  Hilbert-Schmidt (HS) decompositions and Frobenius norms are used to analyze
biseparability of 3-qubit systems, with particular emphasis on density matrices
with maximally disordered subsystems (MDS) and on the W state mixed with white
noise. The biseparable form of a MDS density matrix is obtained by using the
Bell states of a 2-qubit subsystem, multiplied by density matrices of the third
qubit, which include the relevant HS parameters. Using our methods a sufficient
condition and explicit biseparability of the W state mixed with white noise are
given. They are compared with the sufficient condition for explicit full
separability given in a previous work.
