 $0$-periodic trajectories are shown in Figure \[fig:0-periodic\].  (0,0) circle \[x radius=[sqrt(0)]{}, y radius=[sqrt(0)]{}\];  (0,0) circle \[x radius=[sqrt(0-0.000)]{}, y radius=[sqrt(0+0.000)]{}\];  ([sqrt(0)+0.0]{},-0.0) - (-0.0,[sqrt(0)+0.0]{}); ([-sqrt(0)-0.0]{},0.0) - (0.0,[-sqrt(0)-0.0]{}); (0.0,[sqrt(0)+0.0]{}) - ([-sqrt(0)-0.0]{},-0.0); (-0.0,[-sqrt(0)-0.0]{}) - ([sqrt(0)+0.0]{},0.0);  (.0, 