- In between the fields Q and R there is another ( perhaps hypothetical field ) called A, the field of algebraic numbers. It contains of all quotients plus all numbers that are solutions to polynomial equations with coefficients in Q. For example Sqrt(2) is not a quotient but can be expressed as the solution of the equation x^2-2=0. So A is equal to Q plus all numbers like Sqrt(2).
- Something very interesting happens if we add ( adjoin ) Sqrt(2) to Q: Q remains a field! ( The field Q is an abelian group for + and *, the operations + and * are related via the distributive laws. Identities are 0 for + and 1 for * ). It can be proved trivially that { x | x = a + b*Sqrt(2) , a,b in Q } is a field. This field is written like Q(Srt(2)), or Q/(X^2-2) and is called an extension field.
- If we put on our Linear Algebra glasses we could say that a + b*Sqrt(2) is in fact a vector (a,b) over the basis {1, Sqrt(2)}.
- The roots of the equation X^2-2 have a C2 symmetry, the roots of X^3-2 have a Dihedral Group 3 symmetry. Investigating the symmetry of the roots of equations is a task in Galois Theory. The symmetry group is called the Galois Group, Gal(E/F). In our example E=Q(Sqrt(2)) and F=Q.
- Now the Fundamental Theory of Galois Theory ( FTGT ) says that there is a 1-to-1 correspondence between subgroups of Gal(E/F) and fields intermediate E and F.
Fascinating stuff. Unfortunately Galois Theory is not part of any Open University course I know of.
The book I am reading on Galois Theory is:
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