1. Institute of Mathematics with Computing Centre, Ufa Federal Research Centre of Russian Academy of Sciences, Ufa, Russia;
2. Institut de Mathématiques de Jussieu–Paris Rive Gauche, UFR de Mathématiques, Université Paris Diderot - Paris 7, Paris Cedex 13, France;
3. Universität zu Köln, Mathematisches Institut, Köln, Germany;
4. Institute of Mathematics ‘Simion Stoilow’, Romanian Academy, Bucharest, Romania