# Places of function fields¶

The places of a function field correspond, one-to-one, to valuation rings of the function field, each of which defines a discrete valuation for the elements of the function field. “Finite” places are in one-to-one correspondence with the prime ideals of the finite maximal order while places “at infinity” are in one-to-one correspondence with the prime ideals of the infinite maximal order.

EXAMPLES:

All rational places of a function field can be computed:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x + x^3*Y)
sage: L.places()
[Place (1/x, 1/x^3*y^2 + 1/x),
Place (1/x, 1/x^3*y^2 + 1/x^2*y + 1),
Place (x, y)]
```

The residue field associated with a place is given as an extension of the constant field:

```sage: F.<x> = FunctionField(GF(2))
sage: O = F.maximal_order()
sage: p = O.ideal(x^2 + x + 1).place()
sage: k, fr_k, to_k = p.residue_field()
sage: k
Finite Field in z2 of size 2^2
```

The homomorphisms are between the valuation ring and the residue field:

```sage: fr_k
Ring morphism:
From: Finite Field in z2 of size 2^2
To:   Valuation ring at Place (x^2 + x + 1)
sage: to_k
Ring morphism:
From: Valuation ring at Place (x^2 + x + 1)
To:   Finite Field in z2 of size 2^2
```

AUTHORS:

• Kwankyu Lee (2017-04-30): initial version

• Brent Baccala (2019-12-20): function fields of characteristic zero

class sage.rings.function_field.place.FunctionFieldPlace(parent, prime)

Places of function fields.

INPUT:

• `parent` – place set of a function field

• `prime` – prime ideal associated with the place

EXAMPLES:

```sage: K.<x>=FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y>=K.extension(Y^3 + x + x^3*Y)
sage: L.places_finite()
Place (x, y)
```
divisor(multiplicity=1)

Return the prime divisor corresponding to the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(5)); R.<Y> = PolynomialRing(K)
sage: F.<y> = K.extension(Y^2 - x^3 - 1)
sage: O = F.maximal_order()
sage: I = O.ideal(x + 1,y)
sage: P = I.place()
sage: P.divisor()
Place (x + 1, y)
```
function_field()

Return the function field to which the place belongs.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: p = L.places()
sage: p.function_field() == L
True
```
prime_ideal()

Return the prime ideal associated with the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: p = L.places()
sage: p.prime_ideal()
Ideal (1/x^3*y^2 + 1/x) of Maximal infinite order of Function field
in y defined by y^3 + x^3*y + x
```
class sage.rings.function_field.place.FunctionFieldPlace_polymod(parent, prime)

Places of extensions of function fields.

degree()

Return the degree of the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: OK = K.maximal_order()
sage: OL = L.maximal_order()
sage: p = OK.ideal(x^2 + x + 1)
sage: dec = OL.decomposition(p)
sage: q = dec.place()
sage: q.degree()
2
```
gaps()

Return the gap sequence for the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(4)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: O = L.maximal_order()
sage: p = O.ideal(x,y).place()
sage: p.gaps() # a Weierstrass place
[1, 2, 4]

sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3 * Y + x)
sage: [p.gaps() for p in L.places()]
[[1, 2, 4], [1, 2, 4], [1, 2, 4]]
```
is_infinite_place()

Return `True` if the place is above the unique infinite place of the underlying rational function field.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: pls = L.places()
sage: [p.is_infinite_place() for p in pls]
[True, True, False]
sage: [p.place_below() for p in pls]
[Place (1/x), Place (1/x), Place (x)]
```
local_uniformizer()

Return an element of the function field that has a simple zero at the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(4)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: pls = L.places()
sage: [p.local_uniformizer().valuation(p) for p in pls]
[1, 1, 1, 1, 1]
```
place_below()

Return the place lying below the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: OK = K.maximal_order()
sage: OL = L.maximal_order()
sage: p = OK.ideal(x^2 + x + 1)
sage: dec = OL.decomposition(p)
sage: q = dec.place()
sage: q.place_below()
Place (x^2 + x + 1)
```
relative_degree()

Return the relative degree of the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: OK = K.maximal_order()
sage: OL = L.maximal_order()
sage: p = OK.ideal(x^2 + x + 1)
sage: dec = OL.decomposition(p)
sage: q = dec.place()
sage: q.relative_degree()
1
```
residue_field(name=None)

Return the residue field of the place.

INPUT:

• `name` – string; name of the generator of the residue field

OUTPUT:

• a field isomorphic to the residue field

• a ring homomorphism from the valuation ring to the field

• a ring homomorphism from the field to the valuation ring

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^2 + Y + x + 1/x)
sage: p = L.places_finite()
sage: k, fr_k, to_k = p.residue_field()
sage: k
Finite Field of size 2
sage: fr_k
Ring morphism:
From: Finite Field of size 2
To:   Valuation ring at Place (x, x*y)
sage: to_k
Ring morphism:
From: Valuation ring at Place (x, x*y)
To:   Finite Field of size 2
sage: to_k(y)
Traceback (most recent call last):
...
TypeError: y fails to convert into the map's domain
Valuation ring at Place (x, x*y)...
sage: to_k(1/y)
0
sage: to_k(y/(1+y))
1
```
valuation_ring()

Return the valuation ring at the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^2 + Y + x + 1/x)
sage: p = L.places_finite()
sage: p.valuation_ring()
Valuation ring at Place (x, x*y)
```
class sage.rings.function_field.place.FunctionFieldPlace_rational(parent, prime)

Places of rational function fields.

degree()

Return the degree of the place.

EXAMPLES:

```sage: F.<x> = FunctionField(GF(2))
sage: O = F.maximal_order()
sage: i = O.ideal(x^2 + x + 1)
sage: p = i.place()
sage: p.degree()
2
```
is_infinite_place()

Return `True` if the place is at infinite.

EXAMPLES:

```sage: F.<x> = FunctionField(GF(2))
sage: F.places()
[Place (1/x), Place (x), Place (x + 1)]
sage: [p.is_infinite_place() for p in F.places()]
[True, False, False]
```
local_uniformizer()

Return a local uniformizer of the place.

EXAMPLES:

```sage: F.<x> = FunctionField(GF(2))
sage: F.places()
[Place (1/x), Place (x), Place (x + 1)]
sage: [p.local_uniformizer() for p in F.places()]
[1/x, x, x + 1]
```
residue_field(name=None)

Return the residue field of the place.

EXAMPLES:

```sage: F.<x> = FunctionField(GF(2))
sage: O = F.maximal_order()
sage: p = O.ideal(x^2 + x + 1).place()
sage: k, fr_k, to_k = p.residue_field()
sage: k
Finite Field in z2 of size 2^2
sage: fr_k
Ring morphism:
From: Finite Field in z2 of size 2^2
To:   Valuation ring at Place (x^2 + x + 1)
sage: to_k
Ring morphism:
From: Valuation ring at Place (x^2 + x + 1)
To:   Finite Field in z2 of size 2^2
```
valuation_ring()

Return the valuation ring at the place.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^2 + Y + x + 1/x)
sage: p = L.places_finite()
sage: p.valuation_ring()
Valuation ring at Place (x, x*y)
```
class sage.rings.function_field.place.PlaceSet(field)

Sets of Places of function fields.

INPUT:

• `field` – function field

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: L.place_set()
Set of places of Function field in y defined by y^3 + x^3*y + x
```
Element

alias of `FunctionFieldPlace`

function_field()

Return the function field to which this place set belongs.

EXAMPLES:

```sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^3 + x^3*Y + x)
sage: PS = L.place_set()
sage: PS.function_field() == L
True
```