I think the following is true and was hoping someone could help me think up a proof or (hopefully not) a counterexample.

Let $X$ and $Y$ be compact Hausdorff. If $f :X \times Y \to \mathbb{R}$ is separately continuous then $f$ is jointly continuous.

  • $\begingroup$ Does anyone know under what additional conditions on $f$, $X,\,Y,\,Z$ does the statement hold? I would be happy with $X$ and $Y$ being cubes in $\mathbb{R}^n$, $Z=\mathbb{R}^m$ and $f$ being separately analytic (even $m=1$). $\endgroup$
    – Sobolev
    Jan 28, 2019 at 20:02

1 Answer 1



$$f(x,y) = \begin{cases}\qquad 0 &, x = y = 0\\ \dfrac{xy}{x^2+y^2} &, x^2+y^2 > 0\end{cases}$$

is separately continuous on $[-1,1]^2$.


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