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A122782 Nonprimes n such that 5^n==5 (mod n). 3
1, 4, 10, 15, 20, 65, 124, 190, 217, 310, 435, 561, 781, 1105, 1541, 1729, 1891, 2465, 2821, 3565, 3820, 4123, 4495, 5461, 5611, 5662, 5731, 6601, 6735, 7449, 7813, 8029, 8290, 8911, 9881, 10585, 11041, 11476, 12801, 13021, 13333, 13981, 14981 (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=3 or q is of the form 10k+1. 15,1891,88831,146611,218791,721801,873181,... are such terms.

LINKS

Table of n, a(n) for n=1..43.

MATHEMATICA

Select[Range[15000], ! PrimeQ[ # ] && Mod[5^#, # ] == Mod[5, # ] &]

CROSSREFS

Cf. A005936.

Sequence in context: A106668 A063295 A189399 * A153515 A005662 A259262

Adjacent sequences:  A122779 A122780 A122781 * A122783 A122784 A122785

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Sep 12 2006

STATUS

approved

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Last modified March 30 16:23 EDT 2017. Contains 284302 sequences.