The document discusses finding the domain of the function g(u)=square root u + square root 4-u. The domain is the set of values that the function is defined for. For this function, the domain is the interval [0,4], as the function is only defined when u is between 0 and 4, where both terms inside the square roots are positive. This domain is expressed in both set notation as R - [(-inf, 0) U (4, inf)] and interval notation as [0,4].