- published: 29 Sep 2013
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In mathematics, more specifically in abstract algebra, Galois theory, named after Évariste Galois, provides a connection between field theory and group theory. Using Galois theory, certain problems in field theory can be reduced to group theory, which is, in some sense, simpler and better understood.
Originally, Galois used permutation groups to describe how the various roots of a given polynomial equation are related to each other. The modern approach to Galois theory, developed by Richard Dedekind, Leopold Kronecker and Emil Artin, among others, involves studying automorphisms of field extensions.
Further abstraction of Galois theory is achieved by the theory of Galois connections.
The birth and development of Galois theory was caused by the following question, whose answer is known as the Abel–Ruffini theorem.
Galois theory not only provides a beautiful answer to this question, it also explains in detail why it is possible to solve equations of degree four or lower in the above manner, and why their solutions take the form that they do. Further, it gives a conceptually clear, and often practical, means of telling when some particular equation of higher degree can be solved in that manner.
Visual Group Theory, Lecture 6.6: The fundamental theorem of Galois theory The fundamental theorem of Galois theory guarantees a remarkable correspondence between the subfield lattice of a polynomial and the subgroup lattice of its Galois group. After illustrating this with a detailed example, we define what it means for a group to be "solvable". Galois proved that a polynomial is solvable by radicals if and only if its Galois group is solvable. We conclude by finding a degree-5 polynomial f(x) whose Galois group acts on the roots by a 5-cycle and by a 2-cycle. Since these two elements generate the (unsolvable) symmetric group S_5, the roots of f(x) are unsolvable by radicals. Course webpage (with lecture notes, HW, etc.): http://www.math.clemson.edu/~macaule/math4120-online.html
Galois theory gives a beautiful insight into the classical problem of when a given polynomial equation in one variable, such as x^5-3x^2+4=0 has solutions which can be expressed using radicals. Historically the problem of solving algebraic equations is one of the great drivers of algebra, with the quadratic equation going back to antiquity, and the discovery of the cubic solution by Italian mathematicians in the 1500's. Here we look at the quartic equation and give a method for factoring it, which relies on solving a cubic equation. We review the connections between roots and coefficients, which leads to the theory of symmetric functions and the identities of Newton. Lagrange was the key figure that introduced the modern approach to the subject. He realized that symmetries between the ro...
Field Theory: We define the Galois group of a polynomial g(x) as the group of automorphisms of the splitting field K that fix the base field F pointwise. The Galois group acts faithfully on the set of roots of g(x) and is isomorphic to a subgroup of a symmetric group. We also show that this action is transitive when g(x) is irreducible over F.
We continue our historical introduction to the ideas of Galois and others on the fundamental problem of how to solve polynomial equations. In this video we focus on Galois' insights into how extending our field of coefficients, typically by introducing some radicals, the symmetries of the roots diminishes. We get a correspondence between a descending chain of groups of symmetries, and an increasing chain of fields of coefficients. This was the key that allowed Galois to see why some equations were solvable by radicals and others not, and in particular to explain Ruffini and Abel's result on the insolvability of the general quintic equation. My research papers can be found at my Research Gate page, at https://www.researchgate.net/profile/.... I also have a blog at http://njwildberger.com/,...
In this seminar talk, we attempt to show (concretely!) how Galois' theory on permutations of roots helps us to *find* the roots of a cubic equation. In Part 1, an introduction to the theory of symmetric polynomials and a trip around a table of celebrity chefs.
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Visual Group Theory, Lecture 6.5: Galois group actions and normal field extensions If f(x) has a root in an extension field F of Q, then any automorphism of F permutes the roots of f(x). This means that there is a group action of Gal(f(x)) on the roots of f(x), and this action has only one orbit iff f(x) is irreducible. An extension of Q is said to be "normal" if it is the splitting field of some polynomial, and the degree of a normal extension of the order of its Galois group. We ilustrate these concept with several examples: the reducible polynomial x^4-5x^2+6, and the irreducible polynomial x^3-2. Course webpage (with lecture notes, HW, etc.): http://www.math.clemson.edu/~macaule/math4120-online.html
Visual Group Theory, Lecture 6.4: Galois groups The Galois group Gal(f(x)) of a polynomial f(x) is the automorphism group of its splitting field. The degree of a chain of field extensions satisfies a "tower law", analogous to the tower law for the index of a chain of subgroups. This hints at a deep connection between subfields of a splitting field and subgroups of its Galois group, which we will uncover soon. Also in this lecture, we learn how every finite degree extension of the rationals Q is "simple", which means that it is generated by a single element that we call a "primitive element". Course webpage (with lecture notes, HW, etc.): http://www.math.clemson.edu/~macaule/math4120-online.html
Évariste Galois was born 200 years ago and died aged 20, shot in a mysterious early-morning duel in 1832. He left contributions to the theory of equations that changed the direction of mathematics and led directly to what is now broadly described as 'modern' or 'abstract' algebra. In this lecture, designed for a general audience, Dr Peter Neumann will explain Galois' discoveries and place them in their historical context. Little knowledge of mathematics is assumed - the only prerequisite is sympathy for mathematics and its history. The transcript and downloadable versions of the lecture are available from the Gresham College website: http://www.gresham.ac.uk/lectures-and-events/the-memoirs-and-legacy-of-evariste-galois Gresham College has been giving free public lectures since 159...