Hacene Belbachir, Laszlo Szalay
E-mail:
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Address:

Faculty of Mathematics,

University of Sciences and Technology Houari Boumediene,

P.B. 32 El Alia 16111, Algiers, Algeria;

Institute of Mathematics,  University of West Hungary,

H-9400 Sopron, Ady E. ut 5., Hungary;