cp's OEIS Frontend

A204330 a(n) is the number of k satisfying 1 <= k <= n and such that floor(sqrt(k)) divides k.

%I A204330 #24 May 27 2013 07:12:46
%S A204330 1,2,3,4,4,5,5,6,7,7,7,8,8,8,9,10,10,10,10,11,11,11,11,12,13,13,13,13,
%T A204330 13,14,14,14,14,14,15,16,16,16,16,16,16,17,17,17,17,17,17,18,19,19,19,
%U A204330 19,19,19,19,20,20,20,20,20,20,20,21,22,22,22,22,22,22,22
%N A204330 a(n) is the number of k satisfying 1 <= k <= n and such that floor(sqrt(k)) divides k.
%C A204330 a(n) = floor(2*sqrt(n)) + floor(sqrt(n-1)) - 1 if n belongs to A135106 otherwise a(n) = floor(2*sqrt(n)) + floor(sqrt(n-1)) - 2.
%H A204330 B. Cloitre, <a href="http://dl.dropbox.com/u/46675017/divisibility_sequences.pdf">Some divisibility sequences</a>
%F A204330 a(n) = card{j>=1, A006446(j)<=n}.
%t A204330 Accumulate[Boole[Table[IntegerQ[n/Floor[n^(1/2)]], {n, 1, 70}]]]  (* _Geoffrey Critzer_, May 25 2013 *)
%o A204330 (PARI) a(n)=sum(k=1,n,if(k%sqrtint(k),0,1));
%Y A204330 Cf. A006446, A079631.
%K A204330 nonn
%O A204330 1,2
%A A204330 _Benoit Cloitre_, Jan 14 2012
%E A204330 Corrected by _Geoffrey Critzer_, May 25 2013