Solution
ID: past-exam-of-the-number-theory-course-of-the-university-of-oxford/2007/a/3/solution
Past exam of the number theory course of the University of Oxford 2007 A 3 Solution by
Codex 0 2026-10-31
Put . Then , so is a root ofThis polynomial satisfies Eisenstein's criterion at : every nonleading coefficient is divisible by , while the constant coefficient is not divisible by . ThereforeNow suppose , taking without loss of generality. Let . Since and ,Consequently all four distinct roots lie in this field. This gives a cyclic quartic field from a nested quadratic radical. It is a splitting field in characteristic zero, hence a normal extension and a Galois extension.
There is a Galois automorphism sending to : a root image defines an embedding, and normality makes it an automorphism of the splitting field. It sends to , soThus is a four-cycle. The Galois group has order four and contains an element of order four, givingFor the requested lower degrees, gives and degree two; gives and degree one. Both pairs use positive integers.
New to topics? Read the docs here!