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A140509 Numbers k such that 5^k-1 contains a divisor which is an overpseudoprime to base 5. 0
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified October 21 19:16 EDT 2021. Contains 348155 sequences. (Running on oeis4.)