OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..42
FORMULA
EXAMPLE
2047 is a member because 2047 = 2^11 - 1 = 23*89.
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 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Feb 27 2004
EXTENSIONS
One more term from Stefan Steinerberger, Apr 12 2006
STATUS
approved