diff --git a/HepLean/Tensors/ComplexLorentz/PauliMatrices/Basic.lean b/HepLean/Tensors/ComplexLorentz/PauliMatrices/Basic.lean
index 021150b..879f1dd 100644
--- a/HepLean/Tensors/ComplexLorentz/PauliMatrices/Basic.lean
+++ b/HepLean/Tensors/ComplexLorentz/PauliMatrices/Basic.lean
@@ -185,4 +185,63 @@ lemma pauliContrDown_eq_metric_mul_pauliContr :
   apply perm_congr _ rfl
   decide
 
+/-!
+
+## Group actions
+
+-/
+
+/-- The tensor `pauliContr` is invariant under the action of `SL(2,ℂ)`. -/
+lemma action_pauliContr (g : SL(2,ℂ)) : {g •ₐ pauliContr | μ α β}ᵀ.tensor =
+    {pauliContr | μ α β}ᵀ.tensor := by
+  rw [tensorNode_pauliContr, constThreeNodeE]
+  rw [← action_constThreeNode _ g]
+  rfl
+
+/-- The tensor `pauliCo` is invariant under the action of `SL(2,ℂ)`. -/
+lemma action_pauliCo (g : SL(2,ℂ)) : {g •ₐ pauliCo | μ α β}ᵀ.tensor =
+    {pauliCo | μ α β}ᵀ.tensor := by
+  conv =>
+    lhs
+    rw [action_tensor_eq <| tensorNode_pauliCo]
+    rw [(contr_action _ _).symm]
+    rw [contr_tensor_eq <| (prod_action _ _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| action_constTwoNode _ _]
+    rw [contr_tensor_eq <| prod_tensor_eq_snd <| action_constThreeNode _ _]
+  rfl
+
+/-- The tensor `pauliCoDown` is invariant under the action of `SL(2,ℂ)`. -/
+lemma action_pauliCoDown (g : SL(2,ℂ)) : {g •ₐ pauliCoDown | μ α β}ᵀ.tensor =
+    {pauliCoDown | μ α β}ᵀ.tensor := by
+  conv =>
+    lhs
+    rw [action_tensor_eq <| tensorNode_pauliCoDown]
+    rw [(contr_action _ _).symm]
+    rw [contr_tensor_eq <| (prod_action _ _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| (contr_action _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| (prod_action _ _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| prod_tensor_eq_fst <|
+      action_pauliCo _]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| prod_tensor_eq_snd <|
+      action_constTwoNode _ _]
+    rw [contr_tensor_eq <| prod_tensor_eq_snd <| action_constTwoNode _ _]
+  rfl
+
+/-- The tensor `pauliContrDown` is invariant under the action of `SL(2,ℂ)`. -/
+lemma action_pauliContrDown (g : SL(2,ℂ)) : {g •ₐ pauliContrDown | μ α β}ᵀ.tensor =
+    {pauliContrDown | μ α β}ᵀ.tensor := by
+  conv =>
+    lhs
+    rw [action_tensor_eq <| tensorNode_pauliContrDown]
+    rw [(contr_action _ _).symm]
+    rw [contr_tensor_eq <| (prod_action _ _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| (contr_action _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| (prod_action _ _ _).symm]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| prod_tensor_eq_fst <|
+      action_pauliContr _]
+    rw [contr_tensor_eq <| prod_tensor_eq_fst <| contr_tensor_eq <| prod_tensor_eq_snd <|
+      action_constTwoNode _ _]
+    rw [contr_tensor_eq <| prod_tensor_eq_snd <| action_constTwoNode _ _]
+  rfl
+
 end complexLorentzTensor
diff --git a/HepLean/Tensors/Tree/Basic.lean b/HepLean/Tensors/Tree/Basic.lean
index dc42c9e..72b222a 100644
--- a/HepLean/Tensors/Tree/Basic.lean
+++ b/HepLean/Tensors/Tree/Basic.lean
@@ -543,9 +543,13 @@ inductive TensorTree (S : TensorSpecies) : ∀ {n : ℕ}, (Fin n → S.C) → Ty
   | smul {n : ℕ} {c : Fin n → S.C} : S.k → TensorTree S c → TensorTree S c
   /-- The negative of a node. -/
   | neg {n : ℕ} {c : Fin n → S.C} : TensorTree S c → TensorTree S c
+  /-- The contraction of indices. -/
   | contr {n : ℕ} {c : Fin n.succ.succ → S.C} : (i : Fin n.succ.succ) →
     (j : Fin n.succ) → (h : c (i.succAbove j) = S.τ (c i)) → TensorTree S c →
     TensorTree S (c ∘ Fin.succAbove i ∘ Fin.succAbove j)
+  /-- The group action on a tensor. -/
+  | action {n : ℕ} {c : Fin n → S.C} : S.G → TensorTree S c → TensorTree S c
+  /-- The evaluation of an index-/
   | eval {n : ℕ} {c : Fin n.succ → S.C} :
     (i : Fin n.succ) → (x : ℕ) → TensorTree S c →
     TensorTree S (c ∘ Fin.succAbove i)
@@ -642,6 +646,7 @@ def size : ∀ {n : ℕ} {c : Fin n → S.C}, TensorTree S c → ℕ := fun
   | prod t1 t2 => t1.size + t2.size + 1
   | contr _ _ _ t => t.size + 1
   | eval _ _ t => t.size + 1
+  | action _ t => t.size + 1
 
