feat(ModelTheory/Definablity): add syntax-to-definability bridge lemmas

Author: NoneMore

Mathlib/ModelTheory/Definability.lean

@@ -93,35 +93,88 @@ theorem Definable.mono (hAs : A.Definable L s) (hAB : A ⊆ B) : B.Definable L s
   rw [definable_iff_empty_definable_with_params] at *
   exact hAs.map_expansion (L.lhomWithConstantsMap (Set.inclusion hAB))
 
+end Set
+
+namespace FirstOrder.Language
+
+open Set
+
+variable {L : Language.{u, v}} {M : Type w} [L.Structure M]
+
+variable {A : Set M} {α : Type u₁}
+
+namespace Formula
+
+/-- A formula with parameters from `A` defines the set of its realizations. -/
+theorem definable_withConstants (φ : L[[A]].Formula α) :
+    A.Definable L {v | φ.Realize v} :=
+  ⟨φ, rfl⟩
+
+theorem definable (φ : L.Formula α) :
+    A.Definable L {v | φ.Realize v} :=
+  (empty_definable_iff.mpr ⟨φ, rfl⟩).mono (empty_subset A)
+
+end Formula
+
+namespace BoundedFormula
+
+variable {n : ℕ}
+
+theorem definable_withConstants (φ : L[[A]].BoundedFormula α n) :
+    A.Definable L {v | φ.Realize (v ∘ Sum.inl) (v ∘ Sum.inr)} := by
+  simpa using φ.toFormula.definable_withConstants
+
+theorem definable (φ : L.BoundedFormula α n) :
+    A.Definable L {v | φ.Realize (v ∘ Sum.inl) (v ∘ Sum.inr)} := by
+  simpa using φ.toFormula.definable
+
+theorem definable_boundedSection_withConstants (φ : L[[A]].BoundedFormula Empty n) :
+    A.Definable L {xs | φ.Realize default xs} := by
+  convert (φ.toFormula.relabel (Sum.elim Empty.elim id)).definable_withConstants using 1
+  ext xs
+  simp only [Formula.realize_relabel, realize_toFormula]
+  congr!
+
+theorem definable_boundedSection (φ : L.BoundedFormula Empty n) :
+    A.Definable L {xs | φ.Realize default xs} := by
+  convert (φ.toFormula.relabel (Sum.elim Empty.elim id)).definable
+  ext xs
+  simp only [Formula.realize_relabel, realize_toFormula]
+  congr!
+
+end BoundedFormula
+
+end FirstOrder.Language
+
+namespace Set
+
+open FirstOrder FirstOrder.Language
+
+variable {M : Type w} {A : Set M} {L : FirstOrder.Language.{u, v}} [L.Structure M]
+
+variable {α : Type u₁} {β : Type*} {s : Set (α → M)}
+
 @[simp]
 theorem definable_empty : A.Definable L (∅ : Set (α → M)) :=
-  ⟨⊥, by
-    ext
-    simp⟩
+  (⊥ : L[[A]].Formula α).definable_withConstants
 
 @[simp]
-theorem definable_univ : A.Definable L (univ : Set (α → M)) :=
-  ⟨⊤, by
-    ext
-    simp⟩
+theorem definable_univ : A.Definable L (univ : Set (α → M)) := by
+  simpa using (⊤ : L[[A]].Formula α).definable_withConstants
 
 @[simp]
 theorem Definable.inter {f g : Set (α → M)} (hf : A.Definable L f) (hg : A.Definable L g) :
     A.Definable L (f ∩ g) := by
   rcases hf with ⟨φ, rfl⟩
   rcases hg with ⟨θ, rfl⟩
-  refine ⟨φ ⊓ θ, ?_⟩
-  ext
-  simp
+  simpa using (φ ⊓ θ).definable_withConstants
 
