Galois theory

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In , Galois theory, originally introduced by , provides a connection between and . This connection, the , allows reducing certain problems in field theory to group theory, which makes them simpler and easier to understand. Galois introduced the subject for studying s of s. This allowed him to characterize the s that are solvable by radicals in terms of properties of the of their roots—an equation is ''solvable by radicals'' if its roots may be expressed by a formula involving only s, , and the four basic . This widely generalizes the , which asserts that a general polynomial of degree at least five cannot be solved by radicals. Galois theory has been used to solve classic problems including showing that two problems of antiquity cannot be solved as they were stated ( and ), and characterizing the s that are (this characterization was previously given by , but all known proofs that this characterization is complete require Galois theory). Galois' work was published by fourteen years after his death. The theory took longer to become popular among mathematicians and to be well understood. Galois theory has been generalized to s and .

# Application to classical problems

The birth and development of Galois theory was caused by the following question, which was one of the main open mathematical questions until the beginning of 19th century: The provides a counterexample proving that there are polynomial equations for which such a formula cannot exist. Galois' theory provides a much more complete answer to this question, by explaining why it ''is'' possible to solve some equations, including all those of degree four or lower, in the above manner, and why it is not possible for most equations of degree five or higher. Furthermore, it provides a means of determining whether a particular equation can be solved that is both conceptually clear and easily expressed as an . Galois' theory also gives a clear insight into questions concerning problems in construction. It gives an elegant characterization of the ratios of lengths that can be constructed with this method. Using this, it becomes relatively easy to answer such classical problems of as # Which s are ? # Why is it not possible to using a ? # Why is not possible with the same method?

# History

## Pre-history

Galois' theory originated in the study of – the coefficients of a are (up to sign) the s in the roots. For instance, , where 1, and are the elementary polynomials of degree 0, 1 and 2 in two variables. This was first formalized by the 16th-century French mathematician , in , for the case of positive real roots. In the opinion of the 18th-century British mathematician , the expression of coefficients of a polynomial in terms of the roots (not only for positive roots) was first understood by the 17th-century French mathematician ; Hutton writes:
...[Girard was] the first person who understood the general doctrine of the formation of the coefficients of the powers from the sum of the roots and their products. He was the first who discovered the rules for summing the powers of the roots of any equation.
In this vein, the is a symmetric function in the roots that reflects properties of the roots – it is zero if and only if the polynomial has a multiple root, and for quadratic and cubic polynomials it is positive if and only if all roots are real and distinct, and negative if and only if there is a pair of distinct complex conjugate roots. See for details. The cubic was first partly solved by the 15–16th-century Italian mathematician , who did not however publish his results; this method, though, only solved one type of cubic equation. This solution was then rediscovered independently in 1535 by , who shared it with , asking him to not publish it. Cardano then extended this to numerous other cases, using similar arguments; see more details at . After the discovery of del Ferro's work, he felt that Tartaglia's method was no longer secret, and thus he published his solution in his 1545 ''.'' His student solved the quartic polynomial; his solution was also included in ''Ars Magna.'' In this book, however, Cardano did not provide a "general formula" for the solution of a cubic equation, as he had neither at his disposal, nor the algebraic notation to be able to describe a general cubic equation. With the benefit of modern notation and complex numbers, the formulae in this book do work in the general case, but Cardano did not know this. It was who managed to understand how to work with complex numbers in order to solve all forms of cubic equation. A further step was the 1770 paper ''Réflexions sur la résolution algébrique des équations'' by the French-Italian mathematician , in his method of , where he analyzed Cardano's and Ferrari's solution of cubics and quartics by considering them in terms of ''permutations'' of the roots, which yielded an auxiliary polynomial of lower degree, providing a unified understanding of the solutions and laying the groundwork for group theory and Galois' theory. Crucially, however, he did not consider ''composition'' of permutations. Lagrange's method did not extend to quintic equations or higher, because the resolvent had higher degree. The quintic was almost proven to have no general solutions by radicals by in 1799, whose key insight was to use , not just a single permutation. His solution contained a gap, which Cauchy considered minor, though this was not patched until the work of the Norwegian mathematician , who published a proof in 1824, thus establishing the . While Ruffini and Abel established that the ''general'' quintic could not be solved, some ''particular'' quintics can be solved, such as , and the precise criterion by which a ''given'' quintic or higher polynomial could be determined to be solvable or not was given by , who showed that whether a polynomial was solvable or not was equivalent to whether or not the permutation group of its roots – in modern terms, its – had a certain structure – in modern terms, whether or not it was a . This group was always solvable for polynomials of degree four or less, but not always so for polynomials of degree five and greater, which explains why there is no general solution in higher degrees.

