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A260626 a(n) = gcd(m, 2^m-1) where m is the n-th nonprime positive integer. 1
1, 1, 3, 1, 1, 1, 3, 1, 1, 1, 9, 5, 7, 1, 3, 1, 1, 1, 1, 3, 1, 1, 1, 1, 9, 1, 1, 5, 21, 1, 1, 1, 3, 1, 1, 1, 1, 27, 1, 1, 1, 1, 15, 1, 7, 1, 1, 3, 1, 1, 1, 9, 1, 1, 1, 1, 3, 5, 1, 1, 21, 1, 1, 1, 1, 9, 1, 1, 1, 1, 1, 3, 1, 1, 25, 3, 1, 7, 1, 27, 11, 1, 1, 3, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

2^m - 1 is a nonprime number if m is a nonprime number.

LINKS

Michel Lagneau, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = gcd(A018252(n), 2^A018252(n)-1).

a(n) = A014491(A018252(n)). - Michel Marcus, Nov 01 2015

MAPLE

seq(`if`(isprime(m), NULL, igcd(m, 2^m-1)), m=1..150);

MATHEMATICA

GCDnonPrime[n_Integer]:=GCD[2^FixedPoint[n+PrimePi@#&, n+PrimePi@n]-1, FixedPoint[n+PrimePi@#&, n+PrimePi@n]]; Array[GCDnonPrime, 120]

GCD[#, 2^#-1]&/@Select[Range[200], !PrimeQ[#]&] (* Harvey P. Dale, Aug 25 2019 *)

PROG

(PARI) for(n=1, 1e3, if(!isprime(n), print1(gcd(n, 2^n-1)", "))) \\ Altug Alkan, Nov 01 2015

CROSSREFS

Cf. A014491, A018252.

Sequence in context: A306346 A060901 A087612 * A155828 A226203 A327791

Adjacent sequences:  A260623 A260624 A260625 * A260627 A260628 A260629

KEYWORD

nonn,easy

AUTHOR

Michel Lagneau, Oct 31 2015

STATUS

approved

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Last modified December 3 16:20 EST 2020. Contains 338906 sequences. (Running on oeis4.)