Hartogs' theorem

From Wikipedia, the free encyclopedia

NB that the terminology is inconsistent and Hartogs' theorem may also mean Hartogs' lemma on removable singularities, or the result on Hartogs number

In mathematics, Hartogs' theorem is a fundamental result of Friedrich Hartogs in the theory of several complex variables. It states that for complex-valued functions F on Cⁿ, with n > 1, being an analytic function in each variable z_i, 1 ≤ i ≤ n, while the others are held constant, is enough to prove F a continuous function.

A corollary of this is that F is then in fact an analytic function in the n-variable sense (i.e. that locally it has a Taylor expansion). Therefore 'separate analyticity' and 'analyticity' are coincident notions, in the several complex variables theory.

Note that there is no analogue of this theorem for real variables. If we assume that a function

$f \colon {\mathbb{R}}^n \to {\mathbb{R}}$

is differentiable (or even analytic) in each variable separately, it is not true that $f$ will necessarily be continuous. A counterexample in two dimensions is given by

$f(x,y) = \frac{xy}{x^2+y^2}.$

This function has well-defined partial derivatives in $x$ and $y$ at 0, but it is not continuous at 0 (the limits along the lines $x = y$ and $x = - y$ give different results).