sentences:  Harmonic coordinates
  
  In Riemannian geometry, a branch of mathematics, harmonic coordinates are a coordinate system on a Riemannian manifold each of whose coordinate functions "x""i" is harmonic, meaning that it satisfies Laplace's equation
  Here Δ is the Laplace–Beltrami operator.Equivalently, regarding a coordinate system as a local diffeomorphism , the coordinate system is harmonic if and only if φ is a harmonic map of Riemannian manifolds, roughly meaning that it minimizes the elastic energy of 
  "stretching" "M" into R"n".