Hartogs's theorem
In mathematics, Hartogs's theorem is a fundamental result of Friedrich Hartogs in the theory of several complex variables. Roughly speaking, it states that a 'separately analytic' function is continuous. More precisely, if is a function which is analytic in each variable zi, 1 ≤ i ≤ n, while the other variables are held constant, then F is a continuous function.
A corollary is that the function F is then in fact an analytic function in the n-variable sense. Therefore 'separate analyticity' and 'analyticity' are coincident notions, in the theory of several complex variables.
Starting with the extra hypothesis that the function is continuous, the theorem is much easier to prove and in this form is known as Osgood's lemma.
Note that there is no analogue of this theorem for real variables. If we assume that a function
is differentiable in each variable separately, it is not true that will necessarily be continuous. A counterexample in two dimensions is given by
If in addition we define, this function has well-defined partial derivatives in and at the origin, but it is not continuous at origin.
