Intrinsically Lipschitz sections and applications to metric groups

We introduce a notion of intrinsically Lipschitz graphs in the context of metric spaces. This is a broad generalization of what in Carnot groups has been considered by Franchi, Serapioni, and Serra Cassano, and later by many others. We proceed by focusing our attention on the graphs as subsets of a metric space given by the image of a section of a quotient map and we require an intrinsically Lipschitz condition. We shall not have any function on a topological product, nor we shall consider a metric on the base of the quotient map. Our results are an Ascoli–Arzelà compactness theorem, an Ahlfors regularity theorem, and some extension theorems for partially defined intrinsically Lipschitz sections. Known results by Franchi, Serapioni, and Serra Cassano, and by Vittone will be our corollaries.