sentences:  The elastic energy is expressed via the Dirichlet energy functional
  
  In two dimensions, harmonic coordinates have been well understood for more than a century, and are closely related to isothermal coordinates, the latter being a special case of the former.Harmonic coordinates in higher dimensions were developed initially in the context of general relativity by (see harmonic coordinate condition).They were then introduced into Riemannian geometry by and later were studied by .