Lemma 11.4.4. Let A be a k-algebra. Let K be a central k-algebra which is a skew field. Then any two-sided ideal I \subset A \otimes _ k K is of the form J \otimes _ k K for some two-sided ideal J \subset A. In particular, if A is simple, then so is A \otimes _ k K.
Proof. Set J = \{ a \in A \mid a \otimes 1 \in I\} . This is a two-sided ideal of A. And I = J \otimes _ k K by Lemma 11.4.3. \square
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