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Field norm


In mathematics, the (field) norm is a particular mapping defined in field theory, which maps elements of a larger field into a subfield.

Let K be a field and L a finite extension (and hence an algebraic extension) of K. The field L is then a finite dimensional vector space over K. Multiplication by α, an element of L,

is a K-linear transformation of this vector space into itself. The norm, NL/K(α), is defined as the determinant of this linear transformation.

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).

More particularly, if L/K is a Galois extension and α is in L, then the norm of α is the product of all the Galois conjugates of α, i.e.

where Gal(L/K) denotes the Galois group of L/K.

The field norm from the complex numbers to the real numbers sends

to

because the Galois group of over has two elements, the identity element and complex conjugation, and taking the product yields (x + iy)(xiy) = x2 + y2.


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