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A036351 Number of numbers <= 10^n that are products of two distinct primes. 2
2, 30, 288, 2600, 23313, 209867, 1903878, 17426029, 160785135, 1493766851, 13959963049, 131125938680, 1237087821006, 11715901643501, 111329815346924, 1061057287065814, 10139482896634686, 97123037634329553, 932300026078297246, 8966605849186166511, 86389956292394285653 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..21.

Index entries for sequences related to numbers of primes in various ranges

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). - Robert G. Wilson v, May 19 2005

A036351(n) = A036352(n) - A122121(n). - Robert G. Wilson v, Feb 07 2012

MATHEMATICA

f[n_] := Sum[ PrimePi[n/Prime[i]] - i, {i, PrimePi[ Sqrt[ n]] }]; Table[ f[10^n], {n, 14}] (* Robert G. Wilson v, Feb 07 2012 and modified Dec 28 2016 *)

PROG

(PARI) a(n)=my(s); forprime(p=2, sqrt(10^n), s+=primepi(10^n\p)); s-binomial(primepi(sqrt(10^n))+1, 2) \\ Charles R Greathouse IV, Apr 23 2012

CROSSREFS

Cf. A066265.

Sequence in context: A300685 A300608 A301350 * A189770 A245020 A277660

Adjacent sequences:  A036348 A036349 A036350 * A036352 A036353 A036354

KEYWORD

nonn

AUTHOR

Shyam Sunder Gupta

EXTENSIONS

a(14) from Robert G. Wilson v, May 19 2005

a(15)-a(16) from Donovan Johnson, Oct 16 2010

Corrected a(15) and a(16) by Henri Lifchitz, Nov 11 2012

a(17)-a(19) from Henri Lifchitz, Nov 11 2012

a(20)-a(21) from Henri Lifchitz, Jul 03 2015

STATUS

approved

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Last modified January 17 11:16 EST 2022. Contains 350389 sequences. (Running on oeis4.)