Primitive Element Theorem

In field theory, the primitive element theorem is a result characterizing the finite degree field extensions that can be generated by a single element. Such a generating element is called a primitive element of the field extension, and the extension is called a simple extension in this case. The theorem states that a finite extension is simple if and only if there are only finitely many intermediate fields. An older result, also often called "primitive element theorem", states that every finite separable extension is simple; it can be seen as a consequence of the former theorem. These theorems imply in particular that all algebraic number fields over the rational numbers, and all extensions in which both fields are finite, are simple.

Terminology

Let $E/F$ be a ''field extension''. An element $\alpha\in E$ is a ''primitive element'' for $E/F$ if $E=F(\alpha),$ i.e. if every element of $E$ can be written as a rational function in $\alpha$ with coefficients in $F$. If there exists such a primitive element, then $E/F$ is referred to as a '' simple extension''.

If the field extension $E/F$ has primitive element $\alpha$ and is of finite degree n = :F/math>, then every element ''x'' of ''E'' can be written uniquely in the form

:$x=f_^+\cdots+f_1+f_0,$

where $f_i\in F$ for all ''i''. That is, the set

:$\$

is a basis for ''E'' as a vector space over ''F''.

Example

If one adjoins to the rational numbers $F = \mathbb$ the two irrational numbers $\sqrt$ and $\sqrt$ to get the extension field $E=\mathbb(\sqrt,\sqrt)$ of degree 4, one can show this extension is simple, meaning $E=\mathbb(\alpha)$ for a single $\alpha\in E$. Taking $\alpha = \sqrt + \sqrt$, the powers 1, α , α², α³ can be expanded as linear combinations of 1, $\sqrt$, $\sqrt$, $\sqrt$ with integer coefficients. One can solve this system of linear equations for $\sqrt$ and $\sqrt$ over $\mathbb(\alpha)$, to obtain $\sqrt = \tfrac12(\alpha^3-9\alpha)$ and $\sqrt = -\tfrac12(\alpha^3-11\alpha)$. This shows α is indeed a primitive element:
:$\mathbb(\sqrt 2, \sqrt 3)=\mathbb(\sqrt2 + \sqrt3).$

The theorems

The classical primitive element theorem states:
:Every separable field extension of finite degree is simple.

This theorem applies to algebraic number fields, i.e. finite extensions of the rational numbers Q, since Q has characteristic 0 and therefore every finite extension over Q is separable.

The following primitive element theorem ( Ernst Steinitz) is more general:
:A finite field extension $E/F$ is simple if and only if there exist only finitely many intermediate fields ''K'' with $E\supseteq K\supseteq F$.

Using the fundamental theorem of Galois theory, the former theorem immediately follows from the latter.

Characteristic ''p''

For a non-separable extension $E/F$ of characteristic p, there is nevertheless a primitive element provided the degree 'E'' : ''F''is ''p:'' indeed, there can be no non-trivial intermediate subfields since their degrees would be factors of the prime ''p''.

When 'E'' : ''F''= ''p''², there may not be a primitive element (in which case there are infinitely many intermediate fields). The simplest example is $E=\mathbb_p(T,U)$, the field of rational functions in two indeterminates ''T'' and ''U'' over the finite field with ''p'' elements, and $F=\mathbb_p(T^p,U^p)$. In fact, for any α = ''g''(T,U) in ''E'', the Frobenius endomorphism shows that the element ''α''^''p'' lies in ''F'' , so α is a root of f(X)=X^p-\alpha^p\in F /math>, and α cannot be a primitive element (of degree ''p''² over ''F''), but instead ''F''(α) is a non-trivial intermediate field.

Constructive results

Generally, the set of all primitive elements for a finite separable extension ''E'' / ''F'' is the complement of a finite collection of proper ''F''-subspaces of ''E'', namely the intermediate fields. This statement says nothing in the case of finite fields, for which there is a computational theory dedicated to finding a generator of the multiplicative group of the field (a cyclic group), which is ''a fortiori'' a primitive element (see primitive element (finite field)). Where ''F'' is infinite, a pigeonhole principle proof technique considers the linear subspace generated by two elements and proves that there are only finitely many linear combinations

:$\gamma = \alpha + c \beta\$

with ''c'' in ''F'', that fail to generate the subfield containing both elements:

:as $F(\alpha,\beta)/F(\alpha+c\beta)$ is a separable extension, if $F(\alpha+c\beta) \subsetneq F(\alpha,\beta)$ there exists a non-trivial embedding $\sigma : F(\alpha,\beta)\to \overline$ whose restriction to $F(\alpha+c\beta)$ is the identity which means $\sigma(\alpha)+c \sigma(\beta) = \alpha+c \beta$ and $\sigma(\beta) \ne \beta$ so that $c = \frac$. This expression for ''c'' can take only (\alpha):F (\beta):F/math> different values. For all other value of $c\in F$ then $F(\alpha,\beta) = F(\alpha+c\beta)$.

This is almost immediate as a way of showing how Steinitz' result implies the classical result, and a bound for the number of exceptional ''c'' in terms of the number of intermediate fields results (this number being something that can be bounded itself by Galois theory and ''a priori''). Therefore, in this case trial-and-error is a possible practical method to find primitive elements.

History

In his First Memoir of 1831, Évariste Galois sketched a proof of the classical primitive element theorem in the case of a splitting field of a polynomial over the rational numbers. The gaps in his sketch could easily be filled (as remarked by the referee Siméon Denis Poisson; Galois' Memoir was not published until 1846) by exploiting a theorem of Joseph-Louis Lagrange

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