A216593 Number of semiprimes among n-th million natural numbers.

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

Offset: 1

Author

Zak Seidov, Sep 09 2012

Keywords

• nonn

Comments

Let f(m) = number of semiprimes

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

Crossrefs

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

Programs

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 *)

PARI

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

Formula

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

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement.