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Galois theory In mathematics, Galois theory, originally introduced by Évariste Galois, provides a connection between field (mathematics), field theory and group theory. This connection, the fundamental theorem of Galois theory, allows reducing certain problems ...
, a discipline within the field of abstract algebra, a resolvent for a permutation group ''G'' is a polynomial whose coefficients depend polynomially on the coefficients of a given polynomial ''p'' and has, roughly speaking, a rational number, rational root of a polynomial, root if and only if the Galois group of ''p'' is included in ''G''. More exactly, if the Galois group is included in ''G'', then the resolvent has a rational root, and the converse (logic), converse is true if the rational root is a simple root (polynomial), simple root. Resolvents were introduced by Joseph Louis Lagrange and systematically used by Évariste Galois. Nowadays they are still a fundamental tool to compute Galois groups. The simplest examples of resolvents are * X^2-\Delta where \Delta is the discriminant, which is a resolvent for the alternating group. In the case of a cubic equation, this resolvent is sometimes called the quadratic resolvent; its roots appear explicitly in the formulas for the roots of a cubic equation. * The resolvent cubic, cubic resolvent of a quartic function, quartic equation, which is a resolvent for the dihedral group of 8 elements. * The Quintic function#Solvable quintics, Cayley resolvent is a resolvent for the maximal resoluble Galois group in degree five. It is a polynomial of degree of a polynomial, degree 6. These three resolvents have the property of being ''always separable'', which means that, if they have a multiple root, then the polynomial ''p'' is not irreducible polynomial, irreducible. It is not known if there is an always separable resolvent for every group of permutations. For every equation the roots may be expressed in terms of nth root, radicals and of a root of a resolvent for a resoluble group, because, the Galois group of the equation over the field (mathematics), field generated by this root is resoluble.


Definition

Let be a positive integer, which will be the degree of the equation that we will consider, and an ordered list of indeterminate (variable), indeterminates. This defines the ''generic polynomial'' of degree  F(X)=X^n+\sum_^n (-1)^i E_i X^ = \prod_^n (X-X_i), where is the ''i''th elementary symmetric polynomial. The symmetric group group action, acts on the by permuting them, and this induces an action on the polynomials in the . The stabilizer (group theory), stabilizer of a given polynomial under this action is generally trivial, but some polynomials have a bigger stabilizer. For example, the stabilizer of an elementary symmetric polynomial is the whole group (mathematics), group . If the stabilizer is non-trivial, the polynomial is fixed by some non-trivial subgroup ; it is said an ''invariant'' of . Conversely, given a subgroup of , an invariant of is a resolvent invariant for if it is not an invariant of any bigger subgroup of .http://www.alexhealy.net/papers/math250a.pdf Finding invariants for a given subgroup of is relatively easy; one can sum the Orbit (group theory), orbit of a monomial under the action of . However it may occur that the resulting polynomial is an invariant for a larger group. For example, consider the case of the subgroup of of order 4, consisting of , , and the identity (for the notation, see Permutation group). The monomial gives the invariant . It is not a resolvent invariant for , as being invariant by , in fact, it is a resolvent invariant for the dihedral subgroup , and is used to define the resolvent cubic of the quartic equation. If is a resolvent invariant for a group of index (group theory), index , then its orbit under has order . Let be the elements of this orbit. Then the polynomial :R_G=\prod_^m (Y-P_i) is invariant under . Thus, when expanded, its coefficients are polynomials in the that are invariant under the action of the symmetry group and thus may be expressed as polynomials in the elementary symmetric polynomials. In other words, is an irreducible polynomial in whose coefficients are polynomial in the coefficients of . Having the resolvent invariant as a root, it is called a resolvent (sometimes resolvent equation). Consider now an irreducible polynomial :f(X)=X^n+\sum_^n a_i X^ = \prod_^n (X-x_i), with coefficients in a given field (typically the field of rationals) and roots in an algebraically closed extension, algebraically closed field extension. Substituting the by the and the coefficients of by those of in what precedes, we get a polynomial R_G^(Y), also called ''resolvent'' or ''specialized resolvent'' in case of ambiguity). If the Galois group of is contained in , the specialization of the resolvent invariant is invariant by and is thus a root of R_G^(Y) that belongs to (is rational on ). Conversely, if R_G^(Y) has a rational root, which is not a multiple root, the Galois group of is contained in .


Terminology

There are some variants in the terminology. * Depending on the authors or on the context, ''resolvent'' may refer to ''resolvent invariant'' instead of to ''resolvent equation''. * A Galois resolvent is a resolvent such that the resolvent invariant is linear in the roots. * The may refer to the linear polynomial \sum_^ X_i \omega^i where \omega is a primitive nth root of unity, primitive ''n''th root of unity. It is the resolvent invariant of a Galois resolvent for the identity group. * A relative resolvent is defined similarly as a resolvent, but considering only the action of the elements of a given subgroup of , having the property that, if a relative resolvent for a subgroup of has a rational simple root and the Galois group of is contained in , then the Galois group of is contained in . In this context, a usual resolvent is called an absolute resolvent.


Resolvent method

The Galois group of a polynomial of degree n is S_n or a proper subgroup of it. If a polynomial is separable and irreducible, then the corresponding Galois group is a transitive subgroup. Transitive subgroups of S_n form a directed graph: one group can be a subgroup of several groups. One resolvent can tell if the Galois group of a polynomial is a (not necessarily proper) subgroup of given group. The resolvent method is just a systematic way to check groups one by one until only one group is possible. This does not mean that every group must be checked: every resolvent can cancel out many possible groups. For example, for degree five polynomials there is never need for a resolvent of D_5: resolvents for A_5 and M_ give desired information. One way is to begin from maximal (transitive) subgroups until the right one is found and then continue with maximal subgroups of that.


References

* * {{Cite journal , last1 = Girstmair , first1 = K. , title = On the computation of resolvents and Galois groups , doi = 10.1007/BF01165834 , journal = Manuscripta Mathematica , volume = 43 , issue = 2–3 , pages = 289–307 , year = 1983 , s2cid = 123752910 Group theory Galois theory Equations