let be a non-commutative ring, and define to be the set of zero-divisors of the ring . Suppose that , for any . Prove that is an ideal of .

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- March 19th 2010, 02:26 AMxixi[SOLVED] the set of zero-divisors of a ring
let be a non-commutative ring, and define to be the set of zero-divisors of the ring . Suppose that , for any . Prove that is an ideal of .

- March 19th 2010, 08:39 AMhatsoff
[edit: sorry for the error]

- March 19th 2010, 01:53 PMOpalg
Unfortunately, hatsoff's argument only works in the commutative case. In a noncommutative ring, in general.

However, it is true that if then . If you multiply on the left by or on the right by , you get 0. Provided that both and are nonzero, that says that and hence . Therefore (and that result trivially also holds if or is 0).

A similar argument, with and interchanged, shows that . Together with the previous result, that gives . But then , so .

That shows that D(R) is closed under addition. For the other property of an ideal, if and then clearly . But I cannot see any reason why (in a nocommutative ring) there should exist a nonzero element c such that . All I can conclude is that in the ring R, the set of left zero-divisors is a right ideal, and the set of right zero-divisors is a left ideal. - March 21st 2010, 01:21 AMxixithanks
thanks very much , you have already proved it ,as you have showed if and then which means that and this shows that is a right ideal . we also have which shows that is a left ideal and therefore is an ideal. (note that here is the set of left and right zero-divisors together and not just the two-sided ones.)