[math-fun] A018805 references

A018805(n) = gives the number of coprime pairs (x, y) with 1 <= x,y <= n. Based on my authorship of A018805, I have been asked for references for the result lim n->inf a(n)/n^2 = 6/pi^2. Given the difficult part, i.e zeta(2) = pi^2/6, I'm sure I could cobble together a proof. I pointed my friend to http://mathworld.wolfram.com/RelativelyPrime.html, which gives the limit in equation (1), and has copious references, of which I assume at least one or two give an argument for the limit. I personally don't have access to the literature, I was hoping that someone else might add some appropriate references to A018805.