Ramification groups are a refinement of the Galois group of a finite Galois extension of local fields. We shall write for the valuation, the ring of integers and its maximal ideal for. As a consequence of Hensel's lemma, one can write for some where is the ring of integers of. Then, for each integer, we define to be the set of all that satisfies the following equivalent conditions.
operates trivially on
for all
The group is called -th ramification group. They form a decreasing filtration, In fact, the are normal by and trivial for sufficiently large by. For the lowest indices, it is customary to call the inertia subgroup of because of its relation to splitting of prime ideals, while the wild inertia subgroup of. The quotient is called the tame quotient. The Galois group and its subgroups are studied by employing the above filtration or, more specifically, the corresponding quotients. In particular,
The study of ramification groups reduces to the totally ramified case since one has for. One also defines the function. in the above shows is independent of choice of and, moreover, the study of the filtration is essentially equivalent to that of. satisfies the following: for, Fix a uniformizer of. Then induces the injection where. It follows from this
In particular, is a p-group and is solvable. The ramification groups can be used to compute the different of the extension and that of subextensions: If is a normal subgroup of, then, for,. Combining this with the above one obtains: for a subextension corresponding to, If, then. In the terminology of Lazard, this can be understood to mean the Lie algebra is abelian.
The ramification groups for a cyclotomic extension, where is a -th primitive root of unity, can be described explicitly: where e is chosen such that.
Example: a quartic extension
Let K be the extension of generated by. The conjugates of x1 are x2= x3 = −x1, x4 = −x2. A little computation shows that the quotient of any two of these is a unit. Hence they all generate the same ideal; call it. generates 2; =4. Now x1 − x3 = 2x1, which is in 5. and which is in 3. Various methods show that the Galois group of K is, cyclic of order 4. Also: and so that the different x1 satisfies x4 − 4x2 + 2, which has discriminant 2048 = 211.
Ramification groups in upper numbering
If is a real number, let denote where i the least integer. In other words, Define by where, by convention, is equal to if and is equal to for. Then for. It is immediate that is continuous and strictly increasing, and thus has the continuous inverse function defined on. Define is then called the v-th ramification group in upper numbering. In other words,. Note. The upper numbering is defined so as to be compatible with passage to quotients: if is normal in, then
Herbrand's theorem
Herbrand's theorem states that the ramification groups in the lower numbering satisfy , and that the ramification groups in the upper numbering satisfy. This allows one to define ramification groups in the upper numbering for infinite Galois extensions from the inverse system of ramification groups for finite subextensions. The upper numbering for an abelian extension is important because of the Hasse–Arf theorem. It states that if is abelian, then the jumps in the filtration are integers; i.e., whenever is not an integer. The upper numbering is compatible with the filtration of the norm residue group by the unit groups under the Artin isomorphism. The image of under the isomorphism is just
