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A122785 Nonprimes n such that 8^n==8 (mod n). 3
1, 4, 8, 9, 14, 21, 28, 341, 481, 511, 561, 585, 645, 651, 861, 949, 1001, 1016, 1105, 1106, 1281, 1288, 1365, 1387, 1417, 1541, 1649, 1661, 1729, 1736, 1785, 1905, 2044, 2047, 2169, 2465, 2501, 2696, 2701, 2821, 3145, 3171, 3201, 3277, 3605, 3641, 4005 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Theorem: If both numbers q and 2q-1 are primes and n=q*(2q-1) then 8^n==8 (mod n) (n is in the sequence) iff q is of the form 4k+1. 2701,18721,49141,104653,226801,665281,721801,... are such terms.

LINKS

Table of n, a(n) for n=1..47.

MATHEMATICA

Select[Range[6000], ! PrimeQ[ # ] && Mod[8^#, # ] == Mod[8, # ] &]

CROSSREFS

Cf. A020137, A001567.

Sequence in context: A085711 A182491 A281686 * A280450 A137055 A078177

Adjacent sequences:  A122782 A122783 A122784 * A122786 A122787 A122788

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Sep 12 2006

STATUS

approved

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Last modified May 20 03:10 EDT 2019. Contains 323412 sequences. (Running on oeis4.)