A129492 Composite numbers k such that 2^k mod k is a power of 2.

6, 9, 10, 12, 14, 15, 20, 21, 22, 24, 26, 28, 30, 33, 34, 38, 39, 40, 44, 46, 48, 51, 52, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 72, 74, 76, 78, 80, 82, 84, 85, 86, 87, 90, 92, 93, 94, 96, 102, 106, 111, 112, 114, 116, 118, 120, 122, 123, 124, 126, 129, 132, 133, 134, 138

Offset

1,1

Comments

Complement to composite numbers: 4, 8, 16, 18, 25, 27, 32, 35, 36, 42, 45, 49, 50, 54, 55, 64, 70, 75, 77, 81, 88, 91, 95, 98, 99, ....

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

15 is a term since 2^15 mod 15 = 8.

Maple

filter:= proc(n) local k;
  if isprime(n) then return false fi;
  k:= 2 &^ n mod n;
  k > 1 and k = 2^padic:-ordp(k, 2)
end proc:
select(filter, [$4..1000]); # Robert Israel, Dec 03 2019

Mathematica

Select[Range@ 141, IntegerQ@ Log[2, PowerMod[2, #, # ]] &]

Prog

(Magma) [k:k in [2..150]| not IsPrime(k)  and  not IsZero(a)  and (PrimeDivisors(a) eq [2]) where a is 2^k mod k ]; // Marius A. Burtea, Dec 04 2019

Crossrefs

Cf. A036236, A129493, A129494, A129495, A129496, A129497.

Contains A001567.

Sequence in context: A325170 A325229 A036347 * A241913 A053869 A085275

Adjacent sequences: A129489 A129490 A129491 * A129493 A129494 A129495

Keyword

easy,nonn

Author

Robert G. Wilson v, Apr 17 2007

Status

approved