# Quadratic extension of field of characteristic 2 can/cannot be obtained by adjoining a single square root

I understand that every degree 2 extension of fields of characteristic not equal to 2 can be obtained by adjoining the square root of a single element. It seems that for extensions in characteristic 2, this statement can be true and it can be false. May I please ask for two specific examples for either case, please? (P.S. I am a beginner in field theory so it would help if some explanations on the two examples can be given.)

Here are a few confusions:

1. It seems that I cannot adjoin a square which already exists in the smaller field. I can only adjoin a square root of an element (of the smaller field) which does not exist in the smaller field (By the following proposition from Artin: Does that mean I cannot adjoin any square root of the elements in $\Bbb Z_2$ because it contains all its square roots?

1. I still don't know how to formally prove that the field $\Bbb Z_2[x]/<x^2+x+1>$ cannot be obtained by adjoining a square root.

2. In which case an extension field can be obtained by adjoining a square root? Detailed Explanation needed. Thanks so much.

• I'm sure this has been handled on our site earlier. Anyway, show that $p(x)=x^2+x+1$ is irreducible over $\Bbb{F}_2$, but the extension field (of four elemens) you get by adjoining a zero of $p(x)$ cannot be gotten by adjoining a square root. OTOH, it is possible to extend the field $K=\Bbb{F}_2(x)$, $x$ transcendental, by adjoining $\sqrt x$. – Jyrki Lahtonen Mar 19 '17 at 9:28
• – Jyrki Lahtonen Mar 19 '17 at 9:34
• And another. I think this is a duplicate, but I am not sure which candidate is a better pick. – Jyrki Lahtonen Mar 19 '17 at 9:38
• Basically because $\Bbb{F}_2$ already contains the square roots of all its elements. You won't get anything new by adjoining one! The same applies to all the finite fields of characteristic two. All the square roots are already included. – Jyrki Lahtonen Mar 19 '17 at 9:39
• It is sometimes false, sometimes true. You cannot extend a finite field of characteristic two by adjoining a square root, but some infinite fields of characteristic two can be so extended. That's why I also mentioned $\Bbb{F}_2(x)$. – Jyrki Lahtonen Mar 19 '17 at 9:48