  In a recent paper, M. F. Sacchi [Phys. Rev. A 96, 042325 (2017)] addressed
the general problem of approximating an unavailable quantum state by the convex
mixing of different available states. For the case of qubit mixed states, we
show that the analytical solutions in some cases are invalid. In this Comment,
we present complete analytical solutions for the optimal convex approximation.
Our solutions can be viewed as correcting and supplementing the results in the
aforementioned paper.
