OFFSET
1,2
COMMENTS
This sequence differs from A007053 in the offset and in that the latter includes 1 (since the prime 2 is equal to 2^1, not less than it).
Number of primes with at most n bits.
LINKS
David Baugh, Table of n, a(n) for n = 1..90 (terms n = 1..81 from Charles R Greathouse IV).
Wikipedia, Prime-counting function.
EXAMPLE
a(7) = 31 because prime(31) = 127, prime(32) = 131, and 127 < 2^7 < 131.
MATHEMATICA
PrimePi[2^Range[32] - 1] (* Alonso del Arte, May 05 2019 *)
PROG
(PARI) a(n)=primepi(2^n-1) \\ Charles R Greathouse IV, Dec 04 2014
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Washington Bomfim, Jan 23 2012
STATUS
approved