1. Department of Mathematics; Tufts University; 503 Boston Avenue, Medford; MA 02155; U.S.A.
2. Applied Mathematics and Plasma Physics; MS B284, Los Alamos National Laboratory; Los Alamos; NM87545; U.S.A.
3. Department of Mathematics; Davidson College; PO Box 6908, Davidson; NC 28035; U.S.A.