 @[simp]
 theorem Definable.union {f g : Set (α → M)} (hf : A.Definable L f) (hg : A.Definable L g) :
     A.Definable L (f ∪ g) := by
-  rcases hf with ⟨φ, hφ⟩
-  rcases hg with ⟨θ, hθ⟩
-  refine ⟨φ ⊔ θ, ?_⟩
-  ext
-  rw [hφ, hθ, mem_setOf_eq, Formula.realize_sup, mem_union, mem_setOf_eq, mem_setOf_eq]
+  rcases hf with ⟨φ, rfl⟩
+  rcases hg with ⟨θ, rfl⟩
+  simpa using (φ ⊔ θ).definable_withConstants
 
 theorem definable_finset_inf {ι : Type*} {f : ι → Set (α → M)} (hf : ∀ i, A.Definable L (f i))
     (s : Finset ι) : A.Definable L (s.inf f) := by
@@ -161,10 +214,8 @@ theorem definable_iUnion_of_finite {ι : Type*} [Finite ι] {f : ι → Set (α
 
 @[simp]
 theorem Definable.compl {s : Set (α → M)} (hf : A.Definable L s) : A.Definable L sᶜ := by
-  rcases hf with ⟨φ, hφ⟩
-  refine ⟨φ.not, ?_⟩
-  ext v
-  rw [hφ, compl_setOf, mem_setOf, mem_setOf, Formula.realize_not]
+  rcases hf with ⟨φ, rfl⟩
+  simpa using (φ.not).definable_withConstants
 
 @[simp]
 theorem Definable.sdiff {s t : Set (α → M)} (hs : A.Definable L s) (ht : A.Definable L t) :
@@ -177,9 +228,7 @@ theorem Definable.sdiff {s t : Set (α → M)} (hs : A.Definable L s) (ht : A.De
 theorem Definable.preimage_comp (f : α → β) {s : Set (α → M)} (h : A.Definable L s) :
     A.Definable L ((fun g : β → M => g ∘ f) ⁻¹' s) := by
   obtain ⟨φ, rfl⟩ := h
-  refine ⟨φ.relabel f, ?_⟩
-  ext
-  simp only [Set.preimage_setOf_eq, mem_setOf_eq, Formula.realize_relabel]
+  simpa using (φ.relabel f).definable_withConstants
 
 theorem Definable.image_comp_equiv {s : Set (β → M)} (h : A.Definable L s) (f : α ≃ β) :
     A.Definable L ((fun g : β → M => g ∘ f) '' s) := by
@@ -278,19 +327,15 @@ theorem Definable.image_comp {s : Set (β → M)} (h : A.Definable L s) (f : α
 lemma Definable.exists_of_finite [Finite β] {S : Set ((α ⊕ β) → M)}
     (hS : A.Definable L S) :
     A.Definable L { v : α → M | ∃ u : β → M, Sum.elim v u ∈ S } := by
-  obtain ⟨φ, hφ⟩ := hS
-  exists φ.iExs β
-  ext v
-  simp [hφ]
+  obtain ⟨φ, rfl⟩ := hS
+  simpa using (φ.iExs β).definable_withConstants
 
 /-- Finite universal quantifiers preserve definablity. -/
 lemma Definable.forall_of_finite [Finite β] {S : Set ((α ⊕ β) → M)}
     (hS : A.Definable L S) :
     A.Definable L { v : α → M | ∀ u : β → M, Sum.elim v u ∈ S } := by
-  obtain ⟨φ, hφ⟩ := hS
-  exists φ.iAlls β
-  ext v
-  simp [hφ]
+  obtain ⟨φ, rfl⟩ := hS
+  simpa using (φ.iAlls β).definable_withConstants
 
 variable (L A)
 
