Galois theory

In mathematics, Galois theory, originally introduced by Évariste Galois, provides a connection between field theory and group theory. This connection, the fundamental theorem of Galois theory, allows reducing to group theory certain problems in field theory; this makes them simpler in some sense, and allows a better understanding.

Lattice diagram of Q adjoin the positive square roots of 2 and 3, its subfields, and Galois groups.

Galois introduced the subject for studying roots of polynomials. This allowed him to characterize the polynomial equations that are solvable by radicals in terms of properties of the permutation group of their roots—an equation is solvable by radicals if its roots may be expressed by a formula involving only integers, nth roots, and the four basic arithmetic operations. This widely generalizes the Abel–Ruffini theorem, 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 (doubling the cube and trisecting the angle), and characterizing the regular polygons that are constructible (this characterization was previously given by Gauss, but all known proofs that this characterization is complete require Galois theory).

Galois' work was published fourteen years after his death by Joseph Liouville. The theory took longer to become popular among mathematicians and to be well understood.

Galois theory has been generalized to Galois connections and Grothendieck's Galois theory.