A203967 - OEIS

A203967

The number of positive integers <= n that have a prime number of divisors.

0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 8, 9, 10, 10, 11, 11, 11, 11, 12, 12, 13, 13, 13, 13, 14, 14, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 18, 18, 18, 18, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 21, 21, 22, 22, 23, 23, 23, 24, 24, 24, 25, 25, 25

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,3

LINKS

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

FORMULA

a(n) = a(n-1) + A010055(n) * A010051(A100995(n)+1). - Reinhard Zumkeller, Jun 06 2013

MATHEMATICA

Table[Total[Table[PrimePi[m^(1/n)], {n, Table[Prime[n]-1, {n, 1, 20}]}]], {m, 1, 100}]

tot = 0; Table[If[PrimeQ[DivisorSigma[0, n]], tot++]; tot, {n, 100}] (* T. D. Noe, Jan 10 2012 *)

PROG

(Haskell)