Thursday, January 14, 2016

Prescribing norm and trace of elements in a finite field.

Let Fq be the finite field with q elements, where q is a prime power.



Let n1 be an integer and consider Fqn|Fq.




There is a theorem that says the following:



Theorem: There is always an element αFqn that is primitive and normal over Fq.



We say that one can prescribe the norm and the trace of a primitive and normal (over Fq) element αFqn if, for every a,bFq, with b primitive, there is a primitive and normal element αFqn such that TrFqn|Fq(α)=a and NFqn|Fq(α)=b.



The assumption that a is non-zero is because normal elements cannot have zero trace and a primitive element αFqn must have norm a primitive element of Fq.



My point is, the article I'm reading asserts that if n2, such α is already prescribed by its trace and norm, but I cannot see this. Can anyone help me?




The case Fq2|Fq we then have Tr(α)=α+αq and N(α)=αq+1. I cannot see why all possible values for the norm and trace in Fq are achieved by primitive normal elements of Fq2.



Edit: I'm trying to think about it's minimal polynomial. There is a fact (I will not prove here but it is true): In Fq2|Fq, every primitive element is also normal. So, the minimal polynomial of a primitive normal element of Fq2 must be



X2aX+b, where a=Tr(α) and b=N(α). Still cannot see why every possible value for N(α) (any primitive element of Fq) and Tr(α) (any non-zero element of Fq) can be achieved.

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