 noncomputable section
 
@@ -657,6 +662,7 @@ def tensor : ∀ {n : ℕ} {c : Fin n → S.C}, TensorTree S c → S.F.obj (Over
     ((S.F.μ _ _).hom (t1.tensor ⊗ₜ t2.tensor))
   | contr i j h t => (S.contrMap _ i j h).hom t.tensor
   | eval i e t => (S.evalMap i (Fin.ofNat' e Fin.size_pos')) t.tensor
+  | action g t => (S.F.obj (OverColor.mk _)).ρ g t.tensor
 
 /-- Takes a tensor tree based on `Fin 0`, into the field `S.k`. -/
 def field {c : Fin 0 → S.C} (t : TensorTree S c) : S.k := S.castFin0ToField t.tensor
@@ -707,6 +713,10 @@ lemma eval_tensor {n : ℕ} {c : Fin n.succ → S.C} (i : Fin n.succ) (e : ℕ)
 
 lemma smul_tensor {c : Fin n → S.C} (a : S.k) (T : TensorTree S c) :
     (smul a T).tensor = a • T.tensor:= rfl
+
+lemma action_tensor {c : Fin n → S.C} (g : S.G) (T : TensorTree S c) :
+    (action g T).tensor = (S.F.obj (OverColor.mk c)).ρ g T.tensor := rfl
+
 /-!
 
 ## Equality of tensors and rewrites.
@@ -770,6 +780,11 @@ lemma smul_tensor_eq {T1 T2 : TensorTree S c} {a : S.k} (h : T1.tensor = T2.tens
   simp only [smul_tensor]
   rw [h]
 
+lemma action_tensor_eq {T1 T2 : TensorTree S c} {g : S.G} (h : T1.tensor = T2.tensor) :
+    (action g T1).tensor = (action g T2).tensor := by
+  simp only [action_tensor]
+  rw [h]
+
 lemma smul_mul_eq {T1 : TensorTree S c} {a b : S.k} (h : a = b) :
     (smul a T1).tensor = (smul b T1).tensor := by
   rw [h]
diff --git a/HepLean/Tensors/Tree/Dot.lean b/HepLean/Tensors/Tree/Dot.lean
index 07a7c8c..c05551e 100644
--- a/HepLean/Tensors/Tree/Dot.lean
+++ b/HepLean/Tensors/Tree/Dot.lean
@@ -47,6 +47,10 @@ def dotString (m : ℕ) (nt : ℕ) : ∀ {n : ℕ} {c : Fin n → S.C}, TensorTr
     let evalNode := " node" ++ toString m ++ " [label=\"eval\", shape=box];\n"
     let edge1 := " node" ++ toString m ++ " -> node" ++ toString (m + 1) ++ ";\n"
     evalNode ++ dotString (m + 1) nt t1 ++ edge1
+  | action g t =>
+    let actionNode := " node" ++ toString m ++ " [label=\"action\", shape=box];\n"
+    let edge1 := " node" ++ toString m ++ " -> node" ++ toString (m + 1) ++ ";\n"
+    actionNode ++ dotString (m + 1) nt t ++ edge1
   | contr i j _ t1 =>
     let contrNode := " node" ++ toString m ++ " [label=\"contr " ++ toString i ++ " "
       ++ toString j ++ "\", shape=box];\n"
diff --git a/HepLean/Tensors/Tree/Elab.lean b/HepLean/Tensors/Tree/Elab.lean
index ac1a863..9424411 100644
--- a/HepLean/Tensors/Tree/Elab.lean
+++ b/HepLean/Tensors/Tree/Elab.lean
@@ -111,6 +111,9 @@ syntax "(" tensorExpr ")" : tensorExpr
 /-- Scalar multiplication for tensors. -/
 syntax term "•ₜ" tensorExpr : tensorExpr
 
