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A120041 Number of 10-almost primes k such that 2^n < k <= 2^(n+1). 27
0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 5, 8, 22, 47, 103, 233, 487, 1072, 2246, 4803, 10202, 21440, 45115, 94434, 197891, 412010, 858846, 1783610, 3700698, 7665755, 15853990, 32750248, 67564405, 139238488, 286625278, 589472979, 1211146741, 2486322304 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,12

COMMENTS

The partial sum equals the number of Pi_10(2^n).

LINKS

Table of n, a(n) for n=0..38.

FORMULA

a(n) ~ 2^n log^9 n/(725760 n log 2). [Charles R Greathouse IV, Dec 28 2011]

EXAMPLE

(2^10, 2^11] there is one semiprime, namely 1536. 1024 was counted in the previous entry.

MATHEMATICA

AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]]]]]; (* Eric W. Weisstein, Feb 07 2006 *)

t = Table[AlmostPrimePi[10, 2^n], {n, 0, 39}]; Rest@t - Most@t

CROSSREFS

Cf. A046314, A036378, A120033, A120034, A120035, A120036, A120037, A120038, A120039, A120040, A120041, A120042, A120043.

Sequence in context: A120038 A120039 A120040 * A120042 A120043 A063897

Adjacent sequences:  A120038 A120039 A120040 * A120042 A120043 A120044

KEYWORD

nonn

AUTHOR

Jonathan Vos Post and Robert G. Wilson v, Mar 21 2006

STATUS

approved

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Last modified September 28 17:43 EDT 2020. Contains 337393 sequences. (Running on oeis4.)