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A194895 Difference between the number of semiprimes less than 10^n and the number of primes less than 10^n 0
0, -1, 9, 131, 1396, 13786, 131537, 1239745, 11665803, 109941002, 1038723932, 9841935529, 93518105160, 891022511814, 8510960557278, 81485246876212, 781818951793344 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

A066265 minus A006880

LINKS

Table of n, a(n) for n=0..16.

EXAMPLE

There are 2625 semiprimes less than 10^4 and there are 1229 primes less than 10^4, so a(4) = 2625 - 1229 = 1396.

MATHEMATICA

SemiPrimePi[n_] := Sum[ PrimePi[n/Prime@ i] - i + 1, {i, PrimePi@ Sqrt@ n}]; f[n_] := SemiPrimePi[10^n - 1] - PrimePi[10^n - 1]; Array[f, 15, 0] (* Robert G. Wilson v, Jul 05 2012 *)

CROSSREFS

Cf. A006880, A066265

Sequence in context: A060944 A299596 A200407 * A112123 A282820 A296318

Adjacent sequences:  A194892 A194893 A194894 * A194896 A194897 A194898

KEYWORD

sign

AUTHOR

Harvey P. Dale, Sep 04 2011

STATUS

approved

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Last modified May 25 04:50 EDT 2019. Contains 323539 sequences. (Running on oeis4.)