## Galois' writings

In 1830 Galois (at the age of 18) submitted to the a memoir on his theory of solvability by radicals; Galois' paper was ultimately rejected in 1831 as being too sketchy and for giving a condition in terms of the roots of the equation instead of its coefficients. Galois then died in a duel in 1832, and his paper, "''Mémoire sur les conditions de résolubilité des équations par radicaux''", remained unpublished until 1846 when it was published by accompanied by some of his own explanations. Prior to this publication, Liouville announced Galois' result to the Academy in a speech he gave on 4 July 1843. According to Allan Clark, Galois's characterization "dramatically supersedes the work of Abel and Ruffini."

## Aftermath

Galois' theory was notoriously difficult for his contemporaries to understand, especially to the level where they could expand on it. For example, in his 1846 commentary, Liouville completely missed the group-theoretic core of Galois' method. who attended some of Liouville's talks, included Galois' theory in his 1866 (third edition) of his textbook ''Cours d'algèbre supérieure''. Serret's pupil, , had an even better understanding reflected in his 1870 book ''Traité des substitutions et des équations algébriques''. Outside France, Galois' theory remained more obscure for a longer period. In Britain, failed to grasp its depth and popular British algebra textbooks did not even mention Galois' theory until well after the turn of the century. In Germany, Kronecker's writings focused more on Abel's result. Dedekind wrote little about Galois' theory, but lectured on it at Göttingen in 1858, showing a very good understanding. 's books of the 1880s, based on Jordan's ''Traité'', made Galois theory accessible to a wider German and American audience as did 's 1895 algebra textbook.

# Permutation group approach

Given a polynomial, it may be that some of the roots are connected by various s. For example, it may be that for two of the roots, say and , that . The central idea of Galois' theory is to consider s (or rearrangements) of the roots such that ''any'' algebraic equation satisfied by the roots is ''still satisfied'' after the roots have been permuted. Originally, the theory had been developed for algebraic equations whose coefficients are s. It extends naturally to equations with coefficients in any , but this will not be considered in the simple examples below. These permutations together form a , also called the of the polynomial, which is explicitly described in the following examples.

Consider the :$x^2 - 4x + 1 = 0.$ By using the , we find that the two roots are :$\begin A &= 2 + \sqrt,\\ B &= 2 - \sqrt. \end$ Examples of algebraic equations satisfied by and include :$A + B = 4,$ and :$AB = 1.$ If we exchange and in either of the last two equations we obtain another true statement. For example, the equation becomes . It is more generally true that this holds for ''every'' possible between and such that all are ; that is, in any such relation, swapping and yields another true relation. This results from the theory of s, which, in this case, may be replaced by formula manipulations involving the . One might object that and are related by the algebraic equation , which does not remain true when and are exchanged. However, this relation is not considered here, because it has the coefficient which is . We conclude that the Galois group of the polynomial consists of two permutations: the permutation which leaves and untouched, and the permutation which exchanges and . It is a of order two, and therefore to . A similar discussion applies to any quadratic polynomial , where , and are rational numbers. * If the polynomial has rational roots, for example , or , then the Galois group is trivial; that is, it contains only the identity permutation. In this example, if and then is no longer true when are are swapped. * If it has two roots, for example , then the Galois group contains two permutations, just as in the above example.

## Quartic equation

Consider the polynomial :$x^4 - 10x^2 + 1,$ which can also be written as :$\left\left(x^2 - 5\right\right)^2 - 24.$ We wish to describe the Galois group of this polynomial, again over the field of s. The polynomial has four roots: :$\begin A &= \sqrt + \sqrt,\\ B &= \sqrt - \sqrt,\\ C &= -\sqrt + \sqrt,\\ D &= -\sqrt - \sqrt. \end$ There are 24 possible ways to permute these four roots, but not all of these permutations are members of the Galois group. The members of the Galois group must preserve any algebraic equation with rational coefficients involving , , and . Among these equations, we have: :$\begin AB&=-1 \\ AC&=1 \\ A+D&=0 \end$ It follows that, if is a permutation that belongs to the Galois group, we must have: :$\begin \varphi\left(B\right)&=\frac, \\ \varphi\left(C\right)&=\frac, \\ \varphi\left(D\right)&=-\varphi\left(A\right). \end$ This implies that the permutation is well defined by the image of , and that the Galois group has 4 elements, which are: : : : : This implies that the Galois group is isomorphic to the .

