OFFSET
1,1
COMMENTS
Semiprimes in A000295.
LINKS
Hugo Pfoertner, Table of n, a(n) for n = 1..31 (first 19 terms from Vincenzo Librandi)
MATHEMATICA
Select[Table[2^n - n - 1, {n, 300}], PrimeOmega[#] == 2&] (* Vincenzo Librandi, Sep 21 2012 *)
PROG
(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
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Oct 18 2004
STATUS
approved