OFFSET
1,3
COMMENTS
#{p prime: p=n+k*(n-1) and 0<k<=a(n)} = A000720(n).
LINKS
Eric Weisstein's World of Mathematics, Dirichlet's Theorem
EXAMPLE
n=7: #{primes<=7} = #{2,3,5,7} = 4, first 4 primes of the form
7+k*(7-1): 13=7+1*6, 19=7+2*6, 31=7+4*6 and 37=7+5*6: a(7)=5.
CROSSREFS
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Nov 18 2003
STATUS
approved