Y「B P B2 21 P Y B B 则有 Y=XB+u N,(0, o ln)记 则有 , , , u                          =                   1 1 1 2 2 2 n n p Y Y Y= = Y 11 1 21 2 1 1 1 1 p p n np x x x x X x x       =         2 ~ (0, ) n n Y X u u N I    = +  