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A111939 Number of primes < semiprime(n). 1
2, 3, 4, 4, 6, 6, 8, 8, 9, 9, 11, 11, 11, 12, 12, 14, 15, 15, 16, 16, 16, 18, 18, 19, 21, 21, 22, 23, 23, 23, 24, 24, 24, 24, 27, 29, 30, 30, 30, 30, 30, 30, 31, 32, 32, 34, 34, 34, 34, 34, 36, 37, 37, 37, 38, 39, 40, 40, 42, 42, 42, 44, 46, 46, 46, 46, 46, 46, 47, 47, 47, 47, 47 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..73.

FORMULA

a(n) = A000720(A001358(n)).

MATHEMATICA

PrimePi@Select[Range@218, Plus @@ Last /@ FactorInteger@# == 2 &] (* Robert G. Wilson v, Jan 10 2006 *)

PrimePi/@Select[Range[300], PrimeOmega[#]==2&] (* Harvey P. Dale, Nov 16 2014 *)

CROSSREFS

Cf. A122489.

Sequence in context: A304575 A296421 A138768 * A253248 A003962 A102443

Adjacent sequences:  A111936 A111937 A111938 * A111940 A111941 A111942

KEYWORD

nonn

AUTHOR

Giovanni Teofilatto, Nov 27 2005

EXTENSIONS

More terms from Robert G. Wilson v, Lambert Herrgesell (zero815(AT)googlemail.com) and Matthew Conroy, Dec 16 2005

Entry revised by Ray Chandler, Sep 20 2006

STATUS

approved

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Last modified October 23 17:32 EDT 2019. Contains 328373 sequences. (Running on oeis4.)