stiffness tensor with 81 components (3×3×3×3), but because the application of such a rank-4 tensor to a symmetric rank-2 tensor must yield another symmetric rank-2 tensor, not all of the 81 elements are independent. Voigt notation enables such a rank-4 tensor to be represented by a 6×6 matrix. However, Voigt's form does not preserve the sum of the squares, which in the case of Hooke's law has geometric significance. This explains why weights are introduced (to make the mapping an isometry). A discussion of invariance of Voigt's notation and Mandel's notation can be found in Helnwein (2001). References See also Vectorization (mathematics) Hooke's law Tensors Mathematical notation Solid mechanics