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Let 
 be a Field Extension of 
, denoted 
, and let 
 be the set of Automorphisms of
, that is, the set of Automorphisms 
 of 
 such that 
 for every 
,
so that 
 is fixed.  Then 
 is a Group of transformations of 
, called the Galois group of 
.
The Galois group of 
 consists of the Identity Element and Complex Conjugation.  These functions both take a given Real to the same real.
See also Abhyankar's Conjecture, Finite Group, Group
References
Jacobson, N.  Basic Algebra I, 2nd ed.  New York: W. H. Freeman, p. 234, 1985.