Perfect field

A field $k$ over which every polynomial is separable. In other words, every algebraic extension of $k$ is a separable extension. All other fields are called imperfect. Every field of characteristic 0 is perfect. A field $k$ of finite characteristic $p$ is perfect if and only if $k = k^p$, that is, if raising to the power $p$ is an automorphism of $k$. Finite fields and algebraically closed fields are perfect. An example of an imperfect field is the field $\mathbb{F}_q(X)$ of rational functions over the field $\mathbb{F}_q$, where $\mathbb{F}_q$ is the field of $q = p^n$ elements. A perfect field $k$ coincides with the field of invariants of the group of all $k$-automorphisms of the algebraic closure $\bar k$ of $k$. Every algebraic extension of a perfect field is perfect.

For any field $k$ of characteristic $p>0$ with algebraic closure $\bar k$, the field $$ k^{p^{-\infty}} = \bigcup_n k^{p^{-n}} \subset \bar k $$ is the smallest perfect field containing $k$. It is called the perfect closure of the field $k$ in $\bar k$.

References

[1]	N. Bourbaki, "Elements of mathematics. Algèbre" , Masson (1981) pp. Chapts. 4–5
[2]	O. Zariski, P. Samuel, "Commutative algebra" , 1 , Springer (1975)