# Documentation

Mathlib.MeasureTheory.Integral.TorusIntegral

# Integral over a torus in ℂⁿ#

In this file we define the integral of a function f : ℂⁿ → E over a torus {z : ℂⁿ | ∀ i, z i ∈ Metric.sphere (c i) (R i)}. In order to do this, we define torusMap (c : ℂⁿ) (R θ : ℝⁿ) to be the point in ℂⁿ given by $z_k=c_k+R_ke^{θ_ki}$, where $i$ is the imaginary unit, then define torusIntegral f c R as the integral over the cube $[0, (fun _ ↦ 2π)] = {θ|∀ k, 0 ≤ θ_k ≤ 2π}$ of the Jacobian of the torusMap multiplied by f (torusMap c R θ).

We also define a predicate saying that f ∘ torusMap c R is integrable on the cube [0, (fun _ ↦ 2π)].

## Main definitions #

• torusMap c R: the generalized multidimensional exponential map from ℝⁿ to ℂⁿ that sends $θ=(θ_0,…,θ_{n-1})$ to $z=(z_0,…,z_{n-1})$, where $z_k= c_k + R_ke^{θ_k i}$;

• TorusIntegrable f c R: a function f : ℂⁿ → E is integrable over the generalized torus with center c : ℂⁿ and radius R : ℝⁿ if f ∘ torusMap c R is integrable on the closed cube Icc (0 : ℝⁿ) (fun _ ↦ 2 * π);

• torusIntegral f c R: the integral of a function f : ℂⁿ → E over a torus with center c ∈ ℂⁿ and radius R ∈ ℝⁿ defined as $\iiint_{[0, 2 * π]} (∏_{k = 1}^{n} i R_k e^{θ_k * i}) • f (c + Re^{θ_k i}),dθ_0…dθ_{k-1}$.

## Main statements #

• torusIntegral_dim0, torusIntegral_dim1, torusIntegral_succ: formulas for torusIntegral in cases of dimension 0, 1, and n + 1.

## Notations #

• ℝ⁰, ℝ¹, ℝⁿ, ℝⁿ⁺¹: local notation for Fin 0 → ℝ, Fin 1 → ℝ, Fin n → ℝ, and Fin (n + 1) → ℝ, respectively;
• ℂ⁰, ℂ¹, ℂⁿ, ℂⁿ⁺¹: local notation for Fin 0 → ℂ, Fin 1 → ℂ, Fin n → ℂ, and Fin (n + 1) → ℂ, respectively;
• ∯ z in T(c, R), f z: notation for torusIntegral f c R;
• ∮ z in C(c, R), f z: notation for circleIntegral f c R, defined elsewhere;
• ∏ k, f k: notation for Finset.prod, defined elsewhere;
• π: notation for Real.pi, defined elsewhere.

## Tags #

integral, torus

### torusMap, a parametrization of a torus #

def torusMap {n : } (c : Fin n) (R : Fin n) :
(Fin n) → Fin n

The n dimensional exponential map $θ_i ↦ c + R e^{θ_i*I}, θ ∈ ℝⁿ$ representing a torus in ℂⁿ with center c ∈ ℂⁿ and generalized radius R ∈ ℝⁿ, so we can adjust it to every n axis.

Instances For
theorem torusMap_sub_center {n : } (c : Fin n) (R : Fin n) (θ : Fin n) :
torusMap c R θ - c = torusMap 0 R θ
theorem torusMap_eq_center_iff {n : } {c : Fin n} {R : Fin n} {θ : Fin n} :
torusMap c R θ = c R = 0
@[simp]
theorem torusMap_zero_radius {n : } (c : Fin n) :

### Integrability of a function on a generalized torus #

def TorusIntegrable {n : } {E : Type u_1} (f : (Fin n) → E) (c : Fin n) (R : Fin n) :

A function f : ℂⁿ → E is integrable on the generalized torus if the function f ∘ torusMap c R θ is integrable on Icc (0 : ℝⁿ) (fun _ ↦ 2 * π).

Instances For
theorem TorusIntegrable.torusIntegrable_const {n : } {E : Type u_1} (a : E) (c : Fin n) (R : Fin n) :
TorusIntegrable (fun x => a) c R

Constant functions are torus integrable

theorem TorusIntegrable.neg {n : } {E : Type u_1} {f : (Fin n) → E} {c : Fin n} {R : Fin n} (hf : ) :

If f is torus integrable then -f is torus integrable.

theorem TorusIntegrable.add {n : } {E : Type u_1} {f : (Fin n) → E} {g : (Fin n) → E} {c : Fin n} {R : Fin n} (hf : ) (hg : ) :
TorusIntegrable (f + g) c R

If f and g are two torus integrable functions, then so is f + g.

theorem TorusIntegrable.sub {n : } {E : Type u_1} {f : (Fin n) → E} {g : (Fin n) → E} {c : Fin n} {R : Fin n} (hf : ) (hg : ) :
TorusIntegrable (f - g) c R

If f and g are two torus integrable functions, then so is f - g.

