login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A146167 Number of odd squarefree semiprimes (A046388) <= n. 0
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,21

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).

Sequence in context: A279758 A082996 A094382 * A103380 A098708 A067394

Adjacent sequences:  A146164 A146165 A146166 * A146168 A146169 A146170

KEYWORD

nonn

AUTHOR

Washington Bomfim, Oct 27 2008

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified September 20 15:44 EDT 2017. Contains 292275 sequences.