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

1,1

Comments

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

Links

Table of n, a(n) for n=1..67.

Prog

(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

Crossrefs

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

Sequence in context: A070873 A234037 A225836 * A314580 A314581 A272902

Adjacent sequences: A140506 A140507 A140508 * A140510 A140511 A140512

Keyword

nonn

Author

Vladimir Shevelev, Jun 30 2008

Extensions

Corrected and more terms from Michel Marcus, Oct 25 2018

Status

approved