+/-- group action for tensors. -/
+syntax term "•ₐ" tensorExpr : tensorExpr
+
 /-- Negation of a tensor tree. -/
 syntax "-" tensorExpr : tensorExpr
 
@@ -440,6 +443,14 @@ def smulSyntax (c T : Term) : Term :=
 
 end SMul
 
+namespace Action
+
+/-- The syntax associated with the group action of tensors. -/
+def actionSyntax (c T : Term) : Term :=
+  Syntax.mkApp (mkIdent ``TensorTree.action) #[c, T]
+
+end Action
+
 namespace Add
 
 /-- Gets the indices associated with the LHS of an addition. -/
@@ -522,6 +533,8 @@ partial def syntaxFull (stx : Syntax) : TermElabM Term := do
       return negNode.negSyntax (← syntaxFull a)
   | `(tensorExpr| $c:term •ₜ $a:tensorExpr) => do
       return SMul.smulSyntax c (← syntaxFull a)
+  | `(tensorExpr| $c:term •ₐ $a:tensorExpr) => do
+      return Action.actionSyntax c (← syntaxFull a)
   | `(tensorExpr| $a + $b) => do
       let indicesLeft ← Add.getIndicesLeft stx
       let indicesRight ← Add.getIndicesRight stx
diff --git a/HepLean/Tensors/Tree/NodeIdentities/Basic.lean b/HepLean/Tensors/Tree/NodeIdentities/Basic.lean
index ed1f0f6..afae4fa 100644
--- a/HepLean/Tensors/Tree/NodeIdentities/Basic.lean
+++ b/HepLean/Tensors/Tree/NodeIdentities/Basic.lean
@@ -244,4 +244,88 @@ lemma prod_add_both {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
   rw [add_tensor_eq_fst (prod_add _ _ _)]
   rw [add_tensor_eq_snd (prod_add _ _ _)]
 
