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+/-
+Copyright 2025 The Formal Conjectures Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-/
+
+import FormalConjectures.WrittenOnTheWallII.GraphConjecture19
+import Mathlib.Combinatorics.SimpleGraph.Coloring
+import Mathlib.Combinatorics.SimpleGraph.Bipartite
+
+open SimpleGraph
+
+namespace WrittenOnTheWallII.GraphConjecture19
+
+def K3 : SimpleGraph (Fin 3) := completeGraph (Fin 3)
+
+lemma K3_b : b K3 = 2 := by
+  rw [SimpleGraph.b, SimpleGraph.largestInducedBipartiteSubgraphSize]
+  norm_cast
+  apply le_antisymm
+  · -- b K3 <= 2
+    apply csSup_le
+    · use 0
+      use ∅
+      simp
+      dsimp [SimpleGraph.IsBipartite]
+      apply SimpleGraph.colorable_of_isEmpty
+    · rintro n ⟨s, hs⟩
+      by_contra h_gt
+      simp at h_gt
+      have h_card : s.card = 3 := by
+        have : s.card ≤ 3 := Finset.card_le_univ s
+        linarith
+      have h_univ : s = Finset.univ := Finset.eq_univ_of_card s h_card
+      rw [h_univ] at hs
+      have h_bip : K3.IsBipartite := by
+        have h_iso : K3.induce (Set.univ : Set (Fin 3)) ≃g K3 := SimpleGraph.induceUnivIso K3
+        rw [← Finset.coe_univ] at h_iso
+        exact hs.1.of_embedding h_iso.symm.toEmbedding
+      have h_not_bip : ¬ K3.IsBipartite := by
+        dsimp [SimpleGraph.IsBipartite]
+        rw [← SimpleGraph.chromaticNumber_le_iff_colorable]
+        simp [K3, completeGraph, SimpleGraph.chromaticNumber_top]
+        norm_num
+      contradiction
+  · -- b K3 >= 2
+    apply le_csSup
+    · use 3
+      rintro n ⟨s, hs⟩
+      rw [← hs.2]
+      exact Finset.card_le_univ s
+    · use {0, 1}
+      constructor
+      · -- K3.induce {0, 1} is bipartite (it's K2)
+        dsimp [SimpleGraph.IsBipartite]
+        fconstructor
+        refine SimpleGraph.Coloring.mk (fun x => if (x : Fin 3) = 0 then 0 else 1) ?_
+        intro u v huv
+        have huv_ne : u ≠ v := SimpleGraph.Adj.ne huv
+        have u_mem : (u : Fin 3) ∈ ({0, 1} : Finset (Fin 3)) := u.property
+        have v_mem : (v : Fin 3) ∈ ({0, 1} : Finset (Fin 3)) := v.property
+        simp only [Finset.mem_insert, Finset.mem_singleton] at u_mem v_mem
+        rcases u_mem with u_eq_0 | u_eq_1 <;> rcases v_mem with v_eq_0 | v_eq_1
+        · -- u = 0, v = 0
+          exfalso; apply huv_ne; ext; simp only [u_eq_0, v_eq_0]
+        · -- u = 0, v = 1
+          simp only [u_eq_0, v_eq_1, ↓reduceIte]
+          exact zero_ne_one
+        · -- u = 1, v = 0
+          simp only [u_eq_1, v_eq_0, ↓reduceIte, one_ne_zero]
+          exact zero_ne_one.symm
+        · -- u = 1, v = 1
+          exfalso; apply huv_ne; ext; simp only [u_eq_1, v_eq_1]
+      · simp
+
+lemma K3_ecc (v : Fin 3) : ecc K3 {v} = 1 := by
+  unfold ecc
+  have h_dist : ∀ w, w ≠ v → distToSet K3 w {v} = 1 := by
+    intro w hw
