# Places of function fields: extension#

class sage.rings.function_field.place_polymod.FunctionFieldPlace_polymod(parent, prime)#

Places of extensions of function fields.

degree()#

Return the degree of the place.

EXAMPLES:

```sage: # needs sage.rings.finite_rings
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[0][0].place()
sage: q.degree()
2
```
gaps()#

Return the gap sequence for the place.

EXAMPLES:

```sage: # needs sage.rings.finite_rings
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[]                             # needs sage.rings.finite_rings
sage: L.<y> = K.extension(Y^3 + x^3 * Y + x)                                # needs sage.rings.finite_rings
sage: [p.gaps() for p in L.places()]                                        # needs sage.rings.finite_rings
[[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: # needs sage.rings.finite_rings
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: # needs sage.rings.finite_rings
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: # needs sage.rings.finite_rings
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[0][0].place()
sage: q.place_below()
Place (x^2 + x + 1)
```
relative_degree()#

Return the relative degree of the place.

EXAMPLES:

```sage: # needs sage.rings.finite_rings
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[0][0].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: # needs sage.rings.finite_rings
sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^2 + Y + x + 1/x)
sage: p = L.places_finite()[0]
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: # needs sage.rings.finite_rings
sage: K.<x> = FunctionField(GF(2)); _.<Y> = K[]
sage: L.<y> = K.extension(Y^2 + Y + x + 1/x)
sage: p = L.places_finite()[0]
sage: p.valuation_ring()
Valuation ring at Place (x, x*y)
```