On stability of CR-mappings between nilpotent lie groups of step two

A quasi-CR-mapping from a nilpotent Lie group $\mathscr N_b$ of step two to another such group satisfies a Beltrami-type system of partial differential equations which is usually not elliptic but subelliptic when the group $\mathscr N_b$ is strongly 2-pseudoconcave. We derive an integral representation formula for CR-mappings from a strongly 2-pseudoconcave nilpotent Lie group of step two to another such group and establish the Hölder continuity of $\varepsilon$-quasi-CR-mappings and the stability of CR-mappings between such groups.