1. School of Mathematical Sciences, Beihang University, Beijing 100191, China.
2. Department of Civil, Environmental & Architectural Engineering, University of Colorado Boulder, Boulder, CO 80309-0428 USA.
3. Corresponding author. Department of Civil, Environmental & Architectural Engineering, and Department of Applied Mathematics, University of Colorado Boulder, Boulder, CO 80309-0428 USA.
4. INRIA, Saclay Center, Île de France, and UMA, ENSTA Paris Tech, Palaiseau Cedex, France.