International
Tables for Crystallography Volume D Physical properties of crystals Edited by A. Authier © International Union of Crystallography 2013 
International Tables for Crystallography (2013). Vol. D, ch. 1.2, pp. 6768

An invariant element of the tensor space under the group G is a vector v that is left invariant under each generator: If the number of generators is one, . This equation is solved by diagonalization: where . The dimension of the solution space is the number of elements that are equal to zero. The corresponding rows of Q form a basis for the solution space. (See example further on.)