[1] Univ Nevada, Dept Math Sci, Las Vegas, NV 89154 USA
[2] Penn State Univ, Dept Math, University Pk, PA 16802 USA
[3] Penn State Univ, Dept Mech Engn & Mat Sci, ECEC, University Pk, PA 16802 USA
[4] Penn State Univ, Dept Engn, ECEC, University Pk, PA 16802 USA
[5] Peking Univ, Sch Math Sci, Lab Math & Appl Math, Beijing 100871, Peoples R China