+    simp [distToSet, K3, completeGraph, SimpleGraph.dist_eq_one_iff_adj, SimpleGraph.top_adj]
+    exact hw
+  have h_image : (Finset.univ.filter (fun x => x ≠ v)).image (fun x => distToSet K3 x {v}) = {1} := by
+    ext x
+    constructor
+    · intro h
+      rw [Finset.mem_image] at h
+      rcases h with ⟨w, hw1, hw2⟩
+      rw [h_dist w] at hw2
+      · rw [← hw2]
+        simp
+      · simp only [Finset.mem_filter, Finset.mem_univ, true_and] at hw1
+        exact hw1
+    · intro h
+      simp at h
+      rw [h]
+      rw [Finset.mem_image]
+      use (v + 1)
+      constructor
+      · simp
+      · rw [h_dist]
+        simp
+  -- We need to show the condition is true
+  have h_nonempty : (Finset.univ.filter (fun x => x ∉ ({v} : Set (Fin 3)))).Nonempty := by
+    use (v + 1)
+    simp
+  dsimp
+  split_ifs with h_cond
+  · convert Finset.max'_singleton 1
+    simp [h_image]
+  · exact h_cond (by convert h_nonempty)
+
+lemma indepNum_completeGraph {V : Type*} [Fintype V] [DecidableEq V] [Nonempty V] :
+    (completeGraph V).indepNum = 1 := by
+  rw [SimpleGraph.indepNum, completeGraph_eq_top]
+  apply le_antisymm
+  · apply csSup_le
+    · use 1
+      use {Classical.arbitrary V}
+      simp [SimpleGraph.isNIndepSet_iff]
+    · rintro n ⟨s, hs⟩
+      rw [SimpleGraph.isNIndepSet_iff] at hs
+      obtain ⟨h_indep, h_card⟩ := hs
+      rw [SimpleGraph.isIndepSet_iff] at h_indep
+      have h_sub : (s : Set V).Subsingleton := by
+        intro a ha b hb
+        by_contra h_ne
+        have h_adj : ¬ (⊤ : SimpleGraph V).Adj a b := h_indep ha hb h_ne
+        simp [SimpleGraph.top_adj] at h_adj
+        contradiction
+      rw [← h_card]
+      exact Finset.card_le_one.mpr h_sub
+  · apply le_csSup
+    · use Fintype.card V
+      rintro n ⟨s, hs⟩
+      rw [← hs.card_eq]
+      exact Finset.card_le_univ s
+    · use {Classical.arbitrary V}
+      simp [SimpleGraph.isNIndepSet_iff]
+
+lemma K3_indepNeighbors (v : Fin 3) : indepNeighbors K3 v = 1 := by
+  unfold indepNeighbors
+  have h_induced : K3.induce (K3.neighborSet v) = completeGraph (K3.neighborSet v) := by
+    ext a b
+    simp [K3, completeGraph]
+    simp [Subtype.coe_ne_coe]
+  dsimp [indepNeighborsCard]
+  rw [h_induced]
+  haveI : Nonempty (K3.neighborSet v) := by
+    use v + 1
+    simp [K3, completeGraph, SimpleGraph.neighborSet, SimpleGraph.top_adj]
+  haveI : Fintype (K3.neighborSet v) := Set.Finite.fintype (Set.toFinite _)
+  rw [indepNum_completeGraph]
+  norm_cast
+
+theorem K3_not_counterexample :
+    FLOOR ((∑ v ∈ Finset.univ, ecc K3 {v}) / (Fintype.card (Fin 3) : ℝ) + sSup (Set.range (fun (v : Fin 3) => indepNeighbors K3 v)))
+      ≤ b K3 := by
+  rw [K3_b]
+  have h_ecc : ∀ v, ecc K3 {v} = 1 := K3_ecc
+  simp [h_ecc]
+  have h_indep : ∀ v, indepNeighbors K3 v = 1 := K3_indepNeighbors
+  simp [h_indep]
+  rw [FLOOR]
+  norm_num