1. Dipartimento di Matematica e Geoscienze, University of Trieste, Trieste 34127, Italy.
2. Dipartimento di Matematica, University of Trento, Trento 38123, Italy.
3. Dipartimento di Economia e Management, University of Pisa, Pisa 56124, Italy.
4. Gheorghe Mihoc-Caius Iacob Institute of Mathematical Statistics and Applied Mathematics, Romanian Academy, Bucharest 050711, Romania.