Area formula for centered Hausdorff measures in metric spaces

Motivated by an example in Magnani (in press), we study, inside a separable metric space (X,d), the relations between centered and non centered m-dimensional densities of a Radon measure \u3bc in X and their relations with spherical and centered spherical m-dimensional Hausdorff measures. Eventually we give an application to finite perimeter sets in Carnot groups