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ring_theory.discrete_valuation_ring

Discrete valuation rings

This file defines discrete valuation rings (DVRs) and develops a basic interface for them.

Important definitions

There are various definitions of a DVR in the literature; we define a DVR to be a local PID which is not a field (the first definition in Wikipedia) and prove that this is equivalent to being a PID with a unique non-zero prime ideal (the definition in Serre's book "Local Fields").

Let R be an integral domain, assumed to be a principal ideal ring and a local ring.

discrete_valuation_ring R : a predicate expressing that R is a DVR

Definitions

add_val R : R → ℕ : the additive valuation on a DVR (sending 0 to 0 rather than the mathematically correct +∞). TODO -- the multiplicative valuation, taking values in something like with_zero (multiplicative ℤ)?

Implementation notes

It's a theorem that an element of a DVR is a uniformizer if and only if it's irreducible. We do not hence define uniformizer at all, because we can use irreducible instead.

Tags

discrete valuation ring

@[class]

structure discrete_valuation_ring (R : Type u) [integral_domain R] :

Prop

to_is_principal_ideal_ring : is_principal_ideal_ring R
to_local_ring : local_ring R
not_a_field' : local_ring.maximal_ideal R ≠ ⊥

An integral domain is a discrete valuation ring (DVR) if it's a local PID which is not a field.

Instances

padic_int.discrete_valuation_ring

theorem discrete_valuation_ring.not_a_field (R : Type u) [integral_domain R] [discrete_valuation_ring R] :

local_ring.maximal_ideal R ≠ ⊥

theorem discrete_valuation_ring.irreducible_iff_uniformizer {R : Type u} [integral_domain R] [discrete_valuation_ring R] (ϖ : R) :

irreducible ϖ ↔ local_ring.maximal_ideal R = ideal.span {ϖ}

An element of a DVR is irreducible iff it is a uniformizer, that is, generates the maximal ideal of R

theorem discrete_valuation_ring.exists_irreducible (R : Type u) [integral_domain R] [discrete_valuation_ring R] :

∃ (ϖ : R), irreducible ϖ

Uniformisers exist in a DVR

theorem discrete_valuation_ring.iff_pid_with_one_nonzero_prime (R : Type u) [integral_domain R] :

discrete_valuation_ring R ↔ is_principal_ideal_ring R ∧ ∃! (P : ideal R), P ≠ ⊥ ∧ P.is_prime

an integral domain is a DVR iff it's a PID with a unique non-zero prime ideal

theorem discrete_valuation_ring.associated_of_irreducible (R : Type u) [integral_domain R] [discrete_valuation_ring R] {a b : R} (ha : irreducible a) (hb : irreducible b) :

def discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization (R : Type u_1) [integral_domain R] :

Prop

Alternative characterisation of discrete valuation rings.

Equations

discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization R = ∃ (p : R), irreducible p ∧ ∀ {x : R}, x ≠ 0 → (∃ (n : ℕ), associated (p ^ n) x)

theorem discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization.unique_irreducible {R : Type u_1} [integral_domain R] (hR : discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization R) ⦃p q : R⦄ (hp : irreducible p) (hq : irreducible q) :

theorem discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization.to_unique_factorization_monoid {R : Type u_1} [integral_domain R] (hR : discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization R) :

unique_factorization_monoid R

An integral domain in which there is an irreducible element p such that every nonzero element is associated to a power of p is a unique factorization domain. See discrete_valuation_ring.of_has_unit_mul_pow_irreducible_factorization.

theorem discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization.of_ufd_of_unique_irreducible {R : Type u_1} [integral_domain R] [unique_factorization_monoid R] (h₁ : ∃ (p : R), irreducible p) (h₂ : ∀ ⦃p q : R⦄, irreducible p → irreducible q → associated p q) :

discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization R

theorem discrete_valuation_ring.aux_pid_of_ufd_of_unique_irreducible (R : Type u) [integral_domain R] [unique_factorization_monoid R] (h₁ : ∃ (p : R), irreducible p) (h₂ : ∀ ⦃p q : R⦄, irreducible p → irreducible q → associated p q) :

is_principal_ideal_ring R

theorem discrete_valuation_ring.of_ufd_of_unique_irreducible {R : Type u} [integral_domain R] [unique_factorization_monoid R] (h₁ : ∃ (p : R), irreducible p) (h₂ : ∀ ⦃p q : R⦄, irreducible p → irreducible q → associated p q) :

discrete_valuation_ring R

A unique factorization domain with at least one irreducible element in which all irreducible elements are associated is a discrete valuation ring.

