1. Dynamics & Vibrations Group, fnb, TU Darmstadt; Dolivostr. 15, 64293 Darmstadt Germany
2. Department of Mathematics and Statistics; University of Calgary, 2500 University Drive NW; Calgary AB T2N 1N4 Canada
3. Sicherheitstechnische Regelungs- und Messtechnik, University Wuppertal; Gaußstr. 20, 42119 Wuppertal Germany
4. Department of Applied Mechanics, IIT Delhi; Hauz Khas New Delhi 110016 India