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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.
0
11, 23, 29, 41, 59, 67, 101, 137, 149, 293, 523
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
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
KEYWORD
nonn,hard,more
AUTHOR
J. Lowell, Nov 27 2014
STATUS
approved