theorem discrete_valuation_ring.of_has_unit_mul_pow_irreducible_factorization {R : Type u} [integral_domain R] (hR : discrete_valuation_ring.has_unit_mul_pow_irreducible_factorization R) :

discrete_valuation_ring R

An integral domain in which there is an irreducible element p such that every nonzero element is associated to a power of p is a discrete valuation ring.

theorem discrete_valuation_ring.associated_pow_irreducible {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {x : R} (hx : x ≠ 0) {ϖ : R} (hirr : irreducible ϖ) :

∃ (n : ℕ), associated x (ϖ ^ n)

theorem discrete_valuation_ring.eq_unit_mul_pow_irreducible {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {x : R} (hx : x ≠ 0) {ϖ : R} (hirr : irreducible ϖ) :

∃ (n : ℕ) (u : units R), x = (↑u) * ϖ ^ n

theorem discrete_valuation_ring.ideal_eq_span_pow_irreducible {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {s : ideal R} (hs : s ≠ ⊥) {ϖ : R} (hirr : irreducible ϖ) :

∃ (n : ℕ), s = ideal.span {ϖ ^ n}

theorem discrete_valuation_ring.unit_mul_pow_congr_pow {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {p q : R} (hp : irreducible p) (hq : irreducible q) (u v : units R) (m n : ℕ) (h : (↑u) * p ^ m = (↑v) * q ^ n) :

m = n

theorem discrete_valuation_ring.unit_mul_pow_congr_unit {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {ϖ : R} (hirr : irreducible ϖ) (u v : units R) (m n : ℕ) (h : (↑u) * ϖ ^ m = (↑v) * ϖ ^ n) :

u = v

The additive valuation on a DVR

def discrete_valuation_ring.add_val (R : Type u) [integral_domain R] [discrete_valuation_ring R] :

R → ℕ

The ℕ-valued additive valuation on a DVR (returns junk at 0 rather than +∞)

Equations

discrete_valuation_ring.add_val R = λ (r : R), dite (r = 0) (λ (hr : r = 0), 0) (λ (hr : ¬r = 0), classical.some _)

theorem discrete_valuation_ring.add_val_spec {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {r : R} (hr : r ≠ 0) :

let ϖ : R := classical.some _, n : ℕ := classical.some _ in associated r (ϖ ^ n)

theorem discrete_valuation_ring.add_val_def {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] (r : R) (u : units R) {ϖ : R} (hϖ : irreducible ϖ) (n : ℕ) (hr : r = (↑u) * ϖ ^ n) :

discrete_valuation_ring.add_val R r = n

theorem discrete_valuation_ring.add_val_def' {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] (u : units R) {ϖ : R} (hϖ : irreducible ϖ) (n : ℕ) :

discrete_valuation_ring.add_val R ((↑u) * ϖ ^ n) = n

@[simp]

theorem discrete_valuation_ring.add_val_zero {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] :

discrete_valuation_ring.add_val R 0 = 0

@[simp]

theorem discrete_valuation_ring.add_val_one {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] :

discrete_valuation_ring.add_val R 1 = 0

@[simp]

theorem discrete_valuation_ring.add_val_uniformizer {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {ϖ : R} (hϖ : irreducible ϖ) :

discrete_valuation_ring.add_val R ϖ = 1

@[simp]

theorem discrete_valuation_ring.add_val_mul {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {a b : R} (ha : a ≠ 0) (hb : b ≠ 0) :

discrete_valuation_ring.add_val R (a * b) = discrete_valuation_ring.add_val R a + discrete_valuation_ring.add_val R b

theorem discrete_valuation_ring.add_val_pow {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] (a : R) (n : ℕ) :

discrete_valuation_ring.add_val R (a ^ n) = n * discrete_valuation_ring.add_val R a

theorem discrete_valuation_ring.add_val_le_iff_dvd {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {a b : R} (ha : a ≠ 0) (hb : b ≠ 0) :

discrete_valuation_ring.add_val R a ≤ discrete_valuation_ring.add_val R b ↔ a ∣ b

theorem discrete_valuation_ring.add_val_add {R : Type u_1} [integral_domain R] [discrete_valuation_ring R] {a b : R} (ha : a ≠ 0) (hb : b ≠ 0) (hab : a + b ≠ 0) :

min (discrete_valuation_ring.add_val R a) (discrete_valuation_ring.add_val R b) ≤ discrete_valuation_ring.add_val R (a + b)