  The state of an entangled q-bit pair is specified by 15 numerical parameters
that are naturally regarded as the components of two 3-vectors and a
$3\times3$-dyadic. There are easy-to-use criteria to check whether a given pair
of 3-vectors plus a dyadic specify a 2-q-bit state; and if they do, whether the
state is entangled; and if it is, whether it is a separable state. Some
progress has been made in the search for analytical expressions for the degree
of separability. We report, in particular, the answer in the case of vanishing
3-vectors.
