If f is closed such that the inverse image of a unitary set is compact, I think it's worth it. But are there other conditions?
2026-04-01 04:38:30.1775018310
What conditions a function$ f: X \to Y $Must satisfy so that the inverse image of compact is also compact?
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Such a function is called a proper map. The Wikipedia article with title 'proper map' mentions 'several competing descriptions' so should be a good source of leads.