Editing General topology (section)

====Closure operator definition====
Instead of specifying the open subsets of a topological space, the topology can also be determined by a [[Kuratowski closure operator|closure operator]] (denoted cl), which assigns to any subset ''A'' ⊆ ''X'' its [[closure (topology)|closure]], or an [[interior operator]] (denoted int), which assigns to any subset ''A'' of ''X'' its [[interior (topology)|interior]]. In these terms, a function 
:<math>f\colon (X,\mathrm{cl}) \to (X' ,\mathrm{cl}')\, </math>
between topological spaces is continuous in the sense above if and only if for all subsets ''A'' of ''X''
:<math>f(\mathrm{cl}(A)) \subseteq \mathrm{cl}'(f(A)).</math>
That is to say, given any element ''x'' of ''X'' that is in the closure of any subset ''A'', ''f''(''x'') belongs to the closure of ''f''(''A''). This is equivalent to the requirement that for all subsets ''A''<nowiki>'</nowiki> of ''X''<nowiki>'</nowiki>
:<math>f^{-1}(\mathrm{cl}'(A')) \supseteq \mathrm{cl}(f^{-1}(A')).</math>
Moreover, 
:<math>f\colon (X,\mathrm{int}) \to (X' ,\mathrm{int}') \, </math>
is continuous if and only if 
:<math>f^{-1}(\mathrm{int}'(A)) \subseteq \mathrm{int}(f^{-1}(A))</math>
for any subset ''A'' of ''X''.