A122782 Nonprimes n such that 5^n==5 (mod n).

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

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

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

Mathematica

Select[Range[15000], ! PrimeQ[ # ] && Mod[5^#, # ] == Mod[5, # ] &]
Join[{1, 4}, Select[Range[15000], CompositeQ[#]&&PowerMod[5, #, #]==5&]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 13 2021 *)

Crossrefs

Cf. A005936.

Sequence in context: A310465 A310466 A310467 * A153515 A310468 A310469

Adjacent sequences: A122779 A122780 A122781 * A122783 A122784 A122785

Keyword

nonn

Author

Farideh Firoozbakht, Sep 12 2006

Status

approved