where M are constant (2 j 1 +1)(2 j 2 +1) × (2 j 1 +1)(2 j 2 +1) matrices defining the elements of the Lorentz algebra within the D ( j 1 , j 2 ) ⊕ ⊕ D ( j 2 , j 1 ) representations. The Capital Latin letter labels indicate the finite dimensionality of the representation spaces under consideration which describe the internal angular momentum (spin) degrees of freedom.