Stress-deformation relations for anisotropic solids

We consider a body of material to be subjected to a deformation in which a point of the material initially at X i in a rectangular Cartesian coordinate system x i moves to x i in the same coordinate, system. In the theory of finite elasticity it is assumed that there exists a strain-energy function W which is a polynomial function of the displacement gradients ∂x i /∂X j only.