Define the notion aut (k/f)

Suppose K is an extension field of a field F. Consider the set of all automorphisms on K, which infact forms a group with respect to composition as operation denoted by Aut(K). Th...

Here fixes means.... if g is an element of Aut (K/F) , then g is an automorphism on K such g acts as an identity map on the elements of the field F