A146167 Number of odd squarefree semiprimes (A046388) <= n.

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 11

Offset

1,21

Comments

A346622 is a different although very similar sequence. - N. J. A. Sloane, Aug 23 2021

Links

Table of n, a(n) for n=1..77.

Formula

a(n) = A072000(n) - A000720(floor(sqrt(n))) - A000720(floor(n/2)) + 1.

Example

a(33)= 3. The semiprimes <=33 are 15, 21 and 33. Formula gives 11-pi(5)-pi(16)+1 = 3.

Mathematica

Accumulate[Table[If[OddQ[n]&&SquareFreeQ[n]&&PrimeOmega[n]==2, 1, 0], {n, 0, 100}]] (* Harvey P. Dale, Feb 08 2016 *)

Crossrefs

Cf. A046388, A001358 (semiprimes), A072000 (Number of semiprimes <= n), A000720 (pi(n), the number of primes <= n).

Cf. also A346622.

Sequence in context: A279758 A082996 A094382 * A346622 A103380 A309120

Adjacent sequences: A146164 A146165 A146166 * A146168 A146169 A146170

Keyword

nonn

Author

Washington Bomfim, Oct 27 2008

Status

approved