  In this paper, we propose and solve a low phase-rank approximation problem,
which serves as a counterpart to the well-known low-rank approximation problem
and the Schmidt-Mirsky theorem. More specifically, a nonzero complex number can
be specified by its gain and phase, and while it is generally accepted that the
gains of a matrix may be defined by its singular values, there is no widely
accepted definition for its phases. In this work, we consider sectorial
matrices, whose numerical ranges do not contain the origin, and adopt the
canonical angles of such matrices as their phases. Similarly to the rank of a
matrix defined to be the number of its nonzero singular values, we define the
phase-rank of a sectorial matrix as the number of its nonzero phases. While a
low-rank approximation problem is associated with matrix arithmetic means, as a
natural parallel we formulate a low phase-rank approximation problem using
matrix geometric means to measure the approximation error. A characterization
of the solutions to the proposed problem is then obtained, when both the
objective matrix and the approximant are restricted to be positive-imaginary.
Moreover, the obtained solution has the same flavor as the Schmidt-Mirsky
theorem on low-rank approximation problems. In addition, we provide an
alternative formulation of the low phase-rank approximation problem using
geodesic distances between sectorial matrices. The two formulations give rise
to the exact same set of solutions when the involved matrices are additionally
assumed to be unitary.
