We further study algebraic Riccati equations associated with
regular singular M-matrices. An M-matrix M is said to be
regular if M v ≥ 0 for some v > 0, so every irreducible singular
M-matrix is a regular singular M-matrix. We prove a property
about the product of the minimal nonnegative solution of such
an algebraic Riccati equation and the minimal nonnegative
solution of its dual equation. In the critical case, we show that
the alternating-directional doubling algorithm (which includes
the structure-preserving double algorithm as a special case)
has linear convergence with rate 1/2. The results enhance
our understanding of the behaviour of doubling algorithms
for finding the minimal nonnegative solutions.