1 2 Journal of Integer Sequences, Vol. 21 (2018), 3 Article 18.2.8 47 6 23 11 A Generalization of the “Probl`eme des Rencontres” Stefano Capparelli Dipartimento di Scienze di Base e Applicate per l’Ingegneria Universit´adi Roma “La Sapienza” Via A. Scarpa 16 00161 Roma Italy
[email protected] Margherita Maria Ferrari Department of Mathematics and Statistics University of South Florida 4202 E. Fowler Avenue Tampa, FL 33620 USA
[email protected] Emanuele Munarini and Norma Zagaglia Salvi1 Dipartimento di Matematica Politecnico di Milano Piazza Leonardo da Vinci 32 20133 Milano Italy
[email protected] [email protected] Abstract In this paper, we study a generalization of the classical probl`eme des rencontres (problem of coincidences), where you are asked to enumerate all permutations π ∈ 1This work is partially supported by MIUR (Ministero dell’Istruzione, dell’Universit`ae della Ricerca). 1 S with k fixed points, and, in particular, to enumerate all permutations π S n ∈ n with no fixed points (derangements). Specifically, here we study this problem for the permutations of the n + m symbols 1, 2, ..., n, v , v , ..., v , where v 1 2 m i 6∈ 1, 2,...,n for every i = 1, 2,...,m. In this way, we obtain a generalization of the { } derangement numbers, the rencontres numbers and the rencontres polynomials. For these numbers and polynomials, we obtain the exponential generating series, some recurrences and representations, and several combinatorial identities. Moreover, we obtain the expectation and the variance of the number of fixed points in a random permutation of the considered kind.