Jacobi's theta function

This file defines the one-variable Jacobi theta function

$$\theta(\tau) = \sum_{n \in \mathbb{Z}} \exp (i \pi n ^ 2 \tau),$$

and proves the modular transformation properties θ (τ + 2) = θ τ and θ (-1 / τ) = (-I * τ) ^ (1 / 2) * θ τ, using Poisson's summation formula for the latter. We also show that θ is differentiable on ℍ, and θ(τ) - 1 has exponential decay as im τ → ∞.

noncomputable def jacobiTheta (τ : ℂ) : ℂ

Jacobi's one-variable theta function ∑' (n : ℤ), exp (π * I * n ^ 2 * τ).

Equations

• jacobiTheta τ = ∑' (n : ℤ), (↑Real.pi * Complex.I * ↑n ^ 2 * τ).exp

theorem jacobiTheta_eq_jacobiTheta₂ (τ : ℂ) : jacobiTheta τ = jacobiTheta₂ 0 τ

theorem jacobiTheta_two_add (τ : ℂ) : jacobiTheta (2 + τ) = jacobiTheta τ

theorem jacobiTheta_T_sq_smul (τ : UpperHalfPlane) : jacobiTheta ↑(ModularGroup.T ^ 2 • τ) = jacobiTheta ↑τ

theorem jacobiTheta_S_smul (τ : UpperHalfPlane) : jacobiTheta ↑(ModularGroup.S • τ) = (-Complex.I * ↑τ) ^ (1 / 2) * jacobiTheta ↑τ

theorem norm_exp_mul_sq_le {τ : ℂ} (hτ : 0 < τ.im) (n : ℤ) : ‖(↑Real.pi * Complex.I * ↑n ^ 2 * τ).exp‖ ≤ (-Real.pi * τ.im).exp ^ n.natAbs

theorem hasSum_nat_jacobiTheta {τ : ℂ} (hτ : 0 < τ.im) : HasSum (fun (n : ℕ) => (↑Real.pi * Complex.I * (↑n + 1) ^ 2 * τ).exp) ((jacobiTheta τ - 1) / 2)

theorem jacobiTheta_eq_tsum_nat {τ : ℂ} (hτ : 0 < τ.im) : jacobiTheta τ = 1 + 2 * ∑' (n : ℕ), (↑Real.pi * Complex.I * (↑n + 1) ^ 2 * τ).exp

theorem norm_jacobiTheta_sub_one_le {τ : ℂ} (hτ : 0 < τ.im) : ‖jacobiTheta τ - 1‖ ≤ 2 / (1 - (-Real.pi * τ.im).exp) * (-Real.pi * τ.im).exp

An explicit upper bound for ‖jacobiTheta τ - 1‖.

theorem isBigO_at_im_infty_jacobiTheta_sub_one : (fun (τ : ℂ) => jacobiTheta τ - 1) =O[Filter.comap Complex.im Filter.atTop] fun (τ : ℂ) => (-Real.pi * τ.im).exp

The norm of jacobiTheta τ - 1 decays exponentially as im τ → ∞.

theorem differentiableAt_jacobiTheta {τ : ℂ} (hτ : 0 < τ.im) : DifferentiableAt ℂ jacobiTheta τ

theorem continuousAt_jacobiTheta {τ : ℂ} (hτ : 0 < τ.im) : ContinuousAt jacobiTheta τ