OFFSET
1,1
LINKS
Jonathan Bayless, Dominic Klyve, and Tomás Oliveira e Silva, New Bounds and Computations on Prime-Indexed Primes, Integers, Vol.13, July 10, 2013.
FORMULA
Let pi(n) = the number of primes <= n. Then a(n) = pi(pi(n))
MATHEMATICA
Table[ Nest[ PrimePi, 10^n, 2], {n, 13}] (* Robert G. Wilson v, Jul 01 2004 *)
PROG
(PARI) nestpi(n, m) = { for(x=1, n, z=10^x; for(y=1, m, z=abs(primepi(z))); print1(floor(z)", ")) }
CROSSREFS
KEYWORD
nonn
AUTHOR
Cino Hilliard, Jun 30 2004
EXTENSIONS
a(10) - a(15) from Robert G. Wilson v, Jul 01 2004
a(16) - a(19) from Henri Lifchitz, Nov 11 2012
a(20) - a(24) from Bayless, Table 1, p.7, Jonathan Vos Post, Aug 09 2013
STATUS
approved