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A049093 Numbers n such that 2^n - 1 is squarefree. 9
1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 19, 22, 23, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 37, 38, 39, 41, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 55, 56, 57, 58, 59, 61, 62, 64, 65, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77, 79, 81, 82, 83, 85, 86, 87, 88, 89, 91, 92 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that gcd(n, 2^n - 1) = 1 and n is not a multiple of A002326((q - 1)/2), where q is a Wieferich prime A001220. - Thomas Ordowski, Nov 21 2015

If n is in the sequence, then so are all divisors of n. - Robert Israel, Nov 23 2015

LINKS

Max Alekseyev, Table of n, a(n) for n = 1..910

EXAMPLE

a(7) = 8 because 2^8 - 1 = 255 = 3 * 5 * 17 is squarefree.

MAPLE

N:= 400: # to get all terms <= N

# This relies on the fact that the first N+1 members of A000225 have all been factored

# without any further Wieferich primes being found.

V:= Vector(N, 1):

V[364 * [$1..N/364]]:= 0:

V[1755 * [$1..N/1755]]:= 0:

for n from 2 to N do

if V[n] = 0 then next fi;

if igcd(n, 2 &^n - 1 mod n) > 1 then

  V[n * [$1..N/n]]:= 0

fi;

od:

select(t -> V[t] = 1, [$1..N]); # Robert Israel, Nov 23 2015

MATHEMATICA

Select[Range@ 92, SquareFreeQ[2^# - 1] &] (* Michael De Vlieger, Nov 21 2015 *)

PROG

(PARI) isok(n) = issquarefree(2^n - 1); \\ Michel Marcus, Dec 19 2013

(MAGMA) [n: n in [1..100] | IsSquarefree(2^n-1)]; // Vincenzo Librandi, Nov 22 2015

CROSSREFS

Complement of A049094.

Sequence in context: A136447 A005100 A051772 * A098901 A098767 A333559

Adjacent sequences:  A049090 A049091 A049092 * A049094 A049095 A049096

KEYWORD

nonn

AUTHOR

Labos Elemer

EXTENSIONS

Terms a(73)-a(910) in b-file from Max Alekseyev, Nov 15 2014, Sep 28 2015

STATUS

approved

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Last modified September 21 07:06 EDT 2020. Contains 337267 sequences. (Running on oeis4.)