Duality of Lipschitz-free spaces over ultrametric spaces

Abstract: We give a metric characterisation of when the Lipschitz-free space over a separable ultrametric space is a dual Banach space. In the case where the Lipschitz-free space has a predual we show that this predual is M-embedded if and only if the metric space is proper. We show that for ultrametric spaces, the little Lipschitz space is always an M-ideal in the corresponding space of Lipschitz functions and we show that this is not the case for metric spaces in general thus answering a question posed by Werner in the negative. Finally we show that the space of Lipschitz functions of an ultrametric space contains a strongly extreme point.