OFFSET
2,2
EXAMPLE
a(2)=1 because (2^2-1)*2^2-1 = 11 is prime,
a(3)=5 because (2^3-5)*2^3-5 = 19 is prime,
a(4)=7 because (2^4-7)*2^4-7 = 137 is prime.
PROG
(PARI) a(n)=forstep(k=2^n-1, 1, -1, ispseudoprime((2^n-k)*2^n-k) & return(k)) \\ M. F. Hasler, Jun 17 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Jun 16 2011
STATUS
approved