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A125527 Number of semiprimes <= 2^n. 7
0, 1, 2, 6, 10, 22, 42, 82, 157, 304, 589, 1124, 2186, 4192, 8110, 15658, 30253, 58546, 113307, 219759, 426180, 827702, 1608668, 3129211, 6091437, 11868599, 23140878, 45150717, 88157689, 172235073, 336717854, 658662065, 1289149627, 2524532330 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 1..49 [File corrected Mar 12 2007]

Eric Weisstein's World of Mathematics, Semiprime.

FORMULA

a(n) = A072000(2^n). - R. J. Mathar, Aug 26 2011

MATHEMATICA

SemiPrimePi[n_] := Sum[ PrimePi[ n/Prime@i] - i + 1, {i, PrimePi@ Sqrt@n}]; Table[ SemiPrimePi[2^n], {n, 47}]

PROG

(PARI) a(n)=my(s, i, N=2^n); forprime(p=2, sqrtint(N), s+=primepi(N\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, May 12 2013

CROSSREFS

Cf. A126279, A066265, A007053, A127396.

Sequence in context: A182000 A167512 A055895 * A200572 A049750 A134016

Adjacent sequences:  A125524 A125525 A125526 * A125528 A125529 A125530

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Dec 29 2006

STATUS

approved

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Last modified April 27 06:52 EDT 2017. Contains 285508 sequences.