ABSTRACT
The susceptibilities of various orders xin) have to fulfill the Neumann principle [5] stating that any material property must be invariant with respect to symmetry operations constituting the point group which describes the medium symmetry. This principle has far-reaching implications for the susceptibilities. A well-known im plication is that all tensor components of even-order susceptibilities (including ^ (2)) have to vanish for media belonging to centrosymmetric point groups. In gen eral, the number of independent tensor components is strongly reduced by the presence of symmetry operations, some of the components being equal to zero.
