A092561 "Mersenne" semiprimes, semiprimes of the form 2^k-1.

15, 511, 2047, 8388607, 137438953471, 2199023255551, 562949953421311, 576460752303423487, 147573952589676412927, 9671406556917033397649407, 158456325028528675187087900671, 2535301200456458802993406410751

Offset: 1

Author

Zak Seidov, Feb 27 2004

Keywords

• nonn

Examples

			2047 is a member because 2047 = 2^11 - 1 = 23*89.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..42

Crossrefs

Corresponding k: A085724.

Cf. A000225, A092558, A092559, A092562.

Programs

Mathematica

a := Select[Range[2,120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* Stefan Steinerberger, Apr 12 2006 *)
Select[2^Range[0,110]-1, PrimeOmega[#] == 2&] (* Harvey P. Dale, Feb 22 2013 *)

Formula

a(n) = A000225(A085724(n)). - Amiram Eldar, Jun 18 2022

Extensions

One more term from Stefan Steinerberger, Apr 12 2006