The European Physical Journal C - Particles and Fields

We are grateful to the Editor, to the Referee for careful reading of the manuscript, for the interesting and useful remarks, which allow us to improve the text and clarify some of the results.

Evgenij Martynov (Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine) and Basarab Nicolescu (Babes-Bolyai University, Cluj-Napoca, Romania)