

A109251


Number of numbers up to 10^n which are products of three primes.


18



0, 1, 22, 247, 2569, 25556, 250853, 2444359, 23727305, 229924367, 2227121996, 21578747909, 209214982913, 2030133769624, 19717814526785, 191693417109381, 1865380637252270, 18168907486812690, 177123437184971927, 1728190923820610000
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OFFSET

0,3


LINKS

Table of n, a(n) for n=0..19.


FORMULA

a(n) = A072114(10^n).  R. J. Mathar, May 25 2008


EXAMPLE

There are 22 numbers with three prime factors up to 10^2: 8, 12, 18, 20, 27, 28, 30, 42, 44, 45, 50, 52, 63, 66, 68, 70, 75, 76, 78, 92, 98, 99.


MATHEMATICA

ThreeAlmostPrimePi[n_] := Sum[ PrimePi[n/(Prime@i*Prime@j)]  j + 1, {i, PrimePi[n^(1/3)]}, {j, i, PrimePi@ Sqrt[n/Prime@i]}]; Table[ ThreeAlmostPrimePi[10^n], {n, 0, 14}] (* Robert G. Wilson v *)


CROSSREFS

Cf. A014612 = numbers with three prime factors, A036352 = number of numbers up to 10^n which are products of two primes.
Sequence in context: A125410 A200206 A252819 * A268791 A161521 A161900
Adjacent sequences: A109248 A109249 A109250 * A109252 A109253 A109254


KEYWORD

nonn


AUTHOR

Martin Raab, Aug 19 2005


EXTENSIONS

a(10)a(14) from Robert G. Wilson v, Feb 06 2006
a(15)a(17) from Hiroaki Yamanouchi, Aug 30 2014
a(18)a(19) from Henri Lifchitz, Dec 01 2014


STATUS

approved



