Discrete Valuations and Discrete Pseudo-Valuations
==================================================
.. linkall
High-Level Interface
--------------------
Valuations can be defined conveniently on some Sage rings such as p-adic rings
and function fields.
p-adic valuations
~~~~~~~~~~~~~~~~~
Valuations on number fields can be easily specified if they uniquely extend
the valuation of a rational prime::
sage: v = QQ.valuation(2)
sage: v(1024)
10
They are normalized such that the rational prime has valuation 1::
sage: K.<a> = NumberField(x^2 + x + 1)
sage: v = K.valuation(2)
sage: v(1024)
10
If there are multiple valuations over a prime, they can be obtained by
extending a valuation from a smaller ring::
sage: K.<a> = NumberField(x^2 + x + 1)
sage: K.valuation(7)
Traceback (most recent call last):
...
ValueError: The valuation Gauss valuation induced by 7-adic valuation does not approximate a unique extension of 7-adic valuation with respect to x^2 + x + 1
sage: w,ww = QQ.valuation(7).extensions(K)
sage: w(a + 3), ww(a + 3)
(1, 0)
sage: w(a + 5), ww(a + 5)
(0, 1)
Valuations on Function Fields
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Similarly, valuations can be defined on function fields::
sage: K.<x> = FunctionField(QQ)
sage: v = K.valuation(x)
sage: v(1/x)
-1
sage: v = K.valuation(1/x)
sage: v(1/x)
1
On extensions of function fields, valuations can be created by providing a
prime on the underlying rational function field when the extension is unique::
sage: K.<x> = FunctionField(QQ)
sage: R.<y> = K[]
sage: L.<y> = K.extension(y^2 - x)
sage: v = L.valuation(x)
sage: v(x)
1
Valuations can also be extended from smaller function fields::
sage: K.<x> = FunctionField(QQ)
sage: v = K.valuation(x - 4)
sage: R.<y> = K[]
sage: L.<y> = K.extension(y^2 - x)
sage: v.extensions(L)
[[ (x - 4)-adic valuation, v(y + 2) = 1 ]-adic valuation,
[ (x - 4)-adic valuation, v(y - 2) = 1 ]-adic valuation]
Low-Level Interface
-------------------
Mac Lane valuations
~~~~~~~~~~~~~~~~~~~
Internally, all the above is backed by the algorithms described in
[Mac1936I]_ and [Mac1936II]_. Let us consider the extensions of
``K.valuation(x - 4)`` to the field `L` above to outline how this works
internally.
First, the valuation on `K` is induced by a valuation on `\QQ[x]`. To construct
this valuation, we start from the trivial valuation on `\\Q` and consider its
induced Gauss valuation on `\\Q[x]`, i.e., the valuation that assigns to a
polynomial the minimum of the coefficient valuations::
sage: R.<x> = QQ[]
sage: v = GaussValuation(R, valuations.TrivialValuation(QQ))
The Gauss valuation can be augmented by specifying that `x - 4` has valuation 1::
sage: v = v.augmentation(x - 4, 1); v
[ Gauss valuation induced by Trivial valuation on Rational Field, v(x - 4) = 1 ]
This valuation then extends uniquely to the fraction field::
sage: K.<x> = FunctionField(QQ)
sage: v = v.extension(K); v
(x - 4)-adic valuation
Over the function field we repeat the above process, i.e., we define the Gauss
valuation induced by it and augment it to approximate an extension to `L`::
sage: R.<y> = K[]
sage: w = GaussValuation(R, v)
sage: w = w.augmentation(y - 2, 1); w
[ Gauss valuation induced by (x - 4)-adic valuation, v(y - 2) = 1 ]
sage: L.<y> = K.extension(y^2 - x)
sage: ww = w.extension(L); ww
[ (x - 4)-adic valuation, v(y - 2) = 1 ]-adic valuation
Limit valuations
~~~~~~~~~~~~~~~~
In the previous example the final valuation ``ww`` is not merely given by
evaluating ``w`` on the ring `K[y]`::
sage: ww(y^2 - x)
+Infinity
sage: y = R.gen()
sage: w(y^2 - x)
1
Instead ``ww`` is given by a limit, i.e., an infinite sequence of
augmentations of valuations::
sage: ww._base_valuation
[ Gauss valuation induced by (x - 4)-adic valuation, v(y - 2) = 1 , … ]
The terms of this infinite sequence are computed on demand::
sage: ww._base_valuation._approximation
[ Gauss valuation induced by (x - 4)-adic valuation, v(y - 2) = 1 ]
sage: ww(y - 1/4*x - 1)
2
sage: ww._base_valuation._approximation
[ Gauss valuation induced by (x - 4)-adic valuation, v(y + 1/64*x^2 - 3/8*x - 3/4) = 3 ]
Non-classical valuations
~~~~~~~~~~~~~~~~~~~~~~~~
Using the low-level interface we are not limited to classical valuations on
function fields that correspond to points on the corresponding projective
curves. Instead we can start with a non-trivial valuation on the field of
constants::
sage: v = QQ.valuation(2)
sage: R.<x> = QQ[]
sage: w = GaussValuation(R, v) # v is not trivial
sage: K.<x> = FunctionField(QQ)
sage: w = w.extension(K)
sage: w.residue_field()
Rational function field in x over Finite Field of size 2
Mac Lane Approximants
---------------------
The main tool underlying this package is an algorithm by Mac Lane to compute,
starting from a Gauss valuation on a polynomial ring and a monic squarefree
polynomial G, approximations to the limit valuation which send G to infinity::
sage: v = QQ.valuation(2)
sage: R.<x> = QQ[]
sage: f = x^5 + 3*x^4 + 5*x^3 + 8*x^2 + 6*x + 12
sage: v.mac_lane_approximants(f) # random output (order may vary)
[[ Gauss valuation induced by 2-adic valuation, v(x^2 + x + 1) = 3 ],
[ Gauss valuation induced by 2-adic valuation, v(x) = 1/2 ],
[ Gauss valuation induced by 2-adic valuation, v(x) = 1 ]]
From these approximants one can already see the residual degrees and
ramification indices of the corresponding extensions. The approximants can be
pushed to arbitrary precision, corresponding to a factorization of ``f``::
sage: v.mac_lane_approximants(f, required_precision=10) # random output
[[ Gauss valuation induced by 2-adic valuation, v(x^2 + 193*x + 13/21) = 10 ],
[ Gauss valuation induced by 2-adic valuation, v(x + 86) = 10 ],
[ Gauss valuation induced by 2-adic valuation, v(x) = 1/2, v(x^2 + 36/11*x + 2/17) = 11 ]]
References
----------
The theory was originally described in [Mac1936I]_ and [Mac1936II]_. A summary and some algorithmic details can also be found in Chapter 4 of [Rüt2014]_.
More Details
============
.. toctree::
:maxdepth: 2
sage/rings/valuation/value_group
sage/rings/valuation/valuation
sage/rings/valuation/valuation_space
sage/rings/valuation/trivial_valuation
sage/rings/valuation/gauss_valuation
sage/rings/valuation/developing_valuation
sage/rings/valuation/inductive_valuation
sage/rings/valuation/augmented_valuation
sage/rings/valuation/limit_valuation
sage/rings/valuation/mapped_valuation
sage/rings/valuation/scaled_valuation
sage/rings/function_field/function_field_valuation
sage/rings/padics/padic_valuation
.. include:: ../footer.txt
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