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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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified September 25 02:38 EDT 2023. Contains 365582 sequences. (Running on oeis4.)