arXiv subjects

Miklós Bóna

Publications and source records attributed to Miklós Bóna.

25 records · Page 2Linked to original sources

arXiv1999

A self-dual poset on objects counted by the Catalan numbers and a type-B analogue

We introduce two partially ordered sets, $P^A_n$ and $P^B_n$, of the same cardinalities as the type-A and type-B noncrossing partition lattices. The ground sets of $P^A_n$ and $P^B_n$ are subsets of the symmetric and the hyperoctahedral groups, consisting of permutations which avoid certain patterns. The order relation is given by (strict) containment of the descent sets. In each case, by means of an explicit order-preserving bijection, we show that the poset of restricted permutations is an extension of the refinement order on noncrossing partitions. Several structural properties of these permutation posets follow, including self-duality and the strong Sperner property. We also discuss posets $Q^A_n$ and $Q^B_n$ similarly associated with noncrossing partitions, defined by means of the excedence sets of suitable pattern-avoiding subsets of the symmetric and hyperoctahedral groups.

math.CO↗

arXiv1999

A combinatorial proof of the log-concavity of the numbers of permutations with $k$ runs

We combinatorially prove that the number $R(n,k)$ of permutations of length $n$ having $k$ runs is a log-concave sequence in $k$, for all $n$. We also give a new combinatorial proof for the log-concavity of the Eulerian numbers.

math.CO↗

arXiv1998

An Infinite Antichain of Permutations

We constructively prove that the partially ordered set of finite permutations ordered by deletion of entries contains an infinite antichain.

math.CO↗

arXiv1998

Packing Ferrers Shapes

Answering a question of Wilf, we show that if $n$ is sufficiently large, then one cannot cover an $n \times p(n)$ rectangle using each of the $p(n)$ distinct Ferrers shapes of size $n$ exactly once. Moreover, the maximum number of pairwise distinct, non-overlapping Ferrers shapes that can be packed in such a rectangle is only $Θ(p(n)/ \log n).$

math.CO↗

arXiv1997

2-stack sortable permutations with a given number of runs

Using earlier results we prove a formula for the number $W_{(n,k)}$ of 2-stack sortable permutations of length $n$ with $k$ runs, or in other words, $k-1$ descents. This formula will yield the suprising fact that there are as many 2-stack sortable permutations with $k-1$ descents as with $k-1$ ascents. We also prove that $W_{(n,k)}$ is unimodal in $k$, for any fixed $n$.

math.CO↗

arXiv1997

A Combinatorial proof of a result of Hetyei and Reiner on Foata-Strehl type permutation trees

We give a combinatorial proof of the result of Hetyei and Reiner that there are exactly $n!/3$ permutations of length $n$ in the minmax tree representation of which the $i$th node is a leaf. We also prove the new result that the number of $n$-permutations in which this node has one child is $n!/3$ as well, implying that the same holds for those in which this node has two children.

math.CO↗

arXiv1997

Exact enumeration of 1342-avoiding permutations: A close link with labeled trees and planar maps

Solving the first nonmonotonic, longer-than-three instance of a classic enumeration problem, we obtain the generating function $H(x)$ of all 1342-avoiding permutations of length $n$ as well as an {\em exact} formula for their number $S_n(1342)$. While achieving this, we bijectively prove that the number of indecomposable 1342-avoiding permutations of length $n$ equals that of labeled plane trees of a certain type on $n$ vertices recently enumerated by Cori, Jacquard and Schaeffer, which is in turn known to be equal to the number of rooted bicubic maps enumerated by Tutte in 1963. Moreover, $H(x)$ turns out to be algebraic, proving the first nonmonotonic, longer-than-three instance of a conjecture of Zeilberger and Noonan. We also prove that $\sqrt[n]{S_n(1342)}$ converges to 8, so in particular, $lim_{n\rightarrow \infty}(S_n(1342)/S_n(1234))=0$.

math.CO↗