Answer to Question #23466 in Algebra for john.george.milnor
Let A ideal in a semiprime ring R. Show that annr(A) = annl(A).
We shall prove: if A is a leftideal, thenannl(A) ⊆annr(A). (This clearly givesthe desired statement, by symmetry.) Indeed, let B = annl(A), which isalso a left ideal. By definition, BA = 0, it is iff AB = 0, that is, B ⊆annr(A). By symmetry we obtain the result.
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