Sekundarni povzetek: |
Let ▫$R$▫ be a ring. A biadditive symmetric mapping ▫$D(.,.):R \times R \to R$▫ is called a symmetric bi-derivation if, for any fixed ▫$y \in R$▫, a mapping ▫$x \mapsto D(x,y)$▫ is a derivation. The purpose of this paper is to prove some results concerning symmetric bi-derivations on prime and semi-prime rings. We prove that existence of a nonzero symmetric bi-derivation ▫$D(.,.): R\times R \to R$▫ where ▫$R$▫ is a prime ring of characteristic not two, with the property ▫$D(x,x)x = xD(x,x), \; x \in R$▫, forces ▫$R$▫ to be commutative. A theorem in the spirit of a classical result first proved by E. Posner, which states that, if ▫$R$▫ is a prime ring of characteristic not two and ▫$D_1$▫, ▫$D_2$▫ are nonzero derivations on ▫$R$▫, then the mapping ▫$x \mapsto D_1(D_2(x))$▫ cannot be a derivation, is also presented. |