import Mathlib.Analysis.InnerProductSpace.Basic
import Mathlib.Analysis.InnerProductSpace.PiL2
import Mathlib.Algebra.BigOperators.Basic
import Mathlib.Analysis.Calculus.ContDiff.Bounds
import Mathlib.Analysis.Calculus.FDeriv.Symmetric

open BigOperators
open Finset

variable {E : Type*} [NormedAddCommGroup E] [InnerProductSpace ℝ E] [FiniteDimensional ℝ E]
variable {F : Type*} [NormedAddCommGroup F] [NormedSpace ℝ F]

#check EuclideanSpace.norm_eq
#check EuclideanSpace.dist_eq


noncomputable def Laplace₁ (n : ℕ) (f : EuclideanSpace ℝ (Fin n) → F) : EuclideanSpace ℝ (Fin n) → F := by
  let e : Fin n → EuclideanSpace ℝ (Fin n) := fun i ↦ EuclideanSpace.single i (1 : ℝ)
  exact fun z ↦ ∑ i, iteratedFDeriv ℝ 2 f z ![e i, e i]


noncomputable def Laplace₂
  [Fintype ι]
  (v : Basis ι ℝ E)
  (hv : Orthonormal ℝ v)
  (f : E → F) :
  E → F :=
  fun z ↦ ∑ i, iteratedFDeriv ℝ 2 f z ![v i, v i]

theorem LaplaceIndep
  [Fintype ι]
  (v₁ : Basis ι ℝ E)
  (hv₁ : Orthonormal ℝ v₁)
  (v₂ : Basis ι ℝ E)
  (hv₂ : Orthonormal ℝ v₂)
  (f : E → F) :
  ∑ i, iteratedFDeriv ℝ 2 f z ![v₁ i, v₁ i] =   ∑ i, iteratedFDeriv ℝ 2 f z ![v₂ i, v₂ i] := by

  have (v : E) : v = ∑ j, ⟪v₁ j, v⟫_ℝ • (v₁ j) :=
    sorry

  conv =>
    right
    arg 2
    intro i
    rw [this (v₂ i)]
    rw [this (v₂ i)]
  conv =>
    right
    arg 2
    intro i
    rw [ContinuousMultilinearMap.map_sum_finset]
  sorry