December 26th, 2011
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: firstname.lastname@example.org
A very nice summary of Galois Theory, which I have read about before but which I still don’t understand. But, you know, every time I read about it, I feel like I come closer to understanding it.
For a long time, people wondered whether it is possible to write down something like the “quadratic formula” for cubic, quartic and quintic polynomials with integer coefficients. We now know that for cubic and quartic polynomials, this is possible. But for degree 5 polynomials and beyond, it isn’t. A proof of this was scribbled down hastily by Galois the night before his duel. Galois linked together field theory and group theory in a beautiful way to answer this very question.
Galois’s Approach: The Big Idea
What does writing down a “formula” for roots of a polynomial really mean? For one, we’d be writing down the roots in terms of rational numbers and a combination of +, -, x, ÷, and radicals (taking n-th roots). This is a very limited set of operations, and certainly not all real numbers can be written this way — π clearly can’t be written this way. We say that π is not solvable in radicals.
Are the roots of polynomials with integer coefficients solvable in radicals? Those roots aren’t just any real number, and certainly π is not a root of any polynomial with integer coefficients. Yet Galois showed that there are some degree-5 polynomials with roots that are not solvable in radicals. To see how he did this, we first need some terminology about fields, field extensions, and groups.