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A122781 Nonprimes n such that 4^n==4 (mod n). 7
1, 4, 6, 12, 15, 28, 66, 85, 91, 186, 276, 341, 435, 451, 532, 561, 645, 703, 946, 1068, 1105, 1247, 1271, 1387, 1581, 1695, 1729, 1891, 1905, 2044, 2046, 2047, 2071, 2465, 2701, 2821, 2926, 3133, 3277, 3367, 3683, 4033, 4369, 4371, 4681, 4795 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If both numbers q and 2q-1 are prime, then q*(2q-1) is in the sequence. So, A005382(n)*(2*A005382(n)-1) = A129521(n) form a subsequence.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..500

MAPLE

for n from 1 to 5000 do if 4^n mod n = 4 mod n and not isprime(n) then print(n) fi od; # Gary Detlefs, May 14 2012

MATHEMATICA

Select[Range[4800], ! PrimeQ[ # ] && Mod[4^#, # ] == Mod[4, # ] &]

Join[{1, 4}, Select[Range[5000], !PrimeQ[#]&&PowerMod[4, #, #]==4&]] (* Harvey P. Dale, Apr 09 2018 *)

CROSSREFS

Contains A020136, A001567, A006935 (except n=2), and A129521 as subsequences.

Cf. A005382.

Sequence in context: A247632 A104236 A265225 * A153355 A310597 A310598

Adjacent sequences:  A122778 A122779 A122780 * A122782 A122783 A122784

KEYWORD

easy,nonn

AUTHOR

Farideh Firoozbakht, Sep 12 2006

STATUS

approved

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Last modified August 24 14:53 EDT 2019. Contains 326295 sequences. (Running on oeis4.)