Kuiper's theorem

Kuiper's theorem is a result in the field of functional analysis, specifically within the study of Banach spaces and the theory of linear operators. It characterizes when a linear operator between two Banach spaces is compact. The theorem states that if \( X \) and \( Y \) are two Banach spaces, and if \( T: X \to Y \) is a continuous linear operator, then the following are equivalent: 1. The operator \( T \) is compact.