1. Department of Aeronautics & Astronautics, University of Washington, Seattle, WA 98195-2400, USA.
2. Department of Mechanical and Aerospace Engineering, Tohoku University, Sendai, Miyagi 980-8577, Japan.
3. Department of Mathematics and Statistics, University of Massachusetts, Amherst, MA 01003-4515, USA.
4. Department of Mathematics, Bowdoin College, Brunswick, ME 04011, USA.