Local rings #
Define local rings as commutative rings having a unique maximal ideal.
Main definitions #
local_ring: A predicate on commutative semirings, stating that the set of nonunits is closed under the addition. This is shown to be equivalent to the condition that there exists a unique maximal ideal.local_ring.maximal_ideal: The unique maximal ideal for a local rings. Its carrier set is the set of non units.is_local_ring_hom: A predicate on semiring homomorphisms, requiring that it maps nonunits to nonunits. For local rings, this means that the image of the unique maximal ideal is again contained in the unique maximal ideal.local_ring.residue_field: The quotient of a local ring by its maximal ideal.
@[class]
- to_nontrivial : nontrivial R
- nonunits_add : ∀ {a b : R}, a ∈ nonunits R → b ∈ nonunits R → a + b ∈ nonunits R
A semiring is local if it is nontrivial and the set of nonunits is closed under the addition.
Note that local_ring is a predicate.
theorem
local_ring.of_unique_max_ideal
{R : Type u}
[comm_semiring R]
(h : ∃! (I : ideal R), I.is_maximal) :
A semiring is local if it has a unique maximal ideal.
theorem
local_ring.of_unique_nonzero_prime
{R : Type u}
[comm_semiring R]
(h : ∃! (P : ideal R), P ≠ ⊥ ∧ P.is_prime) :
The ideal of elements that are not units.
Equations
- local_ring.maximal_ideal R = {carrier := nonunits R (monoid_with_zero.to_monoid R), add_mem' := _, zero_mem' := _, smul_mem' := _}
@[protected, instance]
theorem
local_ring.maximal_ideal_unique
(R : Type u)
[comm_semiring R]
[local_ring R] :
∃! (I : ideal R), I.is_maximal
theorem
local_ring.eq_maximal_ideal
{R : Type u}
[comm_semiring R]
[local_ring R]
{I : ideal R}
(hI : I.is_maximal) :
theorem
local_ring.le_maximal_ideal
{R : Type u}
[comm_semiring R]
[local_ring R]
{J : ideal R}
(hJ : J ≠ ⊤) :
@[simp]
theorem
local_ring.mem_maximal_ideal
{R : Type u}
[comm_semiring R]
[local_ring R]
(x : R) :
x ∈ local_ring.maximal_ideal R ↔ x ∈ nonunits R
theorem
local_ring.of_is_unit_or_is_unit_one_sub_self
{R : Type u}
[comm_ring R]
[nontrivial R]
(h : ∀ (a : R), is_unit a ∨ is_unit (1 - a)) :
theorem
local_ring.is_unit_or_is_unit_one_sub_self
{R : Type u}
[comm_ring R]
[local_ring R]
(a : R) :
theorem
local_ring.is_unit_of_mem_nonunits_one_sub_self
{R : Type u}
[comm_ring R]
[local_ring R]
(a : R)
(h : 1 - a ∈ nonunits R) :
is_unit a
theorem
local_ring.is_unit_one_sub_self_of_mem_nonunits
{R : Type u}
[comm_ring R]
[local_ring R]
(a : R)
(h : a ∈ nonunits R) :
theorem
local_ring.of_surjective
{R : Type u}
{S : Type v}
[comm_ring R]
[local_ring R]
[comm_ring S]
[nontrivial S]
(f : R →+* S)
(hf : function.surjective ⇑f) :
@[class]
structure
is_local_ring_hom
{R : Type u}
{S : Type v}
[semiring R]
[semiring S]
(f : R →+* S) :
Prop
A local ring homomorphism is a homomorphism f between local rings such that a in the domain
is a unit if f a is a unit for any a. See local_ring.local_hom_tfae for other equivalent
definitions.
@[protected, instance]
@[simp]
theorem
is_unit_map_iff
{R : Type u}
{S : Type v}
[semiring R]
[semiring S]
(f : R →+* S)
[is_local_ring_hom f]
(a : R) :
@[protected, instance]
def
is_local_ring_hom_comp
{R : Type u}
{S : Type v}
{T : Type w}
[semiring R]
[semiring S]
[semiring T]
(g : S →+* T)
(f : R →+* S)
[is_local_ring_hom g]
[is_local_ring_hom f] :
is_local_ring_hom (g.comp f)
@[protected, instance]
@[simp]
theorem
is_unit_of_map_unit
{R : Type u}
{S : Type v}
[semiring R]
[semiring S]
(f : R →+* S)
[is_local_ring_hom f]
(a : R)
(h : is_unit (⇑f a)) :
is_unit a
theorem
of_irreducible_map
{R : Type u}
{S : Type v}
[semiring R]
[semiring S]
(f : R →+* S)
[h : is_local_ring_hom f]
{x : R}
(hfx : irreducible (⇑f x)) :
theorem
is_local_ring_hom_of_comp
{R : Type u}
{S : Type v}
{T : Type w}
[semiring R]
[semiring S]
[semiring T]
(f : R →+* S)
(g : S →+* T)
[is_local_ring_hom (g.comp f)] :
@[protected, instance]
def
CommRing.is_local_ring_hom_comp
{R S T : CommRing}
(f : R ⟶ S)
(g : S ⟶ T)
[is_local_ring_hom g]
[is_local_ring_hom f] :
is_local_ring_hom (f ≫ g)
theorem
ring_hom.domain_local_ring
{R : Type u_1}
{S : Type u_2}
[comm_semiring R]
[comm_semiring S]
[H : local_ring S]
(f : R →+* S)
[is_local_ring_hom f] :
If f : R →+* S is a local ring hom, then R is a local ring if S is.
@[protected, instance]
theorem
map_nonunit
{R : Type u}
{S : Type v}
[comm_semiring R]
[local_ring R]
[comm_semiring S]
[local_ring S]
(f : R →+* S)
[is_local_ring_hom f]
(a : R)
(h : a ∈ local_ring.maximal_ideal R) :
⇑f a ∈ local_ring.maximal_ideal S
The image of the maximal ideal of the source is contained within the maximal ideal of the target.
theorem
local_ring.local_hom_tfae
{R : Type u}
{S : Type v}
[comm_semiring R]
[local_ring R]
[comm_semiring S]
[local_ring S]
(f : R →+* S) :
[is_local_ring_hom f, ⇑f '' (local_ring.maximal_ideal R).carrier ⊆ ↑(local_ring.maximal_ideal S), ideal.map f (local_ring.maximal_ideal R) ≤ local_ring.maximal_ideal S, local_ring.maximal_ideal R ≤ ideal.comap f (local_ring.maximal_ideal S), ideal.comap f (local_ring.maximal_ideal S) = local_ring.maximal_ideal R].tfae
A ring homomorphism between local rings is a local ring hom iff it reflects units, i.e. any preimage of a unit is still a unit. https://stacks.math.columbia.edu/tag/07BJ
The residue field of a local ring is the quotient of the ring by its maximal ideal.
Equations
@[protected, instance]
Equations
@[protected, instance]
Equations
- local_ring.residue_field.inhabited R = {default := 37}
The quotient map from a local ring to its residue field.
Equations
@[protected, instance]
Equations
noncomputable
def
local_ring.residue_field.map
{R : Type u}
{S : Type v}
[comm_ring R]
[local_ring R]
[comm_ring S]
[local_ring S]
(f : R →+* S)
[is_local_ring_hom f] :
The map on residue fields induced by a local homomorphism between local rings
Equations
theorem
local_ring.ker_eq_maximal_ideal
{R : Type u}
{K : Type u'}
[comm_ring R]
[local_ring R]
[field K]
(φ : R →+* K)
(hφ : function.surjective ⇑φ) :