A132090 a(n) = pi(pi(n)), where pi = A000720.

0, 0, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9

Offset: 1

Author

Jonathan Vos Post, Sep 03 2007

Keywords

• nonn

Comments

a(n) = a(n-1) + 1 if n is in A006450, otherwise a(n) = a(n-1). - Robert Israel, Jan 20 2016

Links

• R. Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000720, A006450, A000040, A000720, A137588.

Programs

Haskell

a132090 = a000720 . a000720  -- Reinhard Zumkeller, Jun 23 2015

Maple

map(numtheory:-pi@@2,[$1..100]); # Robert Israel, Jan 20 2016

Mathematica

PrimePi@ PrimePi@ Range@ 105 (* Robert G. Wilson v, Jan 20 2016 *)

PARI

a(n)=primepi(primepi(n)) \\ Charles R Greathouse IV, Aug 12 2014

Formula

a(n) ~ n/log^2 n. - Charles R Greathouse IV, Aug 12 2014