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A127396 Number of 3-almostprimes <= 2^n. 5
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 1..49

Eric Weisstein's World of Mathematics, Semiprime.

FORMULA

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

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}]

CROSSREFS

Cf. A126279, A109251, A125527.

Sequence in context: A297883 A215206 A298215 * A294839 A231478 A320680

Adjacent sequences:  A127393 A127394 A127395 * A127397 A127398 A127399

KEYWORD

nonn,less

AUTHOR

Robert G. Wilson v, Dec 29 2006

STATUS

approved

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Last modified November 27 20:49 EST 2021. Contains 349395 sequences. (Running on oeis4.)