OFFSET
1,1
LINKS
J. Sondow, Ramanujan Prime in MathWorld [From Jonathan Sondow, Aug 10 2008]
FORMULA
a(n)=Card{x; Pi[x]-Pi[x/2]=n}, where Pi()=A000720().
EXAMPLE
n=5,a(5)=7 because in 7 factorials 5 primes arise with exponent 1: in factorials of 31,32,33,37,41,46; e.g. in 37! these are {19,23,29,31,37}, or 10 numbers x, exist such ones that number of unitary prime divisors of x! equals 2, namely in factorials of {3,5,7,8,9,11,12,13,15,16}.
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Feb 21 2003
EXTENSIONS
Definition corrected by Jonathan Sondow, Aug 10 2008
STATUS
approved