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A085770
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Number of odd semiprimes < 10^n. Number of terms of A046315 < 10^n.
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2
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0, 1, 19, 204, 1956, 18245, 168497, 1555811, 14426124, 134432669, 1258822220, 11840335764, 111817881036, 1059796387004, 10076978543513, 96091983644261, 918679875630905, 8803388145953381, 84537081118605467, 813340036541900706, 7838825925851034479, 75669246175972479567
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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..21.
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FORMULA
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a(n) = A066265(n) - A220262(n) for n > 1. - Jinyuan Wang, Jul 30 2021
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EXAMPLE
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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.
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MATHEMATICA
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OddSemiPrimePi[n_] := Sum[ PrimePi[n/Prime@i] - i + 1, {i, 2, PrimePi@ Sqrt@ n}]; Table[ OddSemiPrimePi[10^n], {n, 14}] (* Robert G. Wilson v *)
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CROSSREFS
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Cf. A046315 (odd numbers divisible by exactly 2 primes), A066265 (number of semiprimes < 10^n), A220262, A292785.
Sequence in context: A160452 A113596 A155670 * A002501 A180364 A125407
Adjacent sequences: A085767 A085768 A085769 * A085771 A085772 A085773
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KEYWORD
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nonn
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AUTHOR
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Hugo Pfoertner, Jul 22 2003
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EXTENSIONS
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a(10)-a(14) from Robert G. Wilson v, Feb 02 2006
a(15)-a(16) from Donovan Johnson, Mar 18 2010
a(0) inserted by and a(17)-a(21) from Jinyuan Wang, Jul 30 2021
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STATUS
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approved
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