Barred Preferential Arrangements
  A preferential arrangement of a set is a total ordering of the elements of that set with ties allowed. A barred preferential arrangement is one in which the tied blocks of elements are ordered not only amongst themselves but also with respect to one or more bars. We present various combinatorial identities for r_{m,l}, the number of barred preferential arrangements of l elements with m bars, using both algebraic and combinatorial arguments. Our main result is an expression for r_{m,l} as a linear combination of the r_k (= r_{0,k}, the number of unbarred preferential arrangements of k elements) for l <= k<=l+m. We also study those arrangements in which the sections, into which the blocks are segregated by the bars, must be nonempty. We conclude with an expression of r_l as an infinite series that is both convergent and asymptotic. 
Connor Ahlbach, Jeremy Usatine and Nicholas Pippenger
math.CO