feat(Combinatorics/Graph): infimum (leanprover-community#37620)

Adds `Mathlib/Combinatorics/Graph/Lattice.lean`, which defines intersections, as `Inf`, of Graph α β values and proves the lattice structure induced by binary intersection. 

## Main additions
- `SemilatticeInf (Graph α β)`

Co-authored-by: Peter Nelson [apn.uni@gmail.com](mailto:apn.uni@gmail.com)

Author: Jun2M

Mathlib.lean

@@ -3452,6 +3452,7 @@ public import Mathlib.Combinatorics.Enumerative.Stirling
 public import Mathlib.Combinatorics.Extremal.RuzsaSzemeredi
 public import Mathlib.Combinatorics.Graph.Basic
 public import Mathlib.Combinatorics.Graph.Delete
+public import Mathlib.Combinatorics.Graph.Lattice
 public import Mathlib.Combinatorics.Graph.Subgraph
 public import Mathlib.Combinatorics.HalesJewett
 public import Mathlib.Combinatorics.Hall.Basic

Mathlib/Combinatorics/Graph/Lattice.lean

@@ -0,0 +1,92 @@
+/-
+Copyright (c) 2026 Jun Kwon. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Peter Nelson, Jun Kwon
+-/
+module
+
+public import Mathlib.Combinatorics.Graph.Subgraph
+
+/-!
+# Intersection and union of graphs
+
+This file defines the lattice-like structures on graphs.
+
+## Main results
+
+- `SemilatticeInf (Graph α β)`
+
+## Implementation notes
+
+Intersections are defined here as the maximal mutual subgraph of the given graphs.
+This has the effect of, when taking the intersection of non-compatible graphs,
+**any non-compatible edges are removed**.
+
+## TODO
+
++ Add `ConditionallyCompleteCompleteLatticeInf (Graph α β)` after splitting
+  `ConditionallyCompleteCompleteLattice`.
+
+-/
+
+public section
+
+open Function Set
+
+variable {α β : Type*} {x y : α} {e : β} {G H : Graph α β}
+
+namespace Graph
+
+/-- The infimum of two graphs `G` and `H`. The edges are precisely those on which `G` and `H` agree,
+and the edge set is a subset of `E(G) ∩ E(H)`, with equality if `G` and `H` are compatible. -/
+instance : SemilatticeInf (Graph α β) where
+  inf G H := {
+    vertexSet := V(G) ∩ V(H)
+    edgeSet := {e ∈ E(G) ∩ E(H) | ∀ x y, G.IsLink e x y ↔ H.IsLink e x y}
+    IsLink e x y := G.IsLink e x y ∧ H.IsLink e x y
+    isLink_symm _ _ _ _ h := ⟨h.1.symm, h.2.symm⟩
+    eq_or_eq_of_isLink_of_isLink _ _ _ _ _ h h' := h.1.left_eq_or_eq h'.1
+    edge_mem_iff_exists_isLink e := by
+      simp only [edgeSet_eq_setOf_exists_isLink, mem_inter_iff, mem_setOf_eq]
+      exact ⟨fun ⟨⟨⟨x, y, hexy⟩, ⟨z, w, hezw⟩⟩, h⟩ ↦ ⟨x, y, hexy, by rwa [← h]⟩,
+        fun ⟨x, y, hfG, hfH⟩ ↦ ⟨⟨⟨_, _, hfG⟩, ⟨_, _, hfH⟩⟩,
+        fun z w ↦ by rw [hfG.isLink_iff_sym2_eq, hfH.isLink_iff_sym2_eq]⟩⟩
+    left_mem_of_isLink e x y h := ⟨h.1.left_mem, h.2.left_mem⟩}
+  inf_le_left G H := {
+    vertexSet_mono := inter_subset_left
+    isLink_mono := by simp +contextual}
+  inf_le_right G H := {
+    vertexSet_mono := inter_subset_right
+    isLink_mono := by simp +contextual}
+  le_inf H G₁ G₂ h₁ h₂ := {
+    vertexSet_mono := subset_inter h₁.vertexSet_mono h₂.vertexSet_mono
+    isLink_mono e x y h := by simp [h₁.isLink_mono h, h₂.isLink_mono h]}
+
+@[simp] lemma vertexSet_inf (G H : Graph α β) : V(G ⊓ H) = V(G) ∩ V(H) := rfl
+
+lemma edgeSet_inf (G H : Graph α β) :
+    E(G ⊓ H) = {e ∈ E(G) ∩ E(H) | ∀ x y, G.IsLink e x y ↔ H.IsLink e x y} := rfl
+
+@[simp] lemma inf_isLink : (G ⊓ H).IsLink e x y ↔ G.IsLink e x y ∧ H.IsLink e x y := Iff.rfl
+
+@[simp]
+lemma inf_inc_iff : (G ⊓ H).Inc e x ↔ ∃ y, G.IsLink e x y ∧ H.IsLink e x y := by
+  simp [Inc]
+
+@[simp]
+lemma inf_isLoopAt_iff : (G ⊓ H).IsLoopAt e x ↔ G.IsLoopAt e x ∧ H.IsLoopAt e x := by
+  simp [← isLink_self_iff]
+
+@[simp]
+lemma inf_isNonloopAt_iff : (G ⊓ H).IsNonloopAt e x ↔ ∃ y ≠ x, G.IsLink e x y ∧ H.IsLink e x y := by
+  simp [IsNonloopAt]
+
+@[simp]
+protected lemma disjoint_iff : Disjoint G H ↔ Disjoint V(G) V(H) := by
+  rw [disjoint_iff, ← vertexSet_eq_empty_iff, vertexSet_inf, disjoint_iff_inter_eq_empty]
+
+protected lemma Compatible.edgeSet_inf (h : G.Compatible H) : E(G ⊓ H) = E(G) ∩ E(H) := by
+  rw [G.edgeSet_inf]
+  exact le_antisymm (fun e he ↦ he.1) fun e he ↦ ⟨he, fun _ _ ↦ h.isLink_congr he.1 he.2⟩
+
+end Graph