A122781 Nonprimes n such that 4^n==4 (mod n).

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

Offset: 1

Author

Farideh Firoozbakht, Sep 12 2006

Keywords

• easy
• nonn

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

Crossrefs

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

Cf. A005382.

Programs

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 *)