Fig. 1.
A graphical representation of the L1- and L2-constraints on u in the PMD(L1, L1) criterion. The constraints are as follows: ‖u‖22 ≤ 1 and ‖u‖1 ≤ c. Here, u is two-dimensional, and the grey lines indicate the coordinate axes u1 and u2. Left: the L2-constraint is the solid circle. For both the L1- and L2-constraints to be active, c must be between 1 and 
. The constraints ‖u‖1 = 1 and ‖u‖1= are shown using dashed lines. Right: The L2- and L1-constraints on u are shown for some c between 1 and . Small circles indicate the points where both the L1- and the L2-constraints are active. The solid arcs indicate the solutions that occur when Δ1 = 0 in Algorithm 3. The figure shows that in 2D, the points where both the L1- and L2-constraints are active do not have either u1 or u2 equal to 0.