1. Department of Mathematics and Informatics University of Perugia Perugia Italy
2. ELKH–ELTE Geometric and Algebraic Combinatorics Research Group, Department of Geometry ELTE Eötvös Loránd University Pázmány P. stny. 1/C, Hungary Budapest 1117 Hungary
3. Dipartimento di Matematica e Applicazioni "R. Caccioppoli" Università degli Studi di Napoli "Federico II” Napoli Italy