A247228 - OEIS

A247228

Prime numbers p where 2^p-1 is composite that set records for smallest prime factor of 2^p-1: the smallest prime factor of 2^p-1 is larger than the smallest prime factor of 2^q-1 for all primes q > p.

11, 23, 29, 41, 59, 67, 101, 137, 149, 293, 523

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OFFSET

1,1

COMMENTS

Smallest prime factor of 2^523 - 1 = 160188778313202118610543685368878688932828701136501444932217468039063 (69 digits).

a(12) is probably 1237; 2^1237 - 1 has a 70-digit prime factor and a 303-digit cofactor. - Charles R Greathouse IV, Nov 27 2014

LINKS

Table of n, a(n) for n=1..11.

EXAMPLE

12 is not in the sequence because 12 is not prime.

13 is not in the sequence because 2^13-1 is prime.

37 is not in the sequence because while the smallest prime factor of 2^29-1 is 233, the smallest prime factor of 2^37-1 is only 223.

CROSSREFS

Cf. A054723.

Sequence in context: A091939 A072185 A105898 * A243461 A259560 A136001

Adjacent sequences: A247225 A247226 A247227 * A247229 A247230 A247231

KEYWORD

nonn,hard,more

AUTHOR

J. Lowell, Nov 27 2014

STATUS

approved