Ricerc@Sapienza

In this paper, we study a generalization of the classical \emph{probl\'eme des rencontres} (\emph{problem of coincidences}),
consisting in the enumeration of all permutations $ \pi \in \SS_n $ with $k$ fixed points,
and, in particular, in the enumeration of all permutations $ \pi \in \SS_n $ with no fixed points (derangements).
Specifically, we study this problem for the permutations of the $n+m$
symbols $1$, $2$, \ldots, $n$, $v_1$, $v_2$, \ldots, $v_m$,
where $ v_i \not\in\{1,2,\ldots,n\} $ for every $i=1,2,\ldots,m$.
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.
Finally, we obtain some asymptotic formulas
for the generalized rencontres numbers and the generalized derangement numbers.