OFFSET
0,1
LINKS
Sean A. Irvine, Table of n, a(n) for n = 0..312
Sam Wagstaff, Cunningham Project, Factorizations of 10^n+1, n<=330
EXAMPLE
1001 = 7 * 11 * 13 and has 8 divisors, but only {1, 7, 13, 91} are relatively prime to 11 and 101, so a(3) = 4. - Bernard Schott, May 27 2019
MATHEMATICA
a = {1}; Do[ d = Divisors[ 10^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[ 10^n + 1 ] ][ [ 1 ] ] ] ] ], {n, 0, 46} ]
PROG
(PARI) a(n) = if (n==0, 2, sumdiv(10^n+1, d, vecsum(vector(n-1, k, gcd(d, 10^k+1) == 1)) == n-1)); \\ Michel Marcus, Jun 24 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Sep 10 2001
EXTENSIONS
More terms from Michel Marcus, Jul 02 2018
a(73)-a(82) from Robert Price, May 26 2019
a(73) corrected by Sean A. Irvine, May 26 2019
STATUS
approved