Ramification group
In number theory, more specifically in local class field theory, the ramification groups are a filtration of the Galois group of a local field extension, which gives detailed information on the ramification phenomena of the extension.
Contents
Ramification groups in lower numbering
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
.[1] (This is stronger than the primitive element theorem.) Then, for each integer
, we define
to be the set of all
that satisfies the following equivalent conditions.
- (i)
operates trivially on
- (ii)
for all
- (iii)
The group is called
-th ramification group. They form a decreasing filtration,
In fact, the are normal by (i) and trivial for sufficiently large
by (iii). 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,
where
are the (finite) residue fields of
.[2]
is unramified.
is tamely ramified (i.e., the ramification index is prime to the residue characteristic.)
The study of ramification groups reduces to the totally ramified case since one has for
.
One also defines the function . (ii) in the above shows
is independent of choice of
and, moreover, the study of the filtration
is essentially equivalent to that of
.[3]
satisfies the following: for
,
Fix a uniformizer of
. Then
induces the injection
where
. (The map actually does not depend on the choice of the uniformizer.[4]) It follows from this[5]
is cyclic of order prime to
is a product of cyclic groups of order
.
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:[6]
If is a normal subgroup of
, then, for
,
.[7]
Combining this with the above one obtains: for a subextension corresponding to
,
If , then
.[8] In the terminology of Lazard, this can be understood to mean the Lie algebra
is abelian.
Example
Let K be generated by x1=. 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 π. generates π2; (2)=π4.
Now x1-x3=2x1, which is in π5.
and x1-x2=, which is in π3.
Various methods show that the Galois group of K is , cyclic of order 4. Also:
=
=
=
.
and =
=(13)(24).
= 3+3+3+1+1 = 11. so that the different
=π11.
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[9]
where, by convention, is equal to
if
and is equal to
for
.[10] 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:[11] if
is normal in
, then
for all
(whereas lower numbering is compatible with passage to subgroups.)
Herbrand's theorem states that the ramification groups in the lower numbering satisfy (for
where
is the subextension corresponding to
), and that the ramification groups in the upper numbering satisfy
.[12][13] This allows one to define ramification groups in the upper numbering for infinite Galois extensions (such as the absolute Galois group of a local field) 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.[14]
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[15]
Notes
- ↑ Neukirch (1999) p.178
- ↑ since
is canonically isomorphic to the decomposition group.
- ↑ Serre (1979) p.62
- ↑ Conrad
- ↑ Use
and
- ↑ Serre (1979) 4.1 Prop.4, p.64
- ↑ Serre (1979) 4.1. Prop.3, p.63
- ↑ Serre (1979) 4.2. Proposition 10.
- ↑ Serre (1967) p.156
- ↑ Neukirch (1999) p.179
- ↑ Serre (1967) p.155
- ↑ Neukirch (1999) p.180
- ↑ Serre (1979) p.75
- ↑ Neukirch (1999) p.355
- ↑ Snaith (1994) pp.30-31
See also
References
- B. Conrad, Math 248A. Higher ramification groups
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