A064124 Number of divisors of 7^n - 1 that are relatively prime to 7^m - 1 for all 0 < m < n.

4, 1, 2, 3, 2, 2, 4, 2, 4, 4, 4, 4, 2, 4, 4, 4, 4, 2, 4, 4, 2, 4, 8, 8, 4, 4, 16, 2, 8, 2, 8, 4, 8, 2, 4, 2, 8, 4, 8, 4, 8, 4, 4, 8, 4, 2, 4, 2, 16, 2, 16, 4, 8, 2, 2, 16, 8, 2, 16, 4, 16, 8, 2, 8, 32, 4, 16, 8, 8, 16, 4, 8, 32, 4, 4, 2, 8, 8, 4, 8, 16, 16, 128, 2, 8

Offset

1,1

Links

Table of n, a(n) for n=1..85.

Sam Wagstaff, Cunningham Project, Factorizations of 7^n-1, n odd, n<300

Mathematica

a = {1}; Do[ d = Divisors[ 7^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[ 7^n - 1 ] ][ [ 1 ] ] ] ] ], {n, 1, 70} ]

Prog

(PARI) a(n) = sumdiv(7^n-1, d, vecsum(vector(n-1, k, gcd(d, 7^k-1) == 1)) == n-1); \\ Michel Marcus, Jun 23 2018

Crossrefs

Cf. A063982.

Sequence in context: A378626 A321091 A375576 * A106654 A252094 A196138

Adjacent sequences: A064121 A064122 A064123 * A064125 A064126 A064127

Keyword

nonn

Author

Robert G. Wilson v, Sep 10 2001

Extensions

a(71)-a(85) from Giovanni Resta, Jun 26 2018

Status

approved