[1] Univ Calif Santa Barbara, Dept Mech Engn, Santa Barbara, CA 93106 USA
[2] Univ Calif Santa Barbara, Dept Comp Sci, Santa Barbara, CA 93106 USA
[3] Stanford Univ, Dept Comp Sci, Stanford, CA 94305 USA
[4] Univ Calif Los Angeles, Dept Math, Los Angeles, CA 90095 USA