Dedekind domains

This file defines the notion of a Dedekind domain (or Dedekind ring), giving three equivalent definitions (TODO: and shows that they are equivalent).

Main definitions

is_dedekind_domain defines a Dedekind domain as a commutative ring that is not a field, Noetherian, integrally closed in its field of fractions and has Krull dimension exactly one. is_dedekind_domain_iff shows that this does not depend on the choice of field of fractions.
is_dedekind_domain_dvr alternatively defines a Dedekind domain as an integral domain that is not a field, Noetherian, and the localization at every nonzero prime ideal is a discrete valuation ring.
is_dedekind_domain_inv alternatively defines a Dedekind domain as an integral domain that is not a field, and every nonzero fractional ideal is invertible.
is_dedekind_domain_inv_iff shows that this does note depend on the choice of field of fractions.

Implementation notes

The definitions that involve a field of fractions choose a canonical field of fractions, but are independent of that choice. The ..._iff lemmas express this independence.

References

[D. Marcus, Number Fields][marcus1977number]
[J.W.S. Cassels, A. Frölich, Algebraic Number Theory][cassels1967algebraic]

Tags

dedekind domain, dedekind ring

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def ring.dimension_le_one (R : Type u_1) [comm_ring R] :

Prop

A ring R has Krull dimension at most one if all nonzero prime ideals are maximal.

Equations

ring.dimension_le_one R = ∀ (p : ideal R), p ≠ ⊥ → p.is_prime → p.is_maximal

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theorem ring.dimension_le_one.principal_ideal_ring (A : Type u_2) [integral_domain A] [is_principal_ideal_ring A] :

ring.dimension_le_one A

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theorem ring.dimension_le_one.integral_closure (R : Type u_1) (A : Type u_2) [comm_ring R] [integral_domain A] [nontrivial R] [algebra R A] (h : ring.dimension_le_one R) :

ring.dimension_le_one ↥(integral_closure R A)

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@[class]

structure is_dedekind_domain (A : Type u_2) [integral_domain A] :

Prop

not_is_field : ¬is_field A
is_noetherian_ring : is_noetherian_ring A
dimension_le_one : ring.dimension_le_one A
is_integrally_closed : integral_closure A (fraction_ring A) = ⊥

A Dedekind domain is an integral domain that is Noetherian, integrally closed, and has Krull dimension exactly one (not_is_field and dimension_le_one).

The integral closure condition is independent of the choice of field of fractions: use is_dedekind_domain_iff to prove is_dedekind_domain for a given fraction_map.

This is the default implementation, but there are equivalent definitions, is_dedekind_domain_dvr and is_dedekind_domain_inv. TODO: Prove that these are actually equivalent definitions.

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theorem is_dedekind_domain_iff (A : Type u_2) (K : Type u_3) [integral_domain A] [field K] (f : fraction_map A K) :

is_dedekind_domain A ↔ ¬is_field A ∧ is_noetherian_ring A ∧ ring.dimension_le_one A ∧ integral_closure A (localization_map.codomain f) = ⊥

An integral domain is a Dedekind domain iff and only if it is not a field, is Noetherian, has dimension ≤ 1, and is integrally closed in a given fraction field. In particular, this definition does not depend on the choice of this fraction field.

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structure is_dedekind_domain_dvr (A : Type u_2) [integral_domain A] :

Prop

not_is_field : ¬is_field A
is_noetherian_ring : is_noetherian_ring A
is_dvr_at_nonzero_prime : ∀ (P : ideal A), P ≠ ⊥ → ∀ (a : P.is_prime), discrete_valuation_ring (localization.at_prime P)

A Dedekind domain is an integral domain that is not a field, is Noetherian, and the localization at every nonzero prime is a discrete valuation ring.

This is equivalent to is_dedekind_domain. TODO: prove the equivalence.

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structure is_dedekind_domain_inv (A : Type u_2) [integral_domain A] :

Prop

not_is_field : ¬is_field A
mul_inv_cancel : ∀ (I : ring.fractional_ideal (fraction_ring.of A)), I ≠ ⊥ → I * I⁻¹ = 1

A Dedekind domain is an integral domain that is not a field such that every fractional ideal has an inverse.

This is equivalent to is_dedekind_domain. TODO: prove the equivalence.

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theorem is_dedekind_domain_inv_iff (A : Type u_2) (K : Type u_3) [integral_domain A] [field K] (f : fraction_map A K) :

is_dedekind_domain_inv A ↔ ¬is_field A ∧ ∀ (I : ring.fractional_ideal f), I ≠ ⊥ → I * I⁻¹ = 1