Krasner's lemma (source code)

= Krasner's lemma
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Let $K$ be complete, let $\alpha$ be separable over $K$, and let $\beta$ be algebraic over $K$. If $\beta$ is closer to $\alpha$ than any other $K$-conjugate of $\alpha$, then
$$
K(\alpha)\subseteq K(\beta).
$$