diff --git a/Mathlib.lean b/Mathlib.lean
index efc6092ae7e2d..95501c3b5c701 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -2374,6 +2374,7 @@ import Mathlib.NumberTheory.Liouville.Residual
 import Mathlib.NumberTheory.LucasLehmer
 import Mathlib.NumberTheory.LucasPrimality
 import Mathlib.NumberTheory.ModularForms.CongruenceSubgroups
+import Mathlib.NumberTheory.ModularForms.SlashActions
 import Mathlib.NumberTheory.Multiplicity
 import Mathlib.NumberTheory.NumberField.Basic
 import Mathlib.NumberTheory.NumberField.CanonicalEmbedding
diff --git a/Mathlib/NumberTheory/ModularForms/SlashActions.lean b/Mathlib/NumberTheory/ModularForms/SlashActions.lean
new file mode 100644
index 0000000000000..c9c98c1ba35a4
--- /dev/null
+++ b/Mathlib/NumberTheory/ModularForms/SlashActions.lean
@@ -0,0 +1,252 @@
+/-
+Copyright (c) 2022 Chris Birkbeck. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Chris Birkbeck
+
+! This file was ported from Lean 3 source module number_theory.modular_forms.slash_actions
+! leanprover-community/mathlib commit 738054fa93d43512da144ec45ce799d18fd44248
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.Analysis.Complex.UpperHalfPlane.Basic
+import Mathlib.LinearAlgebra.Matrix.GeneralLinearGroup
+import Mathlib.LinearAlgebra.Matrix.SpecialLinearGroup
+
+/-!
+# Slash actions
+
+This file defines a class of slash actions, which are families of right actions of a given group
+parametrized by some Type. This is modeled on the slash action of `GLPos (Fin 2) ℝ` on the space
+of modular forms.
+
+## Notation
+
+In the `ModularForm` locale, this provides
+
+* `f ∣[k;γ] A`: the `k`th `γ`-compatible slash action by `A` on `f`
+* `f ∣[k] A`: the `k`th `ℂ`-compatible slash action by `A` on `f`; a shorthand for `f ∣[k;ℂ] A`
+-/
+
+
+open Complex UpperHalfPlane
+
+open scoped UpperHalfPlane
+
+local macro_rules | `($x ^ $y) => `(HPow.hPow $x $y) -- Porting note: See issue #2220
+
+local notation "GL(" n ", " R ")" "⁺" => Matrix.GLPos (Fin n) R
+
+local notation "SL(" n ", " R ")" => Matrix.SpecialLinearGroup (Fin n) R
+
+local notation:1024 "↑ₘ" A:1024 =>
+  (((A : GL(2, ℝ)⁺) : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) _)
+-- like `↑ₘ`, but allows the user to specify the ring `R`. Useful to help Lean elaborate.
+local notation:1024 "↑ₘ[" R "]" A:1024 =>
+  ((A : GL (Fin 2) R) : Matrix (Fin 2) (Fin 2) R)
+
+/-- A general version of the slash action of the space of modular forms.-/
+class SlashAction (β G α γ : Type _) [Group G] [AddMonoid α] [SMul γ α] where
+  map : β → G → α → α
+  zero_slash : ∀ (k : β) (g : G), map k g 0 = 0
+  slash_one : ∀ (k : β) (a : α), map k 1 a = a
+  slash_mul : ∀ (k : β) (g h : G) (a : α), map k (g * h) a = map k h (map k g a)
+  smul_slash : ∀ (k : β) (g : G) (a : α) (z : γ), map k g (z • a) = z • map k g a