# Modern approach by field theory

In the modern approach, one starts with a (read " over "), and examines the group of s of that fix . See the article on s for further explanation and examples. The connection between the two approaches is as follows. The coefficients of the polynomial in question should be chosen from the base field . The top field should be the field obtained by adjoining the roots of the polynomial in question to the base field. Any permutation of the roots which respects algebraic equations as described above gives rise to an automorphism of , and vice versa. In the first example above, we were studying the extension , where is the field of s, and is the field obtained from by adjoining . In the second example, we were studying the extension . There are several advantages to the modern approach over the permutation group approach. * It permits a far simpler statement of the . * The use of base fields other than is crucial in many areas of mathematics. For example, in , one often does Galois theory using s, s or s as the base field. * It allows one to more easily study infinite extensions. Again this is important in algebraic number theory, where for example one often discusses the of , defined to be the Galois group of where is an of . * It allows for consideration of s. This issue does not arise in the classical framework, since it was always implicitly assumed that arithmetic took place in zero, but nonzero characteristic arises frequently in number theory and in . * It removes the rather artificial reliance on chasing roots of polynomials. That is, different polynomials may yield the same extension fields, and the modern approach recognizes the connection between these polynomials.

# Solvable groups and solution by radicals

The notion of a in allows one to determine whether a polynomial is solvable in radicals, depending on whether its Galois group has the property of solvability. In essence, each field extension corresponds to a in a of the Galois group. If a factor group in the composition series is of order , and if in the corresponding field extension the field already contains a , then it is a radical extension and the elements of can then be expressed using the th root of some element of . If all the factor groups in its composition series are cyclic, the Galois group is called ''solvable'', and all of the elements of the corresponding field can be found by repeatedly taking roots, products, and sums of elements from the base field (usually ). One of the great triumphs of Galois Theory was the proof that for every , there exist polynomials of degree which are not solvable by radicals (this was proven independently, using a similar method, by a few years before, and is the ), and a systematic way for testing whether a specific polynomial is solvable by radicals. The Abel–Ruffini theorem results from the fact that for the contains a , noncyclic, , namely the .

## A non-solvable quintic example

cites the polynomial . By the this has no rational zeroes. Neither does it have linear factors modulo 2 or 3. The Galois group of modulo 2 is cyclic of order 6, because modulo 2 factors into polynomials of orders 2 and 3, . modulo 3 has no linear or quadratic factor, and hence is irreducible. Thus its modulo 3 Galois group contains an element of order 5. It is known that a Galois group modulo a prime is isomorphic to a subgroup of the Galois group over the rationals. A permutation group on 5 objects with elements of orders 6 and 5 must be the symmetric group , which is therefore the Galois group of . This is one of the simplest examples of a non-solvable quintic polynomial. According to , was fond of this example.

# Inverse Galois problem

The ''inverse Galois problem'' is to find a field extension with a given Galois group. As long as one does not also specify the , the problem is not very difficult, and all finite groups do occur as Galois groups. For showing this, one may proceed as follows. Choose a field and a finite group . says that is (up to isomorphism) a subgroup of the on the elements of . Choose indeterminates , one for each element of , and adjoin them to to get the field . Contained within is the field of symmetric s in the . The Galois group of is , by a basic result of Emil Artin. acts on by restriction of action of . If the of this action is , then, by the , the Galois group of is . On the other hand, it is an open problem whether every finite group is the Galois group of a field extension of the field of the rational numbers. proved that every solvable finite group is the Galois group of some extension of . Various people have solved the inverse Galois problem for selected non-Abelian s. Existence of solutions has been shown for all but possibly one ( ) of the 26 sporadic simple groups. There is even a polynomial with integral coefficients whose Galois group is the .

# Inseparable extensions

In the form mentioned above, including in particular the , the theory only considers Galois extensions, which are in particular separable. General field extensions can be split into a separable, followed by a . For a purely inseparable extension ''F'' / ''K'', there is a Galois theory where the Galois group is replaced by the of , $Der_K\left(F, F\right)$, i.e., ''K''- of ''F'' satisfying the Leibniz rule. In this correspondence, an intermediate field ''E'' is assigned $Der_E\left(F, F\right) \subset Der_K\left(F, F\right)$. Conversely, a $V \subset Der_K\left(F, F\right)$ satisfying appropriate further conditions is mapped to $\$. Under the assumption $F^p \subset K$, showed that this establishes a one-to-one correspondence. The condition imposed by Jacobson has been removed by , by giving a correspondence using notions of .