factor (one not dividing Fibonacci(1), ..., Fibonacci(n-1)). But all primes up to n-2 divide smaller Fibonacci numbers, see A001602, so a(n) >= n-1 for n > 12. This strengthens a more general theorem of Bravo and Luca. - Charles R Greathouse IV, Feb 01 2013

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 3..1000

U. Alfred, On the form of primitive factors of Fibonacci numbers, Fibonacci Quarterly 1:1 (1963), pp. 43-45.

Jhon J. Bravo and Florian Luca, On the largest prime factor of the k-Fibonacci numbers, arXiv:1210.4101 [math.NT], 2012.

D. E. Daykin and L. A. G. Dresel, Factorization of Fibonacci numbers, Fibonacci Quarterly 8:1 (1970), pp. 23-30.

FORMULA

a(n) >= n - 1 for n > 12, see comments. It is not hard to show that a(n) > 1000 for n > 88. Similarly a(n) > 20641 for n > 120. - Charles R Greathouse IV, Feb 01 2013

EXAMPLE

F(82) = 2789 * 59369 * 370248451, so a(82) = 370248451.

MAPLE

with(numtheory): with(combinat): seq(max(op(factorset(fibonacci(n)))), n=3..48); # Paolo P. Lava, Dec 10 2018

MATHEMATICA

Table[First[Last[FactorInteger[Fibonacci[n]]]], {n, 3, 100}] (* Vladimir Joseph Stephan Orlovsky, Feb 03 2012 *)

PROG

(PARI) a(n)=my(f=factor(fibonacci(n))[, 1]); f[#f] \\ Charles R Greathouse IV, Feb 01 2013

(MAGMA) [Maximum(PrimeDivisors(Fibonacci(n))): n in [3..50]]; // Vincenzo Librandi, Dec 25 2016

CROSSREFS

Cf. A000045, A022307, A001605, A060319, A060320, A051694, A193615.

Sequence in context: A060383 A139044 A060442 * A080648 A113195 A069110

Adjacent sequences:  A060382 A060383 A060384 * A060386 A060387 A060388

KEYWORD

nonn

AUTHOR

Labos Elemer, Apr 03 2001

STATUS

approved

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Last modified October 14 12:30 EDT 2019. Contains 328006 sequences. (Running on oeis4.)