+  add_slash : ∀ (k : β) (g : G) (a b : α), map k g (a + b) = map k g a + map k g b
+#align slash_action SlashAction
+
+scoped[ModularForm] notation:100 f " ∣[" k ";" γ "] " a:100 => SlashAction.map γ k a f
+
+scoped[ModularForm] notation:100 f " ∣[" k "] " a:100 => SlashAction.map ℂ k a f
+
+open scoped ModularForm
+
+@[simp]
+theorem SlashAction.neg_slash {β G α γ : Type _} [Group G] [AddGroup α] [SMul γ α]
+    [SlashAction β G α γ] (k : β) (g : G) (a : α) : (-a) ∣[k;γ] g = -a ∣[k;γ] g :=
+  eq_neg_of_add_eq_zero_left <| by
+    rw [← SlashAction.add_slash, add_left_neg, SlashAction.zero_slash]
+#align slash_action.neg_slash SlashAction.neg_slash
+
+@[simp]
+theorem SlashAction.smul_slash_of_tower {R β G α : Type _} (γ : Type _) [Group G] [AddGroup α]
+    [Monoid γ] [MulAction γ α] [SMul R γ] [SMul R α] [IsScalarTower R γ α] [SlashAction β G α γ]
+    (k : β) (g : G) (a : α) (r : R) : (r • a) ∣[k;γ] g = r • a ∣[k;γ] g := by
+  rw [← smul_one_smul γ r a, SlashAction.smul_slash, smul_one_smul]
+#align slash_action.smul_slash_of_tower SlashAction.smul_slash_of_tower
+
+attribute [simp] SlashAction.zero_slash SlashAction.slash_one SlashAction.smul_slash
+  SlashAction.add_slash
+
+/-- Slash_action induced by a monoid homomorphism.-/
+def monoidHomSlashAction {β G H α γ : Type _} [Group G] [AddMonoid α] [SMul γ α] [Group H]
+    [SlashAction β G α γ] (h : H →* G) : SlashAction β H α γ where
+  map k g := SlashAction.map γ k (h g)
+  zero_slash k g := SlashAction.zero_slash k (h g)
+  slash_one k a := by simp only [map_one, SlashAction.slash_one]
+  slash_mul k g gg a := by simp only [map_mul, SlashAction.slash_mul]
+  smul_slash _ _ := SlashAction.smul_slash _ _
+  add_slash _ g _ _ := SlashAction.add_slash _ (h g) _ _
+#align monoid_hom_slash_action monoidHomSlashAction
+
+namespace ModularForm
+
+noncomputable section
+
+/-- The weight `k` action of `GL(2, ℝ)⁺` on functions `f : ℍ → ℂ`. -/
+def slash (k : ℤ) (γ : GL(2, ℝ)⁺) (f : ℍ → ℂ) (x : ℍ) : ℂ :=
+  f (γ • x) * (((↑ₘγ).det : ℝ) : ℂ) ^ (k - 1) * UpperHalfPlane.denom γ x ^ (-k)
+#align modular_form.slash ModularForm.slash
+
+variable {Γ : Subgroup SL(2, ℤ)} {k : ℤ} (f : ℍ → ℂ)
+
+section
+
+-- temporary notation until the instance is built
+local notation:100 f " ∣[" k "]" γ:100 => ModularForm.slash k γ f
+
+private theorem slash_mul (k : ℤ) (A B : GL(2, ℝ)⁺) (f : ℍ → ℂ) :
+    f ∣[k](A * B) = (f ∣[k]A) ∣[k]B := by
+  ext1 x
+  simp_rw [slash, UpperHalfPlane.denom_cocycle A B x]
+  have e3 : (A * B) • x = A • B • x := by convert UpperHalfPlane.mul_smul' A B x
+  rw [e3]
+  simp only [UpperHalfPlane.num, UpperHalfPlane.denom, ofReal_mul, Subgroup.coe_mul,
+    UpperHalfPlane.coe_smul, Units.val_mul, Matrix.mul_eq_mul, Matrix.det_mul,
+    UpperHalfPlane.smulAux, UpperHalfPlane.smulAux', Subtype.coe_mk] at *
+  field_simp
+  have : (((↑(↑A : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) ℝ).det : ℂ) *
