Counting pop-stacked permutations in polynomial time

On partially ordered patterns of length 4 and 5 in permutations

Partially ordered patterns (POPs) generalize the notion of classical patterns studied widely in the literature in the context of permutations, words, compositions and partitions. In an occurrence of a POP, the relative order of some of the elements is not important. Thus, any POP of length $k$ is defined by a partially ordered set on $k$ elements, and classical patterns correspond to $k$-element chains. The notion of a POP provides a convenient language to deal with larger sets of permutation patterns. This paper contributes to a long line of research on classical permutation patterns of lengt…

避免二部偏序模式的排列具有正则插入编码

Permutations avoiding bipartite partially ordered patterns have a regular insertion encoding

该研究证明避免高度≤2的偏序模式（POP）的排列类有正则插入编码及有理生成函数，还通过组合探索得到数百个避免大小为5的POP的排列类的组合规范与生成函数，解决了多个相关猜想。

Sorting by shuffling methods and a queue

We study sorting by queues that can rearrange their content by applying permutations from a predefined set. These new sorting devices are called shuffle queues and we investigate those of them corresponding to sets of permutations defining some well-known shuffling methods. If $\mathbb{Q}_Σ$ is the shuffle queue corresponding to the shuffling method $Σ$, then we find a number of surprising results related to two natural variations of shuffle queues denoted by $\mathbb{Q}_Σ^{\prime}$ and $\mathbb{Q}_Σ^{\textsf{pop}}$. These require the entire content of the device to be unloaded after a permuta…

Lower bound on the running time of Pop-Stack Sorting on a random permutation

Pop-Stack Sorting is an algorithm that takes a permutation as an input and sorts its elements. It consists of several steps. At one step, the algorithm reads the permutation it has to process from left to right and reverses each of its maximal decreasing subsequences of consecutive elements. It terminates at the first step that outputs the identity permutation. In this note, we answer a question of Defant on the running time of Pop-Stack Sorting on the uniform random permutation $σ_n$. More precisely, we show that there is a constant $c > 0.5$ such that asymptotically almost surely, the algori…

使用带旁路的弹出栈对排列进行排序

Sorting permutations using a pop stack with a bypass

本文提出一种带旁路操作的弹出栈排序装置，通过禁止模式刻画可排序排列，并用受限Motzkin路径分析计数序列，同时给出原像计算算法及主类原像的完整描述。

Passing through a stack $k$ times with reversals

We consider a stack sorting algorithm where only the appropriate output values are popped from the stack and then any remaining entries in the stack are run through the stack in reverse order. We identify the basis for the $2$-reverse pass sortable permutations and give computational results for some classes with larger maximal rev-tier. We also show all classes of $(t+1)$-reverse pass sortable permutations are finitely based. Additionally, a new Entringer family consisting of maximal rev-tier permutations of length $n$ was discovered along with a bijection between this family and the collecti…

Passing through a stack $k$ times

We consider the number of passes a permutation needs to take through a stack if we only pop the appropriate output values and start over with the remaining entries in their original order. We define a permutation $π$ to be $k$-pass sortable if $π$ is sortable using $k$ passes through the stack. Permutations that are $1$-pass sortable are simply the stack sortable permutations as defined by Knuth. We define the permutation class of $2$-pass sortable permutations in terms of their basis. We also show all $k$-pass sortable classes have finite bases by giving bounds on the length of a basis elemen…

Flip-sort 与 pop-stack 排序的组合方面

Flip-sort and combinatorial aspects of pop-stack sorting

本文研究自然排序过程 flip-sort 的组合性质，刻画单次迭代后的置换形状及最佳/最差情形相关族，并建立与 pop-stack 排序、自动机、格路径的联系，提出若干双射证明技巧。

Almost all permutation matrices have bounded saturation functions

Saturation problems for forbidden graphs have been a popular area of research for many decades, and recently Brualdi and Cao initiated the study of a saturation problem for 0-1 matrices. We say that 0-1 matrix $A$ is saturating for the forbidden 0-1 matrix $P$ if $A$ avoids $P$ but changing any zero to a one in $A$ creates a copy of $P$. Define $sat(n, P)$ to be the minimum possible number of ones in an $n \times n$ 0-1 matrix that is saturating for $P$. Fulek and Keszegh proved that for every 0-1 matrix $P$, either $sat(n, P) = O(1)$ or $sat(n, P) = Θ(n)$. They found two 0-1 matrices $P$ for …