Institution: | aVienna Institute of Demography, Prinz Eugenstr. 8-10, 1040 Vienna, Austria;bInstitute of Mathematical Methods in Economics (Research Group on Operations Research and Nonlinear Dynamical Systems), Vienna University of Technology, Argentinierstrasse 8, A-1040 Vienna, Austria;cInstitute of Mathematics and Informatics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria |