1. Professor of Mathematics, Dept. of Civil Engineering, Democritus Univ. of Thrace, 64100 Xanthi, Greece. E-mail: papadob@civil.duth.gr.
2. Adjunct Lecturer of Applied Economics and Computer Applications, Dept. of Economics, Univ. of Crete, 74100 Rethymno, University Campus, Crete, Greece (corresponding author). E-mail: kandila@her.forthnet.gr.
3. Assistant Professor of Applied Economics, Dept. of Economics, Univ. of Crete, 74100 Rethymno, University Campus, Crete, Greece. E-mail: ayannop@econ.soc.uoc.gr.