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%I M0653 #25 Apr 23 2019 22:17:29
%S 2,3,5,7,11,13,17,19,23,31,37,41,59,61,67,83,89,97,101,103,107,109,
%T 127,131,137,139,149,167,197,199,227,241,269,271,281,293,347,373,379,
%U 421,457,487,521,523,607,727,809,881,971,983,997,1061,1063
%N Primes p such that 2^p - 1 has at most 2 prime factors.
%C 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
%C a(54) >= 1277. - _Max Alekseyev_, Apr 23 2019
%D J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H J. Brillhart et al., <a href="http://dx.doi.org/10.1090/conm/022">Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>
%t Select[Prime[Range[100]],PrimeOmega[2^#-1]<3&] (* _Harvey P. Dale_, Nov 11 2011 *)
%Y Cf. A000043 (a subsequence), A001348, A088863.
%K nonn
%O 1,1
%A _N. J. A. Sloane_.
%E More terms from _Sean A. Irvine_, May 04 2017
%E Edited by _Max Alekseyev_, Apr 23 2019
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