AB≠BA (18) 典例子:[xP]=x2.-n,x= d)对易式的代数恒等式: A B B. a AB+C=A.B+Ac ABC=BA.C+ABC ACB=ABC+AC B A, B,C+B, A, C+C, A,B=0AB BA (18) 典型例子: (d)对易式的代数恒等式: , x x x x p x p p x i =−= , , , , , , , , (19) , , , , , , , , , 0 A B B A A B C A B A C A BC B A C A B C AC B A B C A C B A B C B A C C A B = − + = + = + = + + + =