... Yen [11 ], P Daniele, A Maugeri va W Oettli [2], M De Luca [22], M Florian, N Hadjisavvas va S Schaible [18 , 20], X.Q Yang va C.J Goh [8], B Ricceri [ 21] , Q.H Ansari [28, 19 ], P.Q Khanh va 1. M ... mot s6 lint v1jCkhac toan hQc nhu la trudng h...