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A066265
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Number of semiprimes < 10^n.
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22
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0, 3, 34, 299, 2625, 23378, 210035, 1904324, 17427258, 160788536, 1493776443, 13959990342, 131126017178, 1237088048653, 11715902308080, 111329817298881, 1061057292827269
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| Apart from the first nonzero term the sequence is identical to A036352. - Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003
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LINKS
| Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
Index entries for sequences related to numbers of primes in various ranges
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FORMULA
| (1/2)*( Pi(10^(n/2)) + Sum_{i=1..Pi(10^n)} Pi( (10^n-1)/P_i) ) = Sum_{i=1..Pi(sqrt(10^n))} Pi( (10^n-1)/P_i ) - Binomial( Pi(sqrt(10^n)), 2) (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005)
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EXAMPLE
| Below 10 there are three semiprimes: 4 (2*2), 6 (2*3) and 9 (3*3).
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MATHEMATICA
| f[n_] := Sum[ PrimePi[(10^n - 1)/Prime[i]], {i, PrimePi[ Sqrt[10^n]]}] - Binomial[ PrimePi[ Sqrt[10^n]], 2]; Do[ Print[ f[n]], {n, 0, 14}] (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005)
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CROSSREFS
| Cf. A001358, A064911, A072000.
Cf. A036352 (identical starting from a(2)).
Sequence in context: A141789 A121077 A024396 * A134491 A045727 A105713
Adjacent sequences: A066262 A066263 A066264 * A066266 A066267 A066268
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KEYWORD
| nonn
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AUTHOR
| Patrick De Geest (pdg(AT)worldofnumbers.com), Dec 10 2001.
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EXTENSIONS
| More terms from Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003
a(14) from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005
a(15)-a(16) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Mar 18 2010
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