It depends.
The domain and codomain can both be the complex planes.
or
domain x ≥ 11 and codomain = reals
or
domain = 11, 12, 15, 20, ... ie {x = 11+n2 | n is an integer} and codomain = Z
≥
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The domain is anything you want it to be. You could define the domain to be integer values only, or it could be {-3, -2.5, .2, 0, sqrt(7), 9}.
The domain can be anything you like: a single element, all real numbers, all complex numbers, etc.
12
12
3