Published online by Cambridge University Press: 18 May 2009
Let A be an associative ring. Given a ∊ A, an element b ∊ A is called a left identity for a if
Given a subset S of A, an element b ∊ A is, called a left identity for S if (1) is satisfied for all a ∊ S. An element of A need not have a left identity; for example, if A is nilpotent then no non-zero element of A has a left identity. If a does have a left identity, the latter need not be unique; if every element of a subset S of A has a left identity, then it is not necessarily true that S has a left identity.