For nonzero α in L, let σ1(α), ..., σn(α) be the roots (counted with multiplicity) of the minimal polynomial of α over K (in some extension field of L), then
If L/K is separable then each root appears only once in the product (the exponent [L:K(α)] may still be greater than 1).
- x + iy
- x2 + y2,
because the Galois group of over has two elements, the identity element and complex conjugation, and taking the product yields (x + iy)(x − iy) = x2 + y2.
In this example the norm was the square of the usual Euclidean distance norm in . In general, the field norm is very different from the usual distance norm. We will illustrate that with an example where the field norm can be negative. Consider the number field . The Galois group of over has order d = 2 and is generated by the element which sends to . So the norm of is:
The field norm can also be obtained without the Galois group. Fix a -basis of , say : then multiplication by the number sends 1 to and to . So the determinant of "multiplying by is the determinant of the matrix which sends the vector (corresponding to the first basis element, i.e. 1) to and the vector (which represents the second basis element ) to , viz.:
The determinant of this matrix is −1.
Properties of the norm
Several properties of the norm function hold for any finite extension.
The norm NL/K : L* → K* is a group homomorphism from the multiplicative group of L to the multiplicative group of K, that is
Furthermore, if a in K:
If a ∈ K then
Additionally, norm behaves well in towers of fields: if M is a finite extension of L, then the norm from M to K is just the composition of the norm from M to L with the norm from L to K, i.e.
In this setting we have the additional properties,
The norm of an algebraic integer is again an integer, because it is equal (up to sign) to the constant term of the characteristic polynomial.
In algebraic number theory one defines also norms for ideals. This is done in such a way that if I is an ideal of OK, the ring of integers of the number field K, N(I) is the number of residue classes in – i.e. the cardinality of this finite ring. Hence this norm of an ideal is always a positive integer. When I is a principal ideal αOK then N(I) is equal to the absolute value of the norm to Q of α, for α an algebraic integer.
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- Mullen, Gary L.; Panario, Daniel (2013), Handbook of Finite Fields, CRC Press, ISBN 978-1-4398-7378-6
- Roman, Steven (2006), Field theory, Graduate Texts in Mathematics 158 (Second ed.), Springer, Chapter 8, ISBN 978-0-387-27677-9, Zbl 1172.12001
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