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

4, 26, 57, 247, 502, 4083, 1073741793, 4294967263, 8589934558, 70368744177617, 4503599627370443, 4611686018427387841, 18889465931478580854709, 75557863725914323419059, 77371252455336267181195177, 316912650057057350374175801245

Offset: 1

Author

Hugo Pfoertner, Oct 18 2004

Keywords

• nonn

Comments

Semiprimes in A000295.

Examples

			a(4) = 247 because 247 = 13*19 = 2^8-8-1 = 2^A099441(4)-A099441(4)-1.
		

Links

• Hugo Pfoertner, Table of n, a(n) for n = 1..31 (first 19 terms from Vincenzo Librandi)

Crossrefs

Cf. A099439, A099440, A099441.

Programs

Mathematica

Select[Table[2^n - n - 1, {n, 300}], PrimeOmega[#] == 2&] (* Vincenzo Librandi, Sep 21 2012 *)

Python

from sympy.ntheory.factor_ import primeomega
def ok(n): return primeomega(2**n-n-1) == 2
print([2**m-m-1 for m in range(2, 100) if ok(m)]) # Michael S. Branicky, Apr 26 2021