A205726 Number of semiprimes <= n^2.

0, 1, 3, 6, 9, 13, 17, 22, 26, 34, 40, 48, 56, 62, 75, 82, 90, 103, 114, 126, 135, 149, 164, 179, 190, 202, 220, 236, 253, 270, 289, 304, 320, 340, 360, 381, 404, 425, 443, 462, 484, 508, 533, 556, 581, 604, 634, 655, 678, 709, 738, 761, 783, 813, 846, 881

Offset

1,3

Comments

See A205727 and A205728 for related sequences and relationship to Goldbach conjecture.

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Formula

a(n) = A072000(A000290(n)). - Michel Marcus, Sep 02 2013

Mathematica

SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; nn = 100;  t = Select[Range[nn^2], SemiPrimeQ]; Table[Length[Select[t, # <= n^2 &]], {n, nn}] (* T. D. Noe, Jan 30 2012 *)
Module[{nn=60, sp}, sp=Accumulate[Table[If[PrimeOmega[n]==2, 1, 0], {n, nn^2}]]; Table[sp[[i^2]], {i, nn}]] (* Harvey P. Dale, May 29 2014 *)

Crossrefs

Cf. A001358, A038108, A205727, A205728.

Sequence in context: A256966 A280944 A330321 * A002815 A342711 A109512

Adjacent sequences: A205723 A205724 A205725 * A205727 A205728 A205729

Keyword

nonn

Author

Keith Backman, Jan 30 2012

Status

approved