OFFSET
0,4
EXAMPLE
a(5)=2 because 2^5+1-2=31 that is a prime.
PROG
(PARI) a(n) = my(k = 0); while (! isprime(2^n+1-k), k++); k; \\ Michel Marcus, Sep 28 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Erich Friedman
STATUS
approved