A138859 Smallest prime factor of Fibonacci(3n)-1, i.e., A020639(A000071(3n)).

7, 3, 11, 3, 3, 5, 199, 13, 13, 3, 37, 3, 3, 89, 5, 11, 7, 3, 59, 3, 3, 11, 11, 37, 37, 3, 370248451, 3, 3, 139, 13, 5, 5, 3, 5, 3, 3, 59, 709, 557, 127, 3, 11, 3, 3, 5, 9375829, 73, 7, 3, 29, 3, 3, 2789, 5, 11, 47, 3, 13, 3, 3, 11, 11, 13, 7, 3, 809, 3, 3, 953, 47927441, 5, 5

Offset: 2

Author

M. F. Hasler, Apr 05 2008

Keywords

• nonn

Comments

F(n)-1 is even if n is not a multiple of 3.

Links

• Amiram Eldar, Table of n, a(n) for n = 2..934 (terms 2..379 from Chai Wah Wu)

Crossrefs

Cf. A000071, A014445, A020639.

Programs

Mathematica

Table[FactorInteger[Fibonacci[3n]-1][[1,1]],{n,2,80}] (* Harvey P. Dale, Aug 08 2017 *)

PARI

A138859(n)=factor(fibonacci(3*n)-1)[1,1] /* A138859 is defined only for n>1 ! */

Formula

a(n) = A020639(A000071(3n)).