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A120035 Number of 4-almost primes f such that 2^n < f <= 2^(n+1). 8
0, 0, 0, 1, 1, 5, 7, 20, 37, 81, 173, 344, 736, 1461, 3065, 6208, 12643, 25662, 52014, 105487, 212566, 430007, 865650, 1744136, 3508335, 7053390, 14167804, 28441899, 57065447, 114418462, 229341261, 459442819, 920097130, 1841946718, 3686197728 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,6

COMMENTS

The partial sum equals the number of Pi_4(2^n) = 0, 0, 0, 1, 2, 7, 14, 34, 71, 152, 325, 669, 1405, 2866,...

LINKS

Table of n, a(n) for n=0..34.

EXAMPLE

(2^4, 2^5] there is one semiprime, namely 24. 16 was counted in the previous entry.

MATHEMATICA

FourAlmostPrimePi[n_] := Sum[ PrimePi[n/(Prime@i*Prime@j*Prime@k)] - k + 1, {i, PrimePi[n^(1/4)]}, {j, i, PrimePi[(n/Prime@i)^(1/3)]}, {k, j, PrimePi@Sqrt[n/(Prime@i*Prime@j)]}]; t = Table[ FourAlmostPrimePi[2^n], {n, 0, 37}]; Rest@t - Most@t

CROSSREFS

Cf. A014613, A082996, A114106, A036378, A120033 - A120043.

Sequence in context: A249047 A258282 A192422 * A198302 A091154 A057424

Adjacent sequences:  A120032 A120033 A120034 * A120036 A120037 A120038

KEYWORD

nonn

AUTHOR

Jonathan Vos Post and Robert G. Wilson v, Mar 20 2006

STATUS

approved

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Last modified July 21 17:20 EDT 2019. Contains 325198 sequences. (Running on oeis4.)