A122784 - OEIS

A122784

Nonprimes k such that 7^k == 7 (mod k).

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

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OFFSET

1,2

COMMENTS

Theorem: If both numbers q and 2q-1 are primes then 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

Amiram Eldar, Table of n, a(n) for n = 1..10000

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, A121014, A122780, A122781, A122782, A122783, A122785, A122786.

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