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%I #24 Apr 26 2021 14:43:01
%S 4,26,57,247,502,4083,1073741793,4294967263,8589934558,70368744177617,
%T 4503599627370443,4611686018427387841,18889465931478580854709,
%U 75557863725914323419059,77371252455336267181195177,316912650057057350374175801245
%N Semiprimes of the form 2^k-k-1.
%C Semiprimes in A000295.
%H Hugo Pfoertner, <a href="/A099442/b099442.txt">Table of n, a(n) for n = 1..31</a> (first 19 terms from Vincenzo Librandi)
%e a(4) = 247 because 247 = 13*19 = 2^8-8-1 = 2^A099441(4)-A099441(4)-1.
%t Select[Table[2^n - n - 1, {n, 300}], PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 21 2012 *)
%o (Python)
%o from sympy.ntheory.factor_ import primeomega
%o def ok(n): return primeomega(2**n-n-1) == 2
%o print([2**m-m-1 for m in range(2, 100) if ok(m)]) # _Michael S. Branicky_, Apr 26 2021
%Y Cf. A099439, A099440, A099441.
%K nonn
%O 1,1
%A _Hugo Pfoertner_, Oct 18 2004
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