Department of Mathematics, University of California, Irvine, CA, United States of America.|Center for Complex Biological Systems, University of California, Irvine, CA, United States of America.|Department of Mathematics, University of Nebraska, Lincoln, NE, United States of America.|Beijing International Center for Mathematical Research, Peking University, Beijing, China.|Department of Mathematics, University of California, Irvine, CA, United States of America.|Center for Complex Biological Systems, University of California, Irvine, CA, United States of America.