A074477 - OEIS

A074477

Largest prime factor of 3^n - 1.

2, 2, 13, 5, 11, 13, 1093, 41, 757, 61, 3851, 73, 797161, 1093, 4561, 193, 34511, 757, 363889, 1181, 368089, 3851, 1001523179, 6481, 391151, 797161, 8209, 16493, 20381027, 4561, 4404047, 21523361, 2413941289, 34511, 2664097031, 530713

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OFFSET

1,1

LINKS

Table of n, a(n) for n = 1..690

S. S. Wagstaff, Jr., The Cunningham Project

FORMULA

a(n) = A006530(A024023(n)). - Michel Marcus, Jul 18 2015

EXAMPLE

3^7 - 1 = 2186 = 2*1093, so a(7) = 1093.

MAPLE

A074477 := proc(n)

A006530( 3^n-1) ;

end proc: # R. J. Mathar, Jul 18 2015

# alternative:

a:= n-> max(seq(i[1], i=ifactors(3^n-1)[2])):

seq(a(n), n=1..40); # Alois P. Heinz, Jul 18 2015

MATHEMATICA

Table[FactorInteger[3^n - 1] [[-1, 1]], {n, 40}] (* Vincenzo Librandi, Aug 23 2013 *)

PROG

(PARI) for(n=1, 40, v=factor(3^n-1); print1(v[matsize(v)[1], 1], ", "))

(Magma) [Maximum(PrimeDivisors(3^n-1)): n in [1..40]]; // Vincenzo Librandi, Aug 23 2013

CROSSREFS

Cf. A006530 (largest prime factor), A024023 (3^n-1).

Cf. A074476 (largest prime factor of 3^n + 1), A005420 (largest prime factor of 2^n - 1), A074479 (largest prime factor of 5^n - 1).

Sequence in context: A130718 A268528 A173331 * A340587 A347080 A141575

Adjacent sequences: A074474 A074475 A074476 * A074478 A074479 A074480

KEYWORD

nonn

AUTHOR

Rick L. Shepherd, Aug 23 2002

EXTENSIONS

Terms to a(100) in b-file from Vincenzo Librandi, Aug 23 2013

a(101)-a(660) in b-file from Amiram Eldar, Feb 01 2020

a(661)-a(690) in b-file from Max Alekseyev, May 22 2022

STATUS

approved