= A splitting-field degree divides the factorial of the polynomial degree
For a <polynomial> $f\in K[x]$ of degree $n$, its <splitting field> $L$ has $[L:K]\mid n!$, including in positive characteristic when the <polynomial> is inseparable. Prove this by induction on the degree. If $f$ is irreducible, adjoining a root gives degree $n$ and leaves a polynomial of degree $n-1$ to split, so the <tower law for field extensions> gives the divisibility. If $f=gh$ with positive degrees $r,s<n$, split $g$ first and then $h$ over the resulting field. The degree divides $r!s!$, which divides $(r+s)!$. This separate reducible case avoids the invalid step of assuming that the degree of an arbitrarily chosen irreducible factor must divide $n$.
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