@@ -478,10 +523,7 @@ theorem definableFun_iff_empty_definableFun_with_params :
 @[fun_prop]
 theorem _root_.FirstOrder.Language.Term.definableFun_realize (t : L.Term α) :
     (∅ : Set M).DefinableFun L (t.realize) := by
-  rw [empty_definableFun_iff]
-  refine ⟨(t.relabel some).equal (Term.var none), ?_⟩
-  ext v
-  simp [tupleGraph]
+  simpa using ((t.relabel some).equal (Term.var none)).definable
 
 /-- A function symbol is a definable function. -/
 @[fun_prop]
@@ -591,9 +633,7 @@ def TermDefinable (f : (α → M) → M) : Prop :=
 theorem TermDefinable.definable_tupleGraph {f : (α → M) → M} (h : A.TermDefinable L f) :
     A.Definable L f.tupleGraph := by
   obtain ⟨φ, rfl⟩ := h
-  use (φ.relabel some).equal (Term.var none)
-  ext
-  simp [Function.tupleGraph]
+  simpa using ((φ.relabel some).equal (Term.var none)).definable_withConstants
 
 variable {L} {A B} {f : (α → M) → M}
 

Mathlib/ModelTheory/ElementarySubstructures.lean

@@ -159,12 +159,9 @@ theorem closure_eq_self (hA : L.MeetsDefinable A) :
     closure L A = A := by
   refine Subset.antisymm ?_ subset_closure
   rw [coe_closure_eq_range_term_realize]
-  intro x hx
-  have : A.Definable₁ L {x} := by
-    obtain ⟨t, rfl⟩ := hx
-    use (Term.var 0).equal (t.relabel Sum.inl).varsToConstants
-    simp [Set.ext_iff]
-  exact singleton_inter_nonempty.mp <| hA _ (singleton_nonempty x) this
+  rintro x ⟨t, rfl⟩
+  exact singleton_inter_nonempty.mp <| hA _ (singleton_nonempty _) <|
+    by simpa using ((Term.var 0).equal (t.relabel Sum.inl).varsToConstants).definable_withConstants
 
 /-- The closure of a set meeting definable sets is an elementary substructure. -/
 theorem isElementary_closure (hA : L.MeetsDefinable A) :
@@ -174,20 +171,15 @@ theorem isElementary_closure (hA : L.MeetsDefinable A) :
   let D : Set M := {y : M | φ.Realize default (Fin.snoc (Subtype.val ∘ x) y)}
   have hD_ne : D.Nonempty := ⟨a,hφ⟩
   have hD : A.Definable₁ L D := by
-    simp only [Definable₁, Definable, Fin.isValue]
-    refine ⟨((L.lhomWithConstants A).onBoundedFormula φ).toFormula.relabel
-      (Sum.elim Empty.elim id) |>.subst fun i => Fin.lastCases (Term.var 0)
-        (fun j => (L.con ⟨x j, by
+    apply (φ.definable_boundedSection).preimage_map
+    simp only [DefinableMap]
+    refine Fin.lastCases ?_ ?_
+    · simp only [Fin.snoc_last]; fun_prop
+    · intro i; simp only [Fin.snoc_castSucc];
+      have : ↑(x i) ∈ A := by
         nth_rw 1 [← hA.closure_eq_self]
-        simp only [Subtype.coe_prop]
-        ⟩).term) i, ?_⟩
-    ext v
-    simp only [Fin.isValue, mem_setOf_eq, Formula.relabel, Formula.Realize,
-      BoundedFormula.realize_subst, BoundedFormula.realize_relabel, Nat.add_zero, Fin.castAdd_zero,
-      Fin.cast_refl, Function.comp_id, Fin.natAdd_zero, D]
-    rw [← Formula.Realize, BoundedFormula.realize_toFormula, LHom.realize_onBoundedFormula]
-    congr! 1
-    ext i; cases i using Fin.lastCases <;> simp
+        simp
+      fun_prop (disch := assumption)
   obtain ⟨b, hbD, hbA⟩ := hA D hD_ne hD
   exact ⟨⟨b, by rwa [← hA.closure_eq_self] at hbA⟩, hbD⟩