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 A099442 Semiprimes of the form 2^k-k-1. 4

%I

%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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Last modified September 20 20:31 EDT 2021. Contains 347590 sequences. (Running on oeis4.)