  Entangling properties of a mixed 2-qubit system can be described by the local
homogeneous unitary invariant polynomials in elements of the density matrix.
The structure of the corresponding invariant polynomial ring for the special
subclass of states, the so-called mixed X-states, is established. It is shown
that for the X-states there is an injective ring homomorphism of the quotient
ring of SU(2)xSU(2) invariant polynomials modulo its syzygy ideal and the
SO(2)xSO(2)-invariant ring freely generated by five homogeneous polynomials of
degrees 1,1,1,2,2.
