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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 (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)

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

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Last modified June 19 05:33 EDT 2019. Contains 324218 sequences. (Running on oeis4.)