theorem TorusIntegrable.torusIntegrable_zero_radius {n : } {E : Type u_1} {f : (Fin n) → E} {c : Fin n} :
theorem TorusIntegrable.function_integrable {n : } {E : Type u_1} {f : (Fin n) → E} {c : Fin n} {R : Fin n} [] (hf : ) :
MeasureTheory.IntegrableOn (fun θ => (Finset.prod Finset.univ fun i => ↑(R i) * Complex.exp (↑(θ i) * Complex.I) * Complex.I) f (torusMap c R θ)) (Set.Icc 0 fun x => )

The function given in the definition of torusIntegral is integrable.

def torusIntegral {n : } {E : Type u_1} [] (f : (Fin n) → E) (c : Fin n) (R : Fin n) :
E

The definition of the integral over a generalized torus with center c ∈ ℂⁿ and radius R ∈ ℝⁿ as the •-product of the derivative of torusMap and f (torusMap c R θ)

Instances For
Instances For
Instances For
theorem torusIntegral_radius_zero {n : } {E : Type u_1} [] (hn : n 0) (f : (Fin n) → E) (c : Fin n) :
(∯ (x : Fin n) in T(c, 0), f x) = 0
theorem torusIntegral_neg {n : } {E : Type u_1} [] (f : (Fin n) → E) (c : Fin n) (R : Fin n) :
(∯ (x : Fin n) in T(c, R), -f x) = -∯ (x : Fin n) in T(c, R), f x
theorem torusIntegral_add {n : } {E : Type u_1} [] {f : (Fin n) → E} {g : (Fin n) → E} {c : Fin n} {R : Fin n} (hf : ) (hg : ) :
(∯ (x : Fin n) in T(c, R), f x + g x) = (∯ (x : Fin n) in T(c, R), f x) + ∯ (x : Fin n) in T(c, R), g x
theorem torusIntegral_sub {n : } {E : Type u_1} [] {f : (Fin n) → E} {g : (Fin n) → E} {c : Fin n} {R : Fin n} (hf : ) (hg : ) :
(∯ (x : Fin n) in T(c, R), f x - g x) = (∯ (x : Fin n) in T(c, R), f x) - ∯ (x : Fin n) in T(c, R), g x
theorem torusIntegral_smul {n : } {E : Type u_1} [] {𝕜 : Type u_2} [] [] [] (a : 𝕜) (f : (Fin n) → E) (c : Fin n) (R : Fin n) :
(∯ (x : Fin n) in T(c, R), a f x) = a ∯ (x : Fin n) in T(c, R), f x
theorem torusIntegral_const_mul {n : } (a : ) (f : (Fin n) → ) (c : Fin n) (R : Fin n) :
(∯ (x : Fin n) in T(c, R), a * f x) = a * ∯ (x : Fin n) in T(c, R), f x
theorem norm_torusIntegral_le_of_norm_le_const {n : } {E : Type u_1} [] {f : (Fin n) → E} {c : Fin n} {R : Fin n} {C : } (hf : ∀ (θ : Fin n), f (torusMap c R θ) C) :
∯ (x : Fin n) in T(c, R), f x (() ^ n * Finset.prod Finset.univ fun i => |R i|) * C

If for all θ : ℝⁿ, ‖f (torusMap c R θ)‖ is less than or equal to a constant C : ℝ, then ‖∯ x in T(c, R), f x‖ is less than or equal to (2 * π)^n * (∏ i, |R i|) * C

@[simp]
theorem torusIntegral_dim0 {E : Type u_1} [] [] (f : (Fin 0) → E) (c : Fin 0) (R : Fin 0) :
(∯ (x : Fin 0) in T(c, R), f x) = f c
theorem torusIntegral_dim1 {E : Type u_1} [] (f : (Fin 1) → E) (c : Fin 1) (R : Fin 1) :
(∯ (x : Fin 1) in T(c, R), f x) = ∮ (z : ) in C(c 0, R 0), f fun x => z

In dimension one, torusIntegral is the same as circleIntegral (up to the natural equivalence between ℂ and Fin 1 → ℂ).

theorem torusIntegral_succAbove {n : } {E : Type u_1} [] [] {f : (Fin (n + 1)) → E} {c : Fin (n + 1)} {R : Fin (n + 1)} (hf : ) (i : Fin (n + 1)) :
(∯ (x : Fin (n + 1)) in T(c, R), f x) = ∮ (x : ) in C(c i, R i), ∯ (y : Fin n) in T(, ), f ()

Recurrent formula for torusIntegral, see also torusIntegral_succ.

theorem torusIntegral_succ {n : } {E : Type u_1} [] [] {f : (Fin (n + 1)) → E} {c : Fin (n + 1)} {R : Fin (n + 1)} (hf : ) :
(∯ (x : Fin (n + 1)) in T(c, R), f x) = ∮ (x : ) in C(c 0, R 0), ∯ (y : Fin n) in T(c Fin.succ, R Fin.succ), f (Fin.cons x y)

Recurrent formula for torusIntegral, see also torusIntegral_succAbove.