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A216593 Number of semiprimes among n-th million natural numbers. 1
210035, 197249, 193162, 190540, 188288, 187308, 185657, 184788, 183856, 183441, 182123, 181556, 181125, 180878, 180234, 179649, 179055, 178710, 178652, 178034, 178015, 177307, 177391, 177003, 176568, 176419, 176021, 175788, 175655, 175189, 174915, 175357 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Let f(m) = number of semiprimes <m then a(n) is the number of semiprimes between 1+(n-1)*10^6 and n*10^6.

a(n) = 0 for almost all n. It seems infeasible to find the first such n. - Charles R Greathouse IV, Sep 09 2012

LINKS

Zak Seidov, Table of n, a(n) for n = 1..1000

FORMULA

a(n) ~ 1000000 n log log n / log n. - Charles R Greathouse IV, Sep 23 2012

MATHEMATICA

f[m_] := Sum[ PrimePi[(m - 1)/Prime[i]], {i, PrimePi[ Sqrt[m]]}] - Binomial[ PrimePi[ Sqrt[m]], 2]; ta=Table[f[n*10^6], {n, 0, 1000}]; s=Rest[ta]-Most[ta] (* for first 1000 terms *)

(* using Mmca code by Robert G. Wilson v in A066265 - Zak Seidov, Sep 09 2012 *)

PROG

(PARI) a(n)=sum(k=10^6*(n-1), 10^6*n, bigomega(k)==2) \\ Charles R Greathouse IV, Sep 09 2012

CROSSREFS

Cf. A001358, A036352, A064911, A066265, A072000.

Sequence in context: A232136 A183602 A015376 * A172825 A072760 A178421

Adjacent sequences:  A216590 A216591 A216592 * A216594 A216595 A216596

KEYWORD

nonn

AUTHOR

Zak Seidov, Sep 09 2012

STATUS

approved

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Last modified April 24 19:49 EDT 2019. Contains 322446 sequences. (Running on oeis4.)