+      ((↑(↑B : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) ℝ).det : ℂ)) ^ (k - 1) =
+      ((↑(↑A : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) ℝ).det : ℂ) ^ (k - 1) *
+        ((↑(↑B : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) ℝ).det : ℂ) ^ (k - 1) := by
+    simp_rw [← mul_zpow]
+  simp_rw [this, ← mul_assoc, ← mul_zpow]
+
+private theorem add_slash (k : ℤ) (A : GL(2, ℝ)⁺) (f g : ℍ → ℂ) :
+    (f + g) ∣[k]A = f ∣[k]A + g ∣[k]A := by
+  ext1
+  simp only [slash, Pi.add_apply, denom, zpow_neg]
+  ring
+
+private theorem slash_one (k : ℤ) (f : ℍ → ℂ) : f ∣[k]1 = f :=
+  funext <| by simp [slash, denom]
+
+variable {α : Type _} [SMul α ℂ] [IsScalarTower α ℂ ℂ]
+
+private theorem smul_slash (k : ℤ) (A : GL(2, ℝ)⁺) (f : ℍ → ℂ) (c : α) :
+    (c • f) ∣[k]A = c • f ∣[k]A := by
+  simp_rw [← smul_one_smul ℂ c f, ← smul_one_smul ℂ c (f ∣[k]A)]
+  ext1
+  simp_rw [slash]
+  simp only [slash, Algebra.id.smul_eq_mul, Matrix.GeneralLinearGroup.det_apply_val, Pi.smul_apply]
+  ring
+
+private theorem zero_slash (k : ℤ) (A : GL(2, ℝ)⁺) : (0 : ℍ → ℂ) ∣[k]A = 0 :=
+  funext fun _ => by simp only [slash, Pi.zero_apply, MulZeroClass.zero_mul]
+
+instance : SlashAction ℤ GL(2, ℝ)⁺ (ℍ → ℂ) ℂ where
+  map := slash
+  zero_slash := zero_slash
+  slash_one := slash_one
+  slash_mul := slash_mul
+  smul_slash := smul_slash
+  add_slash := add_slash
+
+end
+
+theorem slash_def (A : GL(2, ℝ)⁺) : f ∣[k] A = slash k A f :=
+  rfl
+#align modular_form.slash_def ModularForm.slash_def
+
+instance subgroupAction (Γ : Subgroup SL(2, ℤ)) : SlashAction ℤ Γ (ℍ → ℂ) ℂ :=
+  monoidHomSlashAction
+    (MonoidHom.comp Matrix.SpecialLinearGroup.toGLPos
+      (MonoidHom.comp (Matrix.SpecialLinearGroup.map (Int.castRingHom ℝ)) (Subgroup.subtype Γ)))
+#align modular_form.subgroup_action ModularForm.subgroupAction
+
+@[simp]
+theorem subgroup_slash (Γ : Subgroup SL(2, ℤ)) (γ : Γ) : f ∣[k] γ = f ∣[k] (γ : GL(2, ℝ)⁺) :=
+  rfl
+#align modular_form.subgroup_slash ModularForm.subgroup_slash
+
+instance SLAction : SlashAction ℤ SL(2, ℤ) (ℍ → ℂ) ℂ :=
+  monoidHomSlashAction
+    (MonoidHom.comp Matrix.SpecialLinearGroup.toGLPos
+      (Matrix.SpecialLinearGroup.map (Int.castRingHom ℝ)))
+set_option linter.uppercaseLean3 false in
+#align modular_form.SL_action ModularForm.SLAction
+
+@[simp]
+theorem SL_slash (γ : SL(2, ℤ)) : f ∣[k] γ = f ∣[k] (γ : GL(2, ℝ)⁺) :=
+  rfl
+set_option linter.uppercaseLean3 false in
+#align modular_form.SL_slash ModularForm.SL_slash
+
+/-- The constant function 1 is invariant under any element of `SL(2, ℤ)`. -/
+-- @[simp] -- Porting note: simpNF says LHS simplifies to something more complex
+theorem is_invariant_one (A : SL(2, ℤ)) : (1 : ℍ → ℂ) ∣[(0 : ℤ)] A = (1 : ℍ → ℂ) := by
+  have : ((↑ₘ(A : GL(2, ℝ)⁺)).det : ℝ) = 1 := by
