Past exam of the mathematics course of the University of Cambridge 2001 iii Paper 74 2 ii Solution Created 2026-10-03 Updated 2026-10-14
Let . Each field automorphism preserves the ring of integers and the base prime ideal, so it permutes the prime ideals above . Suppose there were more than one orbit, and fix one orbit . By the Chinese remainder theorem for ideals, choose such thatThe field norm lies in . It lies in every prime ideal of , since for a fixed member each conjugate of is zero modulo that member. Thus its contraction lies in . On the other hand, for outside , every is also outside it, so every equals one modulo . The product is consequently one modulo . This contradicts its membership in . Therefore the Galois action on the primes above is transitive. The normal extension of number fields is separable, so its conjugates are exactly the elements of used in the product.
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.