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A129492 Composite numbers n such that 2^n (mod n) is a power of 2. 6
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 (list; graph; refs; listen; history; text; internal format)
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 member of the sequence 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

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Last modified April 17 08:40 EDT 2021. Contains 343064 sequences. (Running on oeis4.)