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A120881 a(n) = number of k's, for 1 <= k <= n, where GCD(k,floor(n/k)) > 1. 2

%I

%S 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,

%T 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,

%U 10,11,16,16,12,12,13,13,10,10,15,15,14,15,16,16,13,13,17

%N a(n) = number of k's, for 1 <= k <= n, where GCD(k,floor(n/k)) > 1.

%C A120881(n) + A120882(n) = n.

%H Vincenzo Librandi, <a href="/A120881/b120881.txt">Table of n, a(n) for n = 1..1000</a>

%e 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.

%t Table[Length[Select[Table[GCD[k, Floor[n/k]], {k, 1, n}], # > 1 &]], {n, 1, 80}] (* _Stefan Steinerberger_, Jul 23 2006 *)

%o (PARI) a(n) = sum(k=1, n, gcd(k, n\k) > 1); \\ _Michel Marcus_, Feb 16 2014

%Y Cf. A120882.

%K nonn

%O 1,8

%A _Leroy Quet_, Jul 12 2006

%E More terms from _Stefan Steinerberger_, Jul 23 2006

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Last modified August 20 18:56 EDT 2019. Contains 326154 sequences. (Running on oeis4.)