What are the minimal conditions on three topological spaces $X,Y$ and $Z$ such that with the compact-open topology the map


$${(X^Y)}^Z \to X^{Y \times Z}$$


given by taking adjoints is a homeomorpism. The map sends $f: Z \to X^Y$ to $g:Y \times Z \to X$ by the relation $g(y,z)=f(z)(y)$. 


This result is known for $Z$ Hausdorff and $Y$ locally compact. I'm interested in a proposition of the form the adjoint construction is a homeomorphism of mapping spaces if and only if some statement regarding the spaces $X$, $Y$ and $Z$. 
It would also be interesting to see some counterexamples, for example for $Z$ not Hausdorff, etc. 



Minimal conditions for the exponential law for compact-open topologies
A very closely related question (and maybe the one you meant to ask?) is: which spaces $Y$ in the category of topological spaces and continuous maps are exponentiable, i.e., for which $Y$ does the functor $- \times Y: Top \to Top$ have a right adjoint? A necessary and sufficient condition is that $Y$ is *core-compact*, as defined at the [nLab](http://ncatlab.org/nlab/show/exponential+law+for+spaces#corecompactness_5). See also the references in that article. 


There are various ways of defining core-compactness; perhaps the fastest is that the topology is a continuous lattice. It is a slightly weaker condition than local compactness (if local compactness is defined as meaning that every point has a basis of compact neighborhoods), and coincides with local compactness if $Y$ is Hausdorff. 


If $Y$ and $Z$ are core-compact, then for every $X$ one can exhibit a canonical homeomorphism 


$$(X^Z)^Y \cong X^{Y \times Z}$$ 


by abstract nonsense (since any two right adjoints to $- \times (Y \times Z)$, in particular $((-)^Y)^Z$ and $(-)^{Y \times Z}$, are canonically naturally isomorphic). 


Your question is also interesting when interpreted for locales. See Johnstone's Stone Spaces, where it is shown that a locale is exponentiable if and only if it is locally compact. 


If $Y$ is not core-compact, then it is possible to show that there is no exponential $\mathbf{2}^Y$ where $\mathbf{2}$ is Sierpinski space (two points, one open, one closed). In other words, the functor $\hom\_{Top}(- \times Y, \mathbf{2})$ is not representable. I once went through the detailed argument (in the case where $Y$ is the space of rational numbers, which I think is illustrative) over at the n-Category Café, see [here](http://golem.ph.utexas.edu/category/2007/12/this_weeks_finds_in_mathematic_20.html#c014330) and the ensuing discussion. 


