OFFSET
1,1
COMMENTS
For a composite n, number 2^n - 1 has at most 2 prime factors only if n = p^2, where p is prime from the intersection of A000043 and A156585. The only known such primes are 2, 3, 7. - Max Alekseyev, Apr 23 2019
a(54) >= 1277. - Max Alekseyev, Apr 23 2019
REFERENCES
J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
S. S. Wagstaff, Jr., The Cunningham Project
MATHEMATICA
Select[Prime[Range[100]], PrimeOmega[2^#-1]<3&] (* Harvey P. Dale, Nov 11 2011 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Sean A. Irvine, May 04 2017
Edited by Max Alekseyev, Apr 23 2019
STATUS
approved