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Places

Subsections

Creation of Structures

Places(F) : FldFun -> PlcFun
The set of places of the algebraic function field F/k.

Creation of Elements

General Function Field Places
Decomposition(P, F) : PlcFunElt, FldFun -> [ PlcFunElt ]
A sequence containing all places of F/k lying above the place P of the coefficient field k(x) of F. F must be a finite extension of k(x).
DecompositionType(P, F) : PlcFunElt, FldFun -> [ <RngIntElt, RngIntElt> ]
Sequence of tuples of ramification indices and residue degrees of the places of F/k lying over the place P of the coefficient field k(x) of F. F must be a finite extension of k(x).
Zeros(a) : FldFunElt -> [ PlcFunElt ]
Zeros(a) : RngFunOrdElt -> [ PlcFunElt ]
A sequence containing all zeros of the algebraic function a.
Poles(a) : FldFunElt -> [ PlcFunElt ]
Poles(a) : RngFunOrdElt -> [ PlcFunElt ]
A sequence containing all poles of the algebraic function a.
S ! I : PlcFun, RngFunOrdIdl -> PlcFunElt
Place(I) : RngFunOrdIdl -> PlcFunElt
The place corresponding to the prime ideal I, where I is defined over the `finite' or `infinite' maximal order.
Support(D) : DivFunElt -> [ PlcFunElt ], [ RngIntElt ]
Sequences containing the places and exponents occurring in the divisor D.
AssignNames(~P, s) : PlcFunElt, [ MonStgElt ] ->
Change the print name employed when displaying P to be the first element in the sequence of strings s which must have length 1.
Global Function Field Places

F/k denotes a global function field in this section.

Place(F, m) : FldFun, RngIntElt -> PlcFunElt
Returns true and a place of degree m if and only if there exists such in F/k; false otherwise.
RandomPlace(F, m) : FldFun, RngIntElt -> BoolElt, PlcFunElt
Returns true and a random place of degree m of F/k (false if there are none).
Places(F, m) : FldFun, RngIntElt -> SeqEnum[PlcFunElt]
A sequence containing the places of degree m of F/k.

Related Structures

Parent and Category

The sets of function field places form the Magma category PlcFun. The notional power structure exists as parent but allows no operations.

FunctionField(S) : PlcFun -> FldFun
The corresponding function field.
DivisorGroup(F) : FldFun -> DivFun
The group of divisors of the algebraic function field F/k, which is the free abelian group generated by the elements of the set of places of F/k.

Structure Invariants

General function fields
WeierstrassPlaces(F) : FldFunG -> [PlcFunElt]
    SeparatingElement: FldFunGElt       Default: 
The Weierstrass places of F/k. The semantics of calling WeierstrassPlaces() with F/k or the zero divisor of F/k are identical. See the description of tttRef{DivFunElt:WeierstrassPlaces}.
Global Function Fields

F/k denotes a global function field in this section.

NumberOfPlacesOfDegreeOne(F, m) : FldFun, RngIntElt -> RngIntElt
The number of places of degree one in the constant field extension of degree m of F/k. Contrary to the Degree() function the degree is here taken over the respective exact constant fields.
NumberOfPlacesOfDegreeOneBound(F, m) : FldFun, RngIntElt -> RngIntElt
The minimum of the Serre and Ihara bound on the number of places of degree one in the constant field extension of degree m of F/k. Contrary to the Degree() function the degree is here taken over the respective exact constant fields.
NumberOfPlaces(F, m) : FldFun, RngIntElt -> RngIntElt
The number of places of degree m of F/k. Contrary to the Degree() function the degree is here taken over the respective exact constant fields.
Places(F, m) : FldFun, RngIntElt -> SeqEnum[PlcFunElt]
A sequence containing the places of degree m of F/k.

