Elastic Media in Spherical Coordinates
When an elastic medium is at equilibrium, the force at each point balances the divergence of the rank-2 stress tensor. The stress tensor, in turn, is the contraction of the symmetric rank-4 stiffness tensor and the rank-2 strain tensor. The strain tensor is the symmetrized gradient of the displacement field. All these fields can be represented and manipulated as symmetrized arrays, in any coordinate system known to the Wolfram Language.