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%I #20 Jun 18 2022 04:04:19
%S 15,511,2047,8388607,137438953471,2199023255551,562949953421311,
%T 576460752303423487,147573952589676412927,9671406556917033397649407,
%U 158456325028528675187087900671,2535301200456458802993406410751
%N "Mersenne" semiprimes, semiprimes of the form 2^k-1.
%H Amiram Eldar, <a href="/A092561/b092561.txt">Table of n, a(n) for n = 1..42</a>
%F a(n) = A000225(A085724(n)). - _Amiram Eldar_, Jun 18 2022
%e 2047 is a member because 2047 = 2^11 - 1 = 23*89.
%t a := Select[Range[2,120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* _Stefan Steinerberger_, Apr 12 2006 *)
%t Select[2^Range[0,110]-1, PrimeOmega[#] == 2&] (* _Harvey P. Dale_, Feb 22 2013 *)
%Y Corresponding k: A085724.
%Y Cf. A000225, A092558, A092559, A092562.
%K nonn
%O 1,1
%A _Zak Seidov_, Feb 27 2004
%E One more term from _Stefan Steinerberger_, Apr 12 2006
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