Thread: Continuity in each of parameters mean joint continuity in the domain?

Dear MHF members, here is my problem.
Suppose that we are given a real- or complex-valued function
on a rectangular domain such that
is continuous on for each fixed , and
is continuous on for each fixed .

• Does this mean that is continuous on ?
• If not, give a counter example, and provide additional
  conditions on so that it continuous on ?

Thank you very much.
bkarpuz

Hey bkarpuz.

You will have to use the delta epsilon mechanism to show its continuous but basically the limits should exist for all points in the region.

If you can show that the limit point of the function at certain values is a defined quantity then you're done. You should be able to directly use the continuity attributes to say lim x->a f(x,y0) = blah and lim y->b f(x0,y) = blah.

Thank you very much chiro, but I was looking for a more detailed explanation such as the following.
Separate continuity of does not mean joint continuity. But the converse is trivially true.
Consider the simple example

Obviously, is separately continuous.
However, it is not joint continuous as the limit does not exist.

The statement that separate continuity implies joint continuity turns out to be true
if separate continuity of is uniformly for at least one of the variables
(i.e., does not depent on the fixed parameter).
For details further see [1, Theorem 5, pp. 102].

Please correct me if I gave something wrong here.
Many thanks.
bkarpuz

References
[1] L. M. Graves, Theory of Functions of Real Variables, New York, 1946.
[2] V. Naik, Separately continuous functions (vs joint continuity) - YouTube