Kummer’s lemma
The following result is a key ingredient in the proof of Fermat’s last theorem for regular primes
. More concretely, the lemma is needed to show the so-called second case of Fermat, i.e. does not have any non-trivial solutions in with a regular prime and . It is due to Ernst Kummer, thus the name.
Theorem (Kummer’s Lemma).
Let be a prime, let be a primitive th root of unity and let be the corresponding cyclotomic field
. Let be the group of algebraic units of the ring of integers . Suppose that is a regular prime. If a unit is congruent modulo to a rational integer, then is the th power of another unit also .
For a proof, see [Washington], Theorem 5.36. The reader may also be interested in generalizations due to [Washington 1992] and [Ozaki 1997].
References
- Ozaki 1997 Ozaki, M., Kummer’s lemma for -extensions
over totally real number fields, Acta Arith. 81 (1997), no. 1, 37–44.
- Washington Washington L. C., Introduction to CyclotomicFields, Second Edition, Springer-Verlag, New York.
- Washington 1992 Washington, L. C., Kummer’s lemma for prime power cyclotomic fields, J. Number Theory
40 (1992), no. 2, 165–173.