A064128 Number of divisors of 6^n - 1 that are relatively prime to 6^m - 1 for all 0 < m < n.

2, 2, 2, 2, 2, 2, 2, 2, 4, 4, 4, 4, 4, 4, 4, 4, 16, 2, 4, 4, 2, 2, 16, 2, 4, 8, 4, 4, 2, 2, 4, 8, 4, 4, 8, 8, 32, 4, 4, 4, 4, 2, 16, 8, 8, 4, 8, 4, 8, 2, 8, 8, 8, 4, 4, 16, 8, 8, 8, 16, 8, 2, 16, 8, 16, 8, 4, 4, 4, 32, 2, 8, 16, 8, 8, 4, 4, 16, 8, 8, 4, 8, 128

Offset

1,1

Links

Table of n, a(n) for n=1..83.

Sam Wagstaff, Cunningham Project, Factorizations of 6^n-1, n odd, n<330

Mathematica

a = {1}; Do[ d = Divisors[ 6^n - 1 ]; l = Length[ d ]; c = 0; k = 1; While[ k < l + 1, If[ Union[ GCD[ a, d[ [ k ] ] ] ] == {1}, c++ ]; k++ ]; Print[ c ]; a = Union[ Flatten[ Append[ a, Transpose[ FactorInteger[ 6^n - 1 ] ][ [ 1 ] ] ] ] ], {n, 1, 66} ]

Prog

(PARI) a(n) = sumdiv(6^n-1, d, vecsum(vector(n-1, k, gcd(d, 6^k-1) == 1)) == n-1); \\ Michel Marcus, Jun 23 2018

Crossrefs

Cf. A063982.

Sequence in context: A189819 A145992 A045818 * A248774 A104274 A008857

Adjacent sequences: A064125 A064126 A064127 * A064129 A064130 A064131

Keyword

nonn

Author

Robert G. Wilson v, Sep 10 2001

Extensions

a(67)-a(83) from Jon E. Schoenfield, Jun 23 2018

Status

approved