Past exam of the mathematics course of the University of Cambridge 2008 iii Paper 32 1 iii Solution Created 2026-10-03 Updated 2026-10-14
Put and . The Eisenstein criterion at shows that is an irreducible polynomial, so . Its derivative is , and the polynomial discriminant isThe discriminant-index formula for an integral lattice implies that the field discriminant has no prime divisor outside . By the discriminant obstruction to ramification, every other prime is unramified.
To prove that neither candidate disappears through an index correction, let lie above and normalize its valuation to have value group . If , the relation givesIn both cases . The fundamental identity for prime decomposition, , now forces exactly one prime ideal above each candidate, with and . This is ramification forced by a coprime root valuation; at it avoids an invalid direct application of the Eisenstein criterion, since . Hence