Dixmier–Ng theorem

In functional analysis, the Dixmier–Ng theorem is a characterization of when a normed space is in fact a dual Banach space.

It was proven by Kung-fu Ng, who called it a variant of a theorem proven earlier by Jacques Dixmier.

That 1. implies 2. is an application of the Bipolar theorem.

The Dixmier-Ng Theorem is applied to show that the Lipschitz space

(endowed with the Lipschitz constant as norm) is a dual Banach space.