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A262994

Smallest number k>2 such that k*2^n-1 is a prime number.

3, 3, 3, 3, 4, 3, 3, 5, 7, 5, 3, 5, 9, 5, 4, 8, 4, 3, 28, 14, 7, 26, 13, 39, 22, 11, 16, 8, 4, 20, 10, 5, 6, 3, 24, 12, 6, 3, 25, 24, 12, 6, 3, 14, 7, 20, 10, 5, 19, 11, 21, 20, 10, 5, 3, 32, 16, 8, 4, 17, 24, 12, 6, 3, 67, 63, 43, 63, 40, 20, 10, 5, 15, 12, 6, 3

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OFFSET

1,1

COMMENTS

If k=2^j then n+j is a Mersenne exponent.

a(n)=3 if and only if 3*2^n-1 is a prime; that is, n belongs to A002235. - Altug Alkan, Oct 08 2015

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..10000

EXAMPLE

3*2^1-1=5 prime so a(1)=3;

3*2^2-1=11 prime so a(2)=3;

3*2^3-1=23 prime so a(3)=3.