This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A120881 a(n) = number of k's, for 1 <= k <= n, where GCD(k,floor(n/k)) > 1. 2
 0, 0, 0, 1, 1, 0, 0, 2, 3, 2, 2, 2, 2, 1, 1, 4, 4, 4, 4, 5, 4, 3, 3, 5, 6, 5, 7, 8, 8, 3, 3, 7, 7, 6, 6, 8, 8, 7, 6, 9, 9, 6, 6, 7, 9, 8, 8, 11, 12, 12, 12, 13, 13, 14, 13, 15, 14, 13, 13, 11, 11, 10, 11, 16, 16, 12, 12, 13, 13, 10, 10, 15, 15, 14, 15, 16, 16, 13, 13, 17 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,8 COMMENTS A120881(n) + A120882(n) = n. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 EXAMPLE For n = 8, we have the pairs {k,floor(n/k)} of {1,8},{2,4},{3,2},{4,2},{5,1},{6,1},{7,1},{8,1}. From these pairs we get the GCD's of 1,2,1,2,1,1,1,1. 2 of these GCD's are > 1. So a(8)= 2. MATHEMATICA Table[Length[Select[Table[GCD[k, Floor[n/k]], {k, 1, n}], # > 1 &]], {n, 1, 80}] (* Stefan Steinerberger, Jul 23 2006 *) PROG (PARI) a(n) = sum(k=1, n, gcd(k, n\k) > 1); \\ Michel Marcus, Feb 16 2014 CROSSREFS Cf. A120882. Sequence in context: A030329 A300139 A300666 * A297930 A031217 A064131 Adjacent sequences:  A120878 A120879 A120880 * A120882 A120883 A120884 KEYWORD nonn AUTHOR Leroy Quet, Jul 12 2006 EXTENSIONS More terms from Stefan Steinerberger, Jul 23 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
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified July 23 13:58 EDT 2019. Contains 325254 sequences. (Running on oeis4.)