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A036351
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Number of numbers <= 10^n which are products of two distinct primes.
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2
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2, 30, 288, 2600, 23313, 209867, 1903878, 17426029, 160785135, 1493766851, 13959963049, 131125938680, 1237087821006, 11715901643501, 111329816634302, 1061057292162690
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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LINKS
| 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) ) -1 = Sum_{i=1..Pi(sqrt(10^n))} (Pi( (10^n-1)/P_i ) -1) - binomial( Pi(sqrt(10^n)), 2) (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 19 2005)
A036351(n) = A036352(n) - A122121(n). - Robert G. Wilson v, Feb 07 2012.
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MATHEMATICA
| SemiPrimePi[n_] := Sum[ PrimePi[n/Prime[i]] - i + 1, {i, PrimePi[Sqrt[n]]}]; f[n_] := SemiPrimePi[n] - PrimePi[Sqrt[n]]; Table[f[10^n], {n, 14}] (* Robert G. Wilson v, Feb 07 2012 *)
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CROSSREFS
| Cf. A066265.
Sequence in context: A157054 A092355 A189103 * A189770 A089433 A152277
Adjacent sequences: A036348 A036349 A036350 * A036352 A036353 A036354
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KEYWORD
| nonn,changed
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AUTHOR
| Shyam Sunder Gupta (guptass(AT)rediffmail.com)
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EXTENSIONS
| a(14) from Robert G. Wilson v (rgwv(AT)rgwv.com), May 19 2005
a(15)-a(16) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Oct 16 2010
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