declare symbol r, θ, φ: MathExpr

-- Euler form of S2

def x : Vector MathExpr := [| θ, φ |]

def X : Vector MathExpr := [| r * sin θ * cos φ, r * sin θ * sin φ, r * cos θ |]

-- Local basis
def e_i_j : Matrix MathExpr := ∂/∂ X_j x~i

-- Metric tensor
def g_i_j : Matrix MathExpr := generateTensor (\[x, y] -> V.* e_x_# e_y_#) [2, 2]
def g~i~j : Matrix MathExpr := M.inverse g_#_#

g_#_#
assertEqual "Metric tensor"
  g_#_#
  [| [| r^2, 0 |], [| 0, r^2 * (sin θ)^2 |] |]_#_#

-- Christoffel symbols
def Γ_i_j_k : Tensor MathExpr := (1 / 2) * (∂/∂ g_i_k x~j + ∂/∂ g_i_j x~k - ∂/∂ g_j_k x~i)

def Γ~i_j_k : Tensor MathExpr := withSymbols [m]
  g~i~m . Γ_m_j_k

-- Connection 1-form
def ω0 : Tensor MathExpr := Γ~#_#_#

def A : Matrix MathExpr := [| [| 1 / r, 0 |], [| 0, 1 / (r * sin θ) |] |]

-- Transformed connection
def d (A : Tensor MathExpr) : Tensor MathExpr := (flip ∂/∂) x~# A_#_#

def ω := withSymbols [i, j, k, l]
  (M.inverse A)~i_j . ω0~j_k . A~k_l + (M.inverse A)~i_j . d A~j_l

-- Curvature form
def wedge {Num a} (X : Tensor a) (Y : Tensor a) : Tensor a := X !. Y

wedge ω~i_k ω~k_j

def Ω : Tensor MathExpr := withSymbols [i, j, k]
--  dfNormalize (d ω~i_j + wedge ω~i_k ω~k_j)
  (d ω~i_j + wedge ω~i_k ω~k_j)

-- Euler form
def eulerForm : MathExpr := (1 / (2 * π)) * (Ω~1_2 - Ω~2_1)

-- The Euler form integrates to the Euler characteristic χ = 2 for S²
assertEqual "Euler form of S2"
  eulerForm
  [| [| sin θ / (r^2 * π), 0 |], [| 0, sin θ / (r^2 * π) |] |]