 In homogeneous and isotropic materials, these define Hooke's law in 3D,

where is the stress, the strain tensor, the identity matrix and the trace function.Hooke's law may be written in terms of tensor components using index notation as

where is the stress tensor, the strain tensor, and the Kronecker delta.The two parameters together constitute a parameterization of the elastic moduli for homogeneous isotropic media, popular in mathematical literature, and are thus related to the other elastic moduli; for instance, the bulk modulus can be expressed as .