[R] non-linear regression problem

[Sascha Benz]

I need to do a non-linear regression in the form of

Y = a0 + a1 * arctan(a2 * x) + error.

A data sample (X,Y) is available, but I can't remember how to run this sort 
of regression through R so that I get a value for a0, a1 and a2.

Can someone please give me a hint?

Thank you in advance.