login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A085770 Number of odd semiprimes < 10^n. Number of terms of A046315 < 10^n. 2
1, 19, 204, 1956, 18245, 168497, 1555811, 14426124, 134432669, 1258822220, 11840335764, 111817881036, 1059796387004, 10076978543513, 96091983644261, 918679875630905 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..16.

EXAMPLE

a(1)=1 because A046315(1)=9=3*3 is the only odd semiprime < 10^1,

a(2)=19 because there are 19 terms of A046315 < 10^2.

MATHEMATICA

OddSemiPrimePi[n_] := Sum[ PrimePi[n/Prime@i] - i + 1, {i, 2, PrimePi@ Sqrt@ n}]; Table[ OddSemiPrimePi[10^n], {n, 14}] (* Robert G. Wilson v *)

CROSSREFS

Cf. A046315 (odd numbers divisible by exactly 2 primes), A066265 (number of semiprimes < 10^n), A292785.

Sequence in context: A160452 A113596 A155670 * A002501 A180364 A125407

Adjacent sequences:  A085767 A085768 A085769 * A085771 A085772 A085773

KEYWORD

more,nonn

AUTHOR

Hugo Pfoertner, Jul 22 2003

EXTENSIONS

a(10)-a(14) from Robert G. Wilson v, Feb 02 2006

a(15)-a(16) from Donovan Johnson, Mar 18 2010

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
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 11 22:01 EDT 2021. Contains 342888 sequences. (Running on oeis4.)