Sorting probability of Catalan posets
We show that the sorting probability of the Catalan poset} $P_n$ satisfies $δ(P_n)= O \bigl(n^{-5/4}\bigr)$.
math.CO↗
arXiv subjects
Igor Pak
Publications and source records attributed to Igor Pak.
Breaking down the reduced Kronecker coefficients
We resolve three interrelated problems on \emph{reduced Kronecker coefficients} $\overline{g}(α,β,γ)$. First, we disprove the \emph{saturation property} which states that $\overline{g}(Nα,Nβ,Nγ)>0$ implies $\overline{g}(α,β,γ)>0$ for all $N>1$. Second, we esimate the maximal $\overline{g}(α,β,γ)$, over all $|α|+|β|+|γ| = n$. Finally, we show that computing $\overline{g}(λ,μ,ν)$ is strongly $\# P$-hard, i.e. $\#P$-hard when the input $(λ,μ,ν)$ is in unary.
math.CO↗
Upper bounds on Kronecker coefficients with few rows
We present three different upper bounds for Kronecker coefficients $g(λ,μ,ν)$ in terms of Kostka numbers, contingency tables and Littlewood--Richardson coefficients. We then give various examples, asymptotic applications, and compare them with existing lower bounds.
math.CO↗
On the number of integer points in translated and expanded polyhedra
We prove that the problem of minