Lemma 11.4.2. Let A be a finite simple k-algebra. Then the center k' of A is a finite field extension of k.
Proof. Write A = \text{Mat}(n \times n, K) for some skew field K finite over k, see Theorem 11.3.3. By Lemma 11.4.1 the center of A is k \otimes _ k k' where k' \subset K is the center of K. Since the center of a skew field is a field, we win. \square
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