+/-!
+
+## Nodes and the group action.
+
+-/
+
+lemma smul_action {n : ℕ} {c : Fin n → S.C} (g : S.G) (a : S.k) (t : TensorTree S c) :
+    (smul a (action g t)).tensor = (action g (smul a t)).tensor := by
+  simp only [smul_tensor, action_tensor, map_smul]
+
+lemma contr_action {n : ℕ} {c : Fin n.succ.succ → S.C} {i : Fin n.succ.succ} {j : Fin n.succ}
+    {h : c (i.succAbove j) = S.τ (c i)} (g : S.G) (t : TensorTree S c) :
+    (contr i j h (action g t)).tensor = (action g (contr i j h t)).tensor := by
+  simp only [Nat.succ_eq_add_one, contr_tensor, action_tensor]
+  change ((S.F.obj (OverColor.mk c)).ρ g ≫ (S.contrMap c i j h).hom) t.tensor = _
+  erw [(S.contrMap c i j h).comm g]
+  rfl
+
+lemma prod_action {n n1 : ℕ} {c : Fin n → S.C} {c1 : Fin n1 → S.C} (g : S.G)
+    (t : TensorTree S c) (t1 : TensorTree S c1) :
+    (prod (action g t) (action g t1)).tensor = (action g (prod t t1)).tensor := by
+  simp only [prod_tensor, action_tensor, map_tmul]
+  change _ = ((S.F.map (equivToIso finSumFinEquiv).hom).hom ≫
+    (S.F.obj (OverColor.mk (Sum.elim c c1 ∘ ⇑finSumFinEquiv.symm))).ρ g)
+    (((S.F.μ (OverColor.mk c) (OverColor.mk c1)).hom (t.tensor ⊗ₜ[S.k] t1.tensor)))
+  erw [← (S.F.map (equivToIso finSumFinEquiv).hom).comm g]
+  simp
+  change _ = (S.F.map (equivToIso finSumFinEquiv).hom).hom
+    (((S.F.μ (OverColor.mk c) (OverColor.mk c1)).hom ≫ (S.F.obj (OverColor.mk (Sum.elim c c1))).ρ g)
+      (t.tensor ⊗ₜ[S.k] t1.tensor))
+  erw [← (S.F.μ (OverColor.mk c) (OverColor.mk c1)).comm g]
+  rfl
+
+lemma add_action {n : ℕ} {c : Fin n → S.C} (g : S.G) (t t1 : TensorTree S c) :
+    (add (action g t) (action g t1)).tensor = (action g (add t t1)).tensor := by
+  simp only [add_tensor, action_tensor, map_add]
+
+lemma perm_action {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
+    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (g : S.G) (t : TensorTree S c) :
+    (perm σ (action g t)).tensor = (action g (perm σ t)).tensor := by
+  simp only [perm_tensor, action_tensor]
+  change (((S.F.obj (OverColor.mk c)).ρ g) ≫ (S.F.map σ).hom) t.tensor = _
+  erw [(S.F.map σ).comm g]
+  rfl
+
+lemma neg_action {n : ℕ} {c : Fin n → S.C} (g : S.G) (t : TensorTree S c) :
+    (neg (action g t)).tensor = (action g (neg t)).tensor := by
+  simp only [neg_tensor, action_tensor, map_neg]
+
+lemma action_action {n : ℕ} {c : Fin n → S.C} (g h : S.G) (t : TensorTree S c) :
+    (action g (action h t)).tensor = (action (g * h) t).tensor := by
+  simp only [action_tensor, map_mul, LinearMap.mul_apply]
+
+lemma action_id {n : ℕ} {c : Fin n → S.C} (t : TensorTree S c) :
+    (action 1 t).tensor = t.tensor := by
+  simp only [action_tensor, map_one, LinearMap.one_apply]
+
+lemma action_constTwoNode {c1 c2 : S.C}
+    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2))
+    (g : S.G) : (action g (constTwoNode v)).tensor = (constTwoNode v).tensor := by
+  simp only [Nat.succ_eq_add_one, Nat.reduceAdd, action_tensor, constTwoNode_tensor,
+    Action.instMonoidalCategory_tensorObj_V, Action.instMonoidalCategory_tensorUnit_V]
+  change ((Discrete.pairIsoSep S.FDiscrete).hom.hom ≫ (S.F.obj (OverColor.mk ![c1, c2])).ρ g)
+    ((v.hom _)) = _
+  erw [← (Discrete.pairIsoSep S.FDiscrete).hom.comm g]
+  change ((v.hom ≫ (S.FDiscrete.obj { as := c1 } ⊗ S.FDiscrete.obj { as := c2 }).ρ g) ≫
+    (Discrete.pairIsoSep S.FDiscrete).hom.hom) _ =_
+  erw [← v.comm g]
+  simp
+
+lemma action_constThreeNode {c1 c2 c3 : S.C}
+    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2) ⊗
+      S.FDiscrete.obj (Discrete.mk c3))
+    (g : S.G) : (action g (constThreeNode v)).tensor = (constThreeNode v).tensor := by
+  simp only [Nat.succ_eq_add_one, Nat.reduceAdd, action_tensor, constThreeNode_tensor,
+    Action.instMonoidalCategory_tensorObj_V, Action.instMonoidalCategory_tensorUnit_V]
+  change ((Discrete.tripleIsoSep S.FDiscrete).hom.hom ≫ (S.F.obj (OverColor.mk ![c1, c2, c3])).ρ g)
+    ((v.hom _)) = _
+  erw [← (Discrete.tripleIsoSep S.FDiscrete).hom.comm g]
+  change ((v.hom ≫ (S.FDiscrete.obj { as := c1 } ⊗ S.FDiscrete.obj { as := c2 } ⊗
+    S.FDiscrete.obj { as := c3 }).ρ g) ≫ (Discrete.tripleIsoSep S.FDiscrete).hom.hom) _ =_
+  erw [← v.comm g]
+  simp
+
 end TensorTree