Pointwise Lipschitz functions on metric spaces

For a metric space X, we study the space D∞(X) of bounded functions on X whose pointwise Lipschitz constant is uniformly bounded. D∞(X) is compared with the space LIP∞(X) of bounded Lipschitz functions on X, in terms of different properties regarding the geometry of X. We also obtain a Banach–Stone theorem in this context. In the case of a metric measure space, we also compare D∞(X) with the Newtonian–Sobolev space N1,∞(X). In particular, if X supports a doubling measure and satisfies a local Poincaré inequality, we obtain that D∞(X)=N1,∞(X).