How to express the domain of y = √cos x in mathematical notation

[Calixto]

What would the domain of y = sqrt(cosx) be in mathematical terms. I know that it is all the reals that lie in the first and fourth quadrant of the unit circle, but how would you express that in mathematical terms?

 

[nicksauce]

Something like
$$D=\{x|x\in[\frac{(4n-1)\pi}{2},\frac{(4n+1)\pi}{2}]\}$$
where n is any integer

 

[unplebeian]

You first need to define the domain of x.
If x is +60 for example then cos(x)>0 but if it is -60, then it is <0. sqrt of that is an imaginary number which is not defined.

So basically for x>0 domain is (0,1)

I would like to know how Calixto got his answer as I may be wrong.