

A203967


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


4



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
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OFFSET

1,3


LINKS

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


FORMULA

a(n) = a(n1) + 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)
a203967 n = length $ takeWhile (<= n) a009087_list
 Reinhard Zumkeller, Jun 06 2013


CROSSREFS

Cf. A009087.
Sequence in context: A047784 A047742 A302778 * A267530 A050292 A181627
Adjacent sequences: A203964 A203965 A203966 * A203968 A203969 A203970


KEYWORD

nonn


AUTHOR

Geoffrey Critzer, Jan 08 2012


STATUS

approved



