An euler's totient-related sequence

[Nick Hobson]

Hi Seqfans,

Is the following sequence of interest?

0, 0, 0, 1, 1, 1, 2, 1, 2, 2, 3, 1, 4, 2, 3, 3, 5, 2, 6, 2, ... .

a(n) = number of integers < n/3 that are coprime to n.  Equivalently, it  
is the number of fractions in their lowest terms that are < 1/3, with  
denominator equal to n.

Is there an easy analytic form for this sequence?  (Comparable to a(n) =  
number of integers < n/2 that are coprime to n = eulerphi(n)/2, for n > 2.)

Nick

PS  Is it necessary to send to *both* addresses: seqfans at seqfan.net,  
seqfan at ext.jussieu.fr?  Are some people on just one of the lists?