A theorem is proved concerning
the existence of a half-trajectory in the neighborhood of a semi-invariant set of a
general dynamical system. A corollary of this theorem strengthens a result of P.
Mendelson. The theorem is further used to obtain a necessary and sufficient
condition for a compact positively invariant set to be positively asymptotically
orbitally stable, and the condition is compared with another one due to S.
Lefschetz.