OFFSET
1,3
COMMENTS
Primes 1 + b^256 are a form of generalized Fermat primes.
It is conjectured that a(n) is asymptotic to 0.0290422*li(10^n)
LINKS
Yves Gallot, How many prime numbers appear in a sequence ?
EXAMPLE
a(3) = 4 because the generalized Fermat numbers F_8(b) where b<10^3 are prime only for b: 278, 614, 892, 898.
PROG
(PARI) a(n) = sum(b=1, 10^n/2-1, isprime((2*b)^256+1))
CROSSREFS
KEYWORD
nonn
AUTHOR
Henryk Dabrowski, Aug 21 2012
STATUS
approved