example of normal extension
Let . Then the extension is normal because is clearly the splitting field of the polynomial
. Furthermore is a Galois extension
with .
Now, let denote the positive real fourth root of and define . Then the extension is normal because is the splitting field of , and as before is a Galois extension with .
However, the extension is neither normal nor Galois. Indeed, the polynomial has one root in (actually two), namely , and yet does not split in into linear factors.
The Galois closure of over is .