Department of Mathematics, University of South Carolina, Columbia, South Carolina, United States of America.|Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Mathematics, University of South Carolina, Columbia, South Carolina, United States of America.|Marsico Lung Institute & Cystic Fibrosis Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.