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A109251
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Number of numbers up to 10^n which are products of three primes.
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18
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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
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0,3
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LINKS
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Table of n, a(n) for n=0..19.
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FORMULA
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a(n) = A072114(10^n). - R. J. Mathar, May 25 2008
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EXAMPLE
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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.
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MATHEMATICA
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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 *)
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CROSSREFS
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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
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KEYWORD
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nonn
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AUTHOR
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Martin Raab, Aug 19 2005
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EXTENSIONS
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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
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STATUS
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approved
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