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A120042 Number of 11-almost primes 11ap such that 2^n < 11ap <= 2^(n+1). 8
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 5, 8, 22, 47, 103, 234, 490, 1078, 2261, 4844, 10294, 21659, 45609, 95580, 200422, 417715, 871452, 1811412, 3761623, 7798409, 16142081, 33373093, 68906782, 142120436, 292797806, 602653984, 1239225631 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,13

COMMENTS

The partial sum equals the number of Pi_11(2^n).

LINKS

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

EXAMPLE

(2^11, 2^12] there is one semiprime, namely 3072. 2048 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[11, 2^n], {n, 0, 30}]; Rest@t - Most@t

CROSSREFS

Cf. A069272, A036378, A120033, A120034, A120035, A120036, A120037, A120038, A120039, A120040, A120041, A120042, A120043.

Sequence in context: A120039 A120040 A120041 * A120043 A063897 A092733

Adjacent sequences: A120039 A120040 A120041 * A120043 A120044 A120045

KEYWORD

nonn

AUTHOR

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

STATUS

approved

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Last modified December 10 01:39 EST 2022. Contains 358711 sequences. (Running on oeis4.)