Free non-archimedean topological groups

We study free topological groups defined over uniform spaces in some subclasses of the class NA of non-archimedean groups. Our descriptions of the corresponding topologies show that for metrizable uniformities the corresponding free balanced, free abelian and free Boolean NA groups are also metrizable. Graev type ultra-metrics determine the corresponding free topologies. Such results are in a striking contrast with free balanced and free abelian topological groups cases (in standard varieties). Another contrasting advantage is that the induced topological group actions on free abelian NA groups frequently remain continuous. One of the main applications is: any epimorphism in the category NA must be dense. Moreover, the same methods improve the following result of T.H. Fay [A note on Hausdorff groups, Bull. Austral. Math. Soc. 13 (1975), 117-119]: the inclusion of a proper open subgroup H → G ∈ TGR is not an epimorphism in the category TGR of all Hausdorff topological groups. A key tool in the proofs is Pestov's test of epimorphisms [V.G. Pestov, Epimorphisms of Hausdorff groups by way of topological dynamics, New Zealand J. Math. 26 (1997), 257-262]. Our results provide a convenient way to produce surjectively universal NA abelian and balanced groups. In particular, we unify and strengthen some recent results of Gao [Graev ultrametrics and surjectively universal non-Archimedean Polish groups, Topology Appl. 160 (2013), no. 6, 862-870] and Gao-Xuan [On non-Archimedean Polish groups with two-sided invariant metrics, preprint, 2012] as well as classical results about profinite groups which go back to Iwasawa and Gildenhuys-Lim [Free pro-C-groups, Math. Z. 125 (1972), 233-254].