6.1 Coordinates transformation
  6.2 Jacobian
  6.3 Orthogonality relation between two sets of basis vectors
  6.4 An example: (ψ,𝜃,ζ) coordinates
  6.5 Gradient and directional derivative in general coordinates (ψ,𝜃,ζ)
  6.6 Divergence operator in general coordinates (ψ,𝜃,ζ)
  6.7 Laplacian operator in general coordinates (ψ,𝜃,ζ)
  6.8 Curl operator in general coordinates (ψ,𝜃,ζ)
  6.9 Metric tensor for general coordinate system
   Special case: metric tensor for (ψ,𝜃,ϕ) coordinate system