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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified October 1 08:39 EDT 2020. Contains 337442 sequences. (Running on oeis4.)