feat(CategoryTheory/Monoidal): tensorμ_braid_swap and tensor-product IsCommComonObj

Mathlib.lean

@@ -3076,6 +3076,7 @@ public import Mathlib.CategoryTheory.Monoidal.Rigid.FunctorCategory
 public import Mathlib.CategoryTheory.Monoidal.Rigid.OfEquivalence
 public import Mathlib.CategoryTheory.Monoidal.Skeleton
 public import Mathlib.CategoryTheory.Monoidal.Subcategory
+public import Mathlib.CategoryTheory.Monoidal.Symmetric.TensorBraidSwap
 public import Mathlib.CategoryTheory.Monoidal.Tor
 public import Mathlib.CategoryTheory.Monoidal.Transport
 public import Mathlib.CategoryTheory.Monoidal.Types.Basic

Mathlib/CategoryTheory/Monoidal/Symmetric/TensorBraidSwap.lean

@@ -0,0 +1,71 @@
+/-
+Copyright (c) 2026 Pedro Cortes. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Pedro Cortes
+-/
+module
+
+public import Mathlib.CategoryTheory.Monoidal.Braided.Basic
+public import Mathlib.CategoryTheory.Monoidal.CommComon_
+
+/-!
+# Tensor-braid swap coherence in symmetric monoidal categories
+
+The canonical rearrangement `tensorμ` intertwines the braiding on `A ⊗ Y`
+with the pair of braidings on `A` and `Y` in any symmetric monoidal
+category. This is the sibling of `CategoryTheory.MonObj.mul_braiding`
+(`Mathlib.CategoryTheory.Monoidal.Mon_`), and yields the tensor-product
+`IsCommComonObj` instance.
+
+Placed in its own file to avoid an instance-diamond issue in `Braided/Basic.lean`,
+where the ambient `[BraidedCategory C]` variable clashes with the
+`[SymmetricCategory C]`-derived `BraidedCategory` instance, confusing
+`rw [SymmetricCategory.symmetry]` motive extraction on Lean v4.30.0-rc2.
+-/
+
+@[expose] public section
+
+universe v u
+
+namespace CategoryTheory
+
+open MonoidalCategory BraidedCategory
+
+variable {C : Type u} [Category.{v} C] [MonoidalCategory C] [SymmetricCategory C]
+
+/-- **Symmetric-monoidal tensor-braid swap coherence.**
+The canonical rearrangement `tensorμ` intertwines the braiding on `A ⊗ Y`
+with the pair of braidings on `A` and `Y`. Both sides implement the
+permutation `(a₁, a₂, y₁, y₂) ↦ (a₂, y₂, a₁, y₁)`. Sibling of
+`MonObj.mul_braiding` (`Mathlib.CategoryTheory.Monoidal.Mon_`). -/
+@[reassoc]
+theorem MonoidalCategory.tensorμ_braid_swap (A Y : C) :
+    tensorμ A A Y Y ≫ (β_ (A ⊗ Y) (A ⊗ Y)).hom =
+      ((β_ A A).hom ⊗ₘ (β_ Y Y).hom) ≫ tensorμ A A Y Y := by
+  simp only [tensorμ, Category.assoc,
+    BraidedCategory.braiding_tensor_right_hom,
+    BraidedCategory.braiding_tensor_left_hom, comp_whiskerRight,
+    whisker_assoc, whiskerLeft_comp, pentagon_assoc,
+    pentagon_inv_hom_hom_hom_inv_assoc, Iso.inv_hom_id_assoc,
+    whiskerLeft_hom_inv_assoc]
+  slice_lhs 3 4 => rw [← whiskerLeft_comp, ← comp_whiskerRight,
+                       SymmetricCategory.symmetry]
+  simp only [id_whiskerRight, whiskerLeft_id, Category.id_comp, Category.assoc,
+             tensorHom_def, tensor_whiskerLeft, whiskerRight_tensor]
+  monoidal
+
+open scoped ComonObj
+
+/-- The tensor product of two commutative comonoid objects is again a
+commutative comonoid object. The proof reduces the commutativity
+equation for `Δ[A ⊗ B]` to `tensorμ_braid_swap` plus the two pointwise
+commutativity hypotheses `IsCommComonObj.comul_comm A`, `.comul_comm B`. -/
+instance (A B : C) [ComonObj A] [ComonObj B]
+    [IsCommComonObj A] [IsCommComonObj B] : IsCommComonObj (A ⊗ B) where
+  comul_comm := by
+    rw [Comon.tensorObj_comul, Category.assoc,
+        MonoidalCategory.tensorμ_braid_swap,
+        ← Category.assoc, tensorHom_comp_tensorHom,
+        IsCommComonObj.comul_comm A, IsCommComonObj.comul_comm B]
+
+end CategoryTheory