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A122784 Nonprimes n such that 7^n == 7 (mod n). 3
1, 6, 14, 21, 25, 42, 105, 133, 231, 301, 325, 525, 561, 703, 817, 1105, 1729, 1825, 2101, 2353, 2465, 2821, 3277, 3325, 3486, 3913, 4011, 4525, 4825, 5565, 5719, 5901, 6601, 6697, 7525, 8321, 8911, 9331, 10225, 10325, 10585, 10621, 11041, 11521 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Theorem: If both numbers q and 2q-1 are primes then n=q*(2q-1) is in the sequence iff q=2 or mod(q,14) is in the set {1, 5, 13}. 6,703,18721,38503,88831,104653,146611,188191,... are such terms.

LINKS

Table of n, a(n) for n=1..44.

MATHEMATICA

Select[Range[20000], ! PrimeQ[#] && PowerMod[7, #, #] == Mod[7, #] &]

With[{nn=12000}, Select[Complement[Range[nn], Prime[Range[PrimePi[ nn]]]], PowerMod[7, #, #]==Mod[7, #]&]] (* Harvey P. Dale, Jul 12 2012 *)

CROSSREFS

Cf. A005938.

Sequence in context: A046712 A162823 A020171 * A244175 A063299 A184924

Adjacent sequences:  A122781 A122782 A122783 * A122785 A122786 A122787

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Sep 12 2006

STATUS

approved

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Last modified May 24 15:25 EDT 2019. Contains 323532 sequences. (Running on oeis4.)