+    simp only [Matrix.SpecialLinearGroup.coe_GLPos_coe_GL_coe_matrix,
+      Matrix.SpecialLinearGroup.det_coe]
+  funext
+  rw [SL_slash, slash_def, slash, zero_sub, this]
+  simp
+#align modular_form.is_invariant_one ModularForm.is_invariant_one
+
+/-- A function `f : ℍ → ℂ` is slash-invariant, of weight `k ∈ ℤ` and level `Γ`,
+  if for every matrix `γ ∈ Γ` we have `f(γ • z)= (c*z+d)^k f(z)` where `γ= ![![a, b], ![c, d]]`,
+  and it acts on `ℍ` via Möbius transformations. -/
+theorem slash_action_eq'_iff (k : ℤ) (Γ : Subgroup SL(2, ℤ)) (f : ℍ → ℂ) (γ : Γ) (z : ℍ) :
+    (f ∣[k] γ) z = f z ↔ f (γ • z) = ((↑ₘ[ℤ] γ 1 0 : ℂ) * z + (↑ₘ[ℤ] γ 1 1 : ℂ)) ^ k * f z := by
+  simp only [subgroup_slash, slash_def, ModularForm.slash]
+  convert inv_mul_eq_iff_eq_mul₀ (G₀ := ℂ) _ using 2
+  · rw [mul_comm]
+    simp only [denom, Matrix.SpecialLinearGroup.coe_GLPos_coe_GL_coe_matrix, zpow_neg,
+      Matrix.SpecialLinearGroup.det_coe, ofReal_one, one_zpow, mul_one, subgroup_to_sl_moeb,
+      sl_moeb]
+    rfl
+  · convert zpow_ne_zero k (denom_ne_zero γ z)
+#align modular_form.slash_action_eq'_iff ModularForm.slash_action_eq'_iff
+
+theorem mul_slash (k1 k2 : ℤ) (A : GL(2, ℝ)⁺) (f g : ℍ → ℂ) :
+    (f * g) ∣[k1 + k2] A = ((↑ₘA).det : ℝ) • f ∣[k1] A * g ∣[k2] A := by
+  ext1 x
+  simp only [slash_def, slash, Matrix.GeneralLinearGroup.det_apply_val,
+    Pi.mul_apply, Pi.smul_apply, Algebra.smul_mul_assoc, real_smul]
+  set d : ℂ := ↑((↑ₘA).det : ℝ)
+  have h1 : d ^ (k1 + k2 - 1) = d * d ^ (k1 - 1) * d ^ (k2 - 1) := by
+    have : d ≠ 0 := by
+      dsimp
+      norm_cast
+      exact Matrix.GLPos.det_ne_zero A
+    rw [← zpow_one_add₀ this, ← zpow_add₀ this]
+    congr; ring
+  have h22 : denom A x ^ (-(k1 + k2)) = denom A x ^ (-k1) * denom A x ^ (-k2) := by
+    rw [Int.neg_add, zpow_add₀]
+    exact UpperHalfPlane.denom_ne_zero A x
+  rw [h1, h22]
+  ring
+#align modular_form.mul_slash ModularForm.mul_slash
+
+set_option maxHeartbeats 1000000 in
+-- @[simp] -- Porting note: simpNF says LHS simplifies to something more complex
+theorem mul_slash_SL2 (k1 k2 : ℤ) (A : SL(2, ℤ)) (f g : ℍ → ℂ) :
+    (f * g) ∣[k1 + k2] A = f ∣[k1] A * g ∣[k2] A :=
+  calc
+    (f * g) ∣[k1 + k2] (A : GL(2, ℝ)⁺) =
+        ((↑ₘA).det : ℝ) • f ∣[k1] A * g ∣[k2] A := by
+      apply mul_slash
+    _ = (1 : ℝ) • f ∣[k1] A * g ∣[k2] A := by
+      simp only [Matrix.SpecialLinearGroup.coe_GLPos_coe_GL_coe_matrix,
+        Matrix.SpecialLinearGroup.det_coe]
+    _ = f ∣[k1] A * g ∣[k2] A := by simp
+set_option linter.uppercaseLean3 false in
+#align modular_form.mul_slash_SL2 ModularForm.mul_slash_SL2
+
+theorem mul_slash_subgroup (k1 k2 : ℤ) (Γ : Subgroup SL(2, ℤ)) (A : Γ) (f g : ℍ → ℂ) :
+    (f * g) ∣[k1 + k2] A = f ∣[k1] A * g ∣[k2] A :=
+  mul_slash_SL2 k1 k2 A f g
+#align modular_form.mul_slash_subgroup ModularForm.mul_slash_subgroup
+
+end
+
+end ModularForm