Discrete Valuations and Discrete Pseudo-Valuations

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()
Fraction Field of Univariate Polynomial Ring in x over Finite Field of size 2 (using ...)

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].

Indices and Tables