OFFSET
1,6
EXAMPLE
1*(2^2-1)*(1*(2^2-1)-1)-1=5 prime, 2^2-1 first Mersenne prime, a(1)=1;
1*(2^3-1)*(1*(2^3-1)-1)-1=41 prime, 2^3-1 second Mersenne prime, a(2)=1.
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Pierre CAMI, Feb 22 2008
STATUS
approved