login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual appeal: Please make a donation to keep the OEIS running! Over 6000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A120043 Number of 12-almost primes 12ap such that 2^n < 12ap <= 2^(n+1). 12
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 5, 8, 22, 47, 103, 234, 492, 1082, 2271, 4867, 10349, 21794, 45907, 96293, 202006, 421287, 879388, 1828931, 3800227, 7882784, 16325796, 33771056, 69767214, 143971956, 296771231, 611156696, 1257374970 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,14

COMMENTS

The partial sum equals the number of Pi_12(2^n).

LINKS

Table of n, a(n) for n=0..39.

EXAMPLE

(2^12, 2^13] there is one semiprime, namely 6144. 4096 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[12, 2^n], {n, 0, 30}]; Rest@t - Most@t

CROSSREFS

Cf. A069273, A036378, A120033, A120034, A120035, A120036, A120037, A120038, A120039, A120040, A120041, A120042, A120043.

Sequence in context: A120040 A120041 A120042 * A063897 A092733 A116884

Adjacent sequences:  A120040 A120041 A120042 * A120044 A120045 A120046

KEYWORD

nonn

AUTHOR

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

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified November 18 14:12 EST 2017. Contains 294892 sequences.