1. a Department of Mechanics , Beijing University , China
2. b Department of Mathematics and Statistics , University of Massachusetts , Amherst, MA, 01003, U.S.A.
3. c Department of Electrical and Computer Engineering , University of Massachusetts , Amherst, MA, 01003, U.S.A.
4. d Electrical, Computer and Systems Engineering Department , Rensselaer Polytechnic Institute , Troy, NY, 12180, U.S.A.