Structure Predicates

S1 eq S2 : PlcFun, PlcFun -> BoolElt
S1 ne S2 : PlcFun, PlcFun -> BoolElt

Element Operations

Parent and Category
Parent(P) : PlcFunElt -> PlcFun
Category(P) : PlcFunElt -> Cat
Arithmetic Operators
- P : PlcFunElt -> DivFunElt
P1 + P2 : PlcFunElt, PlcFunElt -> DivFunElt
P1 - P2 : PlcFunElt, PlcFunElt -> DivFunElt
k * P : RngIntElt, PlcFunElt -> DivFunElt
P div k : PlcFunElt, RngIntElt -> DivFunElt
P mod k : PlcFunElt, RngIntElt -> DivFunElt
Quotrem(P, k) : PlcFunElt, RngIntElt -> DivFunElt, DivFunElt
Returns divisors D_1, D_2 such that P = kD_1 + D_2 and the exponents in D_2 are of absolute value less than |k|. The operations div and mod yield D_1 resp. D_2.
Equality and Membership

P1 eq P2 : PlcFunElt, PlcFunElt -> BoolElt
P1 ne P2 : PlcFunElt, PlcFunElt -> BoolElt
P in S : PlcFunElt, PlcFun -> BoolElt
P notin S : PlcFunElt, PlcFun -> BoolElt
Predicates on Elements
IsFinite(P) : PlcFunElt -> BoolElt
Returns true if the place P is a `finite' place.
IsWeierstrassPlace(P) : PlcFunElt -> BoolElt
IsWeierstrassPlace(F, P) : FldFunG, PlcFunElt -> BoolElt
Whether the degree one place P is a Weierstraß place of its function field F. See the description of tttRef{DivFunElt:WeierstrassPlaces}.
Other Element Operations
FunctionField(P) : PlcFunElt -> FldFun
The function field that corresponds to the place P.
Degree(P) : PlcFunElt -> RngIntElt
The degree of the place P over the constant field of definition k.
RamificationIndex(P) : PlcFunElt -> RngIntElt
RamificationDegree(P) : PlcFunElt -> RngIntElt
The ramification index of the place P over its subplace of the rational function field k(x) (the function field of P must be a finite extension of k(x)).
InertiaDegree(P) : PlcFunElt -> RngIntElt
ResidueClassDegree(P) : PlcFunElt -> RngIntElt
The degree of inertia (or residue class degree) of a place P over the corresponding subplace of the rational function field (the function field of P must be a finite extension of k(x))
Minimum(P) : PlcFunElt -> RngElt
A monic prime polynomial in k[x] or 1/x, corresponding to the place of the rational function field P lies above (the function field of P must be a finite extension of k(x)).
ResidueClassField(P) : PlcFunElt -> Rng
The residue class field of the place P.
Evaluate(a, P) : RngElt, PlcFunElt -> RngElt
Evaluate the algebraic function a at the place P. If it is not defined at P, infinity is returned.
Lift(a, P) : RngElt, PlcFunElt -> FldFunElt
Lift the element a of the residue class field of the place P (including infinity) to an algebraic function.
TwoGenerators(P) : PlcFunElt -> FldFunGElt, FldFunGElt
Two algebraic functions having P as their unique common zero.
LocalUniformizer(P) : PlcFunElt -> FldFunGElt
UniformizingElement(P) : PlcFunElt -> FldFunGElt
A local uniformizing parameter at the place P.
Valuation(a, P) : FldFunElt, PlcFunElt -> RngIntElt
The valuation of the element a at the place P.
Ideal(P) : PlcFunElt -> RngFunOrdIdl
Create a prime ideal corresponding to the place P.

Example FldFunG_places (H57E11)

> R<x> := FunctionField(GF(9));
> P<y> := PolynomialRing(R);
> f := y^4 + (2*x^5 + x^4 + 2*x^3 + x^2)*y^2 + x^8 
>      + 2*x^6 + x^5 +x^4 + x^3 + x^2;
> F<a> := FunctionField(f);
> Genus(F);
7
> NumberOfPlaces(F, 2);
28
> _, P := RandomPlace(F, 2);
> P;
Place of F defined by x^2 + w1^6*x + w1^2 and a + w1*x + 1
> LocalUniformizer(P);
x^2 + w1^6*x + w1^2
> TwoGenerators(P);
x^2 + w1^6*x + w1^2 a + w1*x + 1
> ResidueClassField(P);
Finite field of size 3^4
> Evaluate(1/LocalUniformizer(P), P);
Infinity
> Valuation(1/LocalUniformizer(P), P);
-1

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