A140509 Numbers k such that 5^k-1 contains a divisor which is an overpseudoprime to base 5.

5, 9, 10, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80

Offset: 1

Author

Vladimir Shevelev, Jun 30 2008

Keywords

• nonn

Comments

An odd prime p is in the sequence iff p is not in A004061.

Crossrefs

Cf. A141390, A020231, A141232, A141350, A003463, A004061.

Programs

PARI

isokd(n) = (n>5) && !isprime(n) && (gcd(n,5)==1) && (znorder(Mod(5,n)) * (sumdiv(n, d, eulerphi(d)/znorder(Mod(5, d))) - 1) + 1 == n); \\ A141390
isok(n) = {fordiv (5^n-1, dd, if (isokd(dd), return (1));); return (0);} \\ Michel Marcus, Oct 25 2018

Extensions

Corrected and more terms from Michel Marcus, Oct 25 2018