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Let $X = A \cup B$, where $A$ and $B$ are subspaces of $X$. let $f: X \to Y$; suppose that the restricted functions $f\upharpoonright A:A\to Y$ and $f\upharpoonright B:B\to Y$ are continuous. Show that if both $A$ and $B$ are closed in $X$, then $f$ is continuous.

How does using h and g, as arbitrary functions in the hint below work?

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    Use the fact that co$n$ti$n$uity ca$n$ be equivale$n$tly defined as: the inverse image of a closed set is closed.2011-11-06

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So... $X$ and $Y$ are topological spaces and you want to show that, for any closed subset $S$ of $Y$, $R=f^{-1}(S)$ is a closed subset of $X$.

Hints: obviously, $R=(R\cap A)\cup (R\cap B)$. Furthermore, $R\cap A=g^{-1}(S)$ and $R\cap B=h^{-1}(S)$ for some suitable functions $g$ and $h$. But you will need to define precisely the functions $g$ and $h$, in particular, the spaces $g$ and $h$ are defined on. A last hint: the union of two closed subsets is a closed subset.

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    So, I can then write: g^1(S) U h^-1(S) = g^-1h^-1(S) and I am trying to see how I can organize the rest2011-11-06