-- Mathlib/NumberTheory/LSeries/ZMod.lean
/--
The `LFunction` of the function `x ↦ e (j * x)`, where `e : ZMod N → ℂ` is the standard additive
character, is `expZeta (j / N)`.

Note this is not at all obvious from the definitions, and we prove it by analytic continuation
from the convergence range.
-/
lemma LFunction_stdAddChar_eq_expZeta (j : ZMod N) (s : ℂ) (hjs : j ≠ 0 ∨ s ≠ 1) :
    LFunction (fun k ↦ 𝕖 (j * k)) s = expZeta (ZMod.toAddCircle j) s

Proof state:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
⊢ AnalyticOnNhd ℂ f U

Tactic:
refine DifferentiableOn.analyticOnNhd (fun u hu ↦ ?_) hUo

Result:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
u : ℂ
hu : u ∈ U
⊢ DifferentiableWithinAt ℂ f U u

Proof state:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
u : ℂ
⊢ u ∈ U ↔ u ≠ 1 ∨ j ≠ 0

Tactic:
simp only [mem_ite_univ_right, U]

Result:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
u : ℂ
⊢ j = 0 → u ∈ {z | z ≠ 1} ↔ u ≠ 1 ∨ j ≠ 0

Proof state:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
hf : AnalyticOnNhd ℂ f U
hg : AnalyticOnNhd ℂ g U
⊢ IsPreconnected U

Tactic:
by_cases h : j = 0

Result:
case pos
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
hf : AnalyticOnNhd ℂ f U
hg : AnalyticOnNhd ℂ g U
h : j = 0
⊢ IsPreconnected U

case neg
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
hf : AnalyticOnNhd ℂ f U
hg : AnalyticOnNhd ℂ g U
h : ¬j = 0
⊢ IsPreconnected U

Proof state:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
u : ℂ
hu : u ∈ U
⊢ DifferentiableWithinAt ℂ f U u

Tactic:
refine (differentiableAt_LFunction _ _ ((hU.mp hu).imp_right fun h ↦ ?_)).differentiableWithinAt

Result:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
u : ℂ
hu : u ∈ U
h : j ≠ 0
⊢ ∑ j_1 : ZMod N, 𝕖 (j * j_1) = 0

Proof state:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
hf : AnalyticOnNhd ℂ f U
⊢ AnalyticOnNhd ℂ g U

Tactic:
refine DifferentiableOn.analyticOnNhd (fun u hu ↦ ?_) hUo

Result:
N : ℕ
inst✝ : NeZero N
j : ZMod N
s : ℂ
hjs : j ≠ 0 ∨ s ≠ 1
U : Set ℂ := if j = 0 then {z | z ≠ 1} else Set.univ
V : Set ℂ := {z | 1 < z.re}
hUo : IsOpen U
f : ℂ → ℂ := LFunction fun k => 𝕖 (j * k)
g : ℂ → ℂ := expZeta (toAddCircle j)
hU : ∀ {u : ℂ}, u ∈ U ↔ u ≠ 1 ∨ j ≠ 0
hf : AnalyticOnNhd ℂ f U
u : ℂ
hu : u ∈ U
⊢ DifferentiableWithinAt ℂ g U u