Show that x -> x^p is an automorphism in K

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Homework Statement
Let K be a finite field of characteristic p > 0. Show that x -> x^p is an automorphism in K.
Relevant Equations
s(x) = x^p
It is clear in case K is a prime field, because then s(x) is just an identity. If K is not prime, s(x) is identity on its prime subfield. But how to show the automorphism of s() for the rest of the field, when it's not prime?
 
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Hill said:
Homework Statement: Let K be a finite field of characteristic p > 0. Show that x -> x^p is an automorphism in K.
Relevant Equations: s(x) = x^p

It is clear in case K is a prime field, because then s(x) is just an identity. If K is not prime, s(x) is identity on its prime subfield. But how to show the automorphism of s() for the rest of the field, when it's not prime?
##s(x\cdot y)=(x\cdot y)^p=x^p\cdot y^p=s(x)\cdot s(y)## is obvious since ##K## is Abelian. To see that ##s(x+y)=s(x)+s(y)## use the binomial theorem. ##s(x)## is called Frobenius homomorphism.
 
fresh_42 said:
##s(x\cdot y)=(x\cdot y)^p=x^p\cdot y^p=s(x)\cdot s(y)## is obvious since ##K## is Abelian. To see that ##s(x+y)=s(x)+s(y)## use the binomial theorem.
Thank you. It shows that s() is homomorphism. But how to show that it is injective and surjective in K?
 
Hill said:
Homework Statement: Let K be a finite field of characteristic p > 0. Show that x -> x^p is an automorphism in K.
Relevant Equations: s(x) = x^p

It is clear in case K is a prime field, because then s(x) is just an identity. If K is not prime, s(x) is identity on its prime subfield. But how to show the automorphism of s() for the rest of the field, when it's not prime?
What about:

Suppose ##x^p = y^p##, then ##(xy^{-1})^p = 1##. Therefore ##xy^{-1}## has order ##1## or ##p##.

The order of the field is ##p^n##, hence the order of the multiplicative group is ##p^n -1##. As ##p## cannot divide ##p^n - 1##, the order of ##xy^{-1}## is ##1##. Hence ##x = y##, and the mapping is injective.
 
PeroK said:
What about:

Suppose ##x^p = y^p##, then ##(xy^{-1})^p = 1##. Therefore ##xy^{-1}## has order ##1## or ##p##.

The order of the field is ##p^n##, hence the order of the multiplicative group is ##p^n -1##. As ##p## cannot divide ##p^n - 1##, the order of ##xy^{-1}## is ##1##. Hence ##x = y##, and the mapping is injective.
Thank you. This is good. Now, for the surjective part ...?
 
Hill said:
Thank you. This is good. Now, for the surjective part ...?
That follows automatically for a finite set.
 
PeroK said:
That follows automatically for a finite set.
I am sorry. I guess it should be obvious, but I don't see it. Could you elaborate, how?
 
Hill said:
I am sorry. I guess it should be obvious, but I don't see it. Could you elaborate, how?
A mapping from a finite set to itself is injective iff it is surjective. You only have to count the elements in the range.
 
PeroK said:
A mapping from a finite set to itself is injective iff it is surjective. You only have to count the elements in the range.
Thanks a lot. Just wanted to tell you that I got it, but you have already answered.
 
  • #10
Just to point out that a homomorphism between fields is always injective. If not it will have a kernel, which is an ideal. Fields have no proper ideals.
 
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  • #11
Notice too, the Binomial Theorem assumes commutativity of the product, as in ##(a+b)^2= a^2+ab +ba + b^2## assumes ##ab=ba##, so this won't hold for general rings. Same for the argument ##(ab)^p=a^pb^p##.
 
  • #12
"Just to point out that a homomorphism between fields is always injective. If not it will have a kernel, which is an ideal. Fields have no proper ideals"
nice, and I guess you have to check the homomorphism is unitary, i.e. as here, that 1-->1. (so that the kernel is a proper ideal.) but of course you knew that.
 
  • #13
mathwonk said:
"Just to point out that a homomorphism between fields is always injective. If not it will have a kernel, which is an ideal. Fields have no proper ideals"
nice, and I guess you have to check the homomorphism is unitary, i.e. as here, that 1-->1. (so that the kernel is a proper ideal.) but of course you knew that.
I guess outside of the Complex Numbers, ##1^x=1## always holds.
 
  • #14
you just checked it!
 
  • #15
Hill said:
I am sorry. I guess it should be obvious, but I don't see it. Could you elaborate, how?
Pigeons and pigeon housing. :smile:
 
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