Affiliation: | aDepartment of Mathematics and Mathematical Statistics, Umeå University, SE-901 87 Umeå, Sweden;bJagiellonian University, Institute of Mathematics, Łojasiewicza 6, 30-348 Kraków, Poland;cDepartment of Mathematics, Truong Dai Hoc Su Pham, 136 Xuan Thuy, Cau Giay, Hanoi, Viet Nam;dUniversité Paul Sabatier, Institut de Mathématiques, 118 Route de Narbonne, 31062 Toulouse cedex, France |