Show that the Galois group of a polynomial of degree n has orderdividing n!

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter4: More On Groups
Section4.5: Normal Subgroups
Problem 8E: Show that every subgroup of an abelian group is normal.
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Show that the Galois group of a polynomial of degree n has order
dividing n!

Expert Solution
Step 1

Let us consider that the polynomial,

fx=i=0ncixin, ciF

Also let K be the splitting field of fx over F and ϕGalK/F.

Let S=a1,a2,,an be the set of root of fx then we see that ϕa1,ϕa2,,ϕan are distinct.

Let lmn.

Since, ϕ fixes F, we can conclude that,

ϕfam=ϕi=0lciami=i=0lciϕami

Since, am is root of fx, so fm=0.

It follows that fϕam=0.

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