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We show that the number of horizons of static black holes can be stronglyconstrained by energy conditions of matter fields. After a carefulclarification on the "interior" of a black hole, we prove that if the interiorof a static black hole satisfies strong energy condition or null energycondition, there is at most one non-degenerated inner Killing horizon behindthe non-degenerated event horizon. Our result offers some universalrestrictions on the number of horizons. Interestingly and importantly, it alsosuggests that matter not only promotes the formation of event horizon but alsoprevents the appearance of multiple horizons inside black holes. Furthermore,using the geometrical construction, we obtain a radially conserved quantitywhich is valid for general static spacetimes.

Constraining the number of horizons with energy conditions