The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A071520 Number of 5-smooth numbers (A051037) <= n. 9
 1, 2, 3, 4, 5, 6, 6, 7, 8, 9, 9, 10, 10, 10, 11, 12, 12, 13, 13, 14, 14, 14, 14, 15, 16, 16, 17, 17, 17, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 21, 22, 22, 22, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A 5-smooth number is a number of the form 2^x*3^y*5^z (x,y,z) >= 0. LINKS FORMULA a(n) = Card{ k | A051037(k) <= n }. Asymptotically : let a = 1/(6*log(2)*log(3)*log(5)) and b = sqrt(30) then a(n) = a*log(b*n)^3 + O(log(n)). a(n) = -Sum_{k=1,n} mu(30*k)*floor(n/k). - Benoit Cloitre, Jun 14 2007 a(n) = Sum_{i=0..floor(log_5(n))} Sum_{j=0..floor(log_3(n/5^i))} floor(log_2(2*n/(5^i*3^j))). - Ridouane Oudra, Jul 17 2020 PROG (PARI) for(n=1, 100, print1(sum(k=1, n, if(sum(i=4, n, if(k%prime(i), 0, 1)), 0, 1)), ", ")) (PARI) a(n)=-sum(k=1, n, moebius(2*3*5*k)*floor(n/k)) \\ Benoit Cloitre, Jun 14 2007 CROSSREFS Cf. A051037, A106598, A112751. Sequence in context: A287354 A025548 A121604 * A195918 A176842 A278055 Adjacent sequences:  A071517 A071518 A071519 * A071521 A071522 A071523 KEYWORD easy,nonn AUTHOR Benoit Cloitre, Jun 02 2002 EXTENSIONS Title corrected by Rainer Rosenthal, Aug 30 2020 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
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 25 16:17 EDT 2021. Contains 347658 sequences. (Running on oeis4.)