1. Norbert Wiener Center, Department of Mathematics, University of Maryland College Park, MD 20742-4015, USA
2. Section on Medical Biophysics, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, 9 Memorial Dr, Bethesda, MD 20892-0924, USA
3. Department of Mathematics and Computer Science, Philipps-Universität Marburg, Hans-Meerwein Str., 35037 Marburg, Germany