A function f: X -> Y mapping elements in the [[topological space]] X to the topological space Y is continuous if for every [[open set]] in Y, the inverse image of Y under f is an open subset of X.
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A function f: X -> Y mapping elements in a [[topological space]] X to a topological space Y is continuous if for every [[open set]] in Y, the inverse image of Y under f is an open subset of X.