  The (complex) two-qubit systems comprise a 15-dimensional convex set and the
real two-qubit systems, a 9-dimensional convex set. While formulas for the
Hilbert-Schmidt volumes of these two sets are known -- owing to recent
important work of Sommers and Zyczkowski (J. Phys. A {36}, 10115 [2003]) --
formulas have not been so far obtained for the volumes of the separable
subsets. We reduce these two problems to the determination of certain functions
of a single variable.
