A127396 Number of 3-almostprimes <= 2^n.

0, 0, 1, 2, 7, 13, 30, 60, 125, 256, 513, 1049, 2082, 4214, 8401, 16771, 33427, 66550, 132405, 262865, 522296, 1036033, 2055256, 4075039, 8078110, 16008485, 31720903, 62847087, 124501149, 246638355, 488559079, 967785236, 1917099175, 3797688543

Offset: 1

Author

Robert G. Wilson v, Dec 29 2006

Keywords

• nonn
• less

Links

• Robert G. Wilson v, Table of n, a(n) for n = 1..53
• Eric Weisstein's World of Mathematics, Semiprime.

Crossrefs

Cf. A126279, A109251, A125527.

Programs

Mathematica

ThreeAlmostPrimePi[n_] := Sum[PrimePi[n/(Prime@i*Prime@j)] - j + 1, {i, PrimePi[n^(1/3)]}, {j, i, PrimePi[Sqrt[n/Prime@i]]}]; Table[ ThreeAlmostPrimePi[2^n], {n, 30}]

Formula

a(n) = A072114(2^n). - R. J. Mathar, Aug 26 2011