TITLE: Is a finite field extension of a imperfect field imperfect
QUESTION [5 upvotes]: Let $K$ be a imperfect field. Let $L/K$ be a finite field extension.
Is $L$ imperfect?
Suppose that $L/K$ is separable. Is $L$ imperfect?
Suppose that $L/K$ is Galois. Is $L$ imperfect?
I'm looking for examples, counterexamples, explanations, and proofs of course.
Note: The converse holds. If $K$ is perfect, then any finite field extension of $K$ is perfect.

REPLY [10 votes]: Yes, if $K$ is imperfect then any  finite extension $L$ of $K$ is also imperfect.
Indeed if $char.K=p$ and $a\in K$ has no $p$-th root in $K$, then any perfect field $P$ containing $K$  must contain all $a^{p^{-n}} \: (n\geq 1)$ and, since $[K(a^{p^{-n}}):K]=p^n$,  the field $P$ must be of infinite dimension over  $K$ (since its dimension is $\geq p^n$ for all $n$).
The fact that $L$ is separable or Galois over $K$ will not help: I'm sorry to say that $L$ will still be imperfect (but aren't we all...)