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A111457 Number of semiprimes smaller than the n-th prime. 1
0, 0, 1, 2, 4, 4, 6, 6, 8, 10, 10, 13, 15, 15, 16, 18, 21, 21, 23, 24, 24, 26, 27, 30, 34, 34, 34, 35, 35, 36, 42, 43, 45, 45, 50, 50, 51, 54, 55, 56, 58, 58, 61, 61, 62, 62, 68, 75, 76, 76, 76, 78, 78, 80, 82, 84, 86, 86, 87, 88, 88, 91, 98, 99, 99, 100, 106, 108, 111, 111 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

M. F. Hasler, Table of n, a(n) for n = 1..9592

MATHEMATICA

s = Select[Range@1000, Plus @@ Last /@ FactorInteger@ # == 2 &]; Table[ Length@Select[s, # < Prime@n &], {n, 70}] (* Robert G. Wilson v *)

Module[{nn=500, sp}, sp=Table[If[PrimeOmega[n]==2, 1, 0], {n, nn}]; Table[ Total[ Take[sp, p]], {p, Prime[Range[PrimePi[nn]]]}]] (* Harvey P. Dale, Aug 22 2019 *)

PROG

(PARI) n=c=0; forprime(p=1, 999, while(n<p, bigomega(n++)==2&&c++); print1(c", ")) \\ M. F. Hasler, Jan 02 2015

CROSSREFS

Cf. A001358, A000040.

Sequence in context: A187323 A262504 A211514 * A161765 A232091 A164798

Adjacent sequences:  A111454 A111455 A111456 * A111458 A111459 A111460

KEYWORD

nonn

AUTHOR

Giovanni Teofilatto, Nov 15 2005

EXTENSIONS

More terms from Robert G. Wilson v and Stefan Steinerberger, Nov 18 2005

Title edited by M. F. Hasler, Jan 02 2015

STATUS

approved

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Last modified April 20 15:21 EDT 2021. Contains 343135 sequences. (Running on oeis4.)