OFFSET
1,1
EXAMPLE
2*(2^2-1)*(2^3-1)*(2^5-1) - 1 = 1301, which is prime, so k(3)=2.
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Pierre CAMI, Oct 17 2004
EXTENSIONS
a(15)-a(20) from Donovan Johnson, Mar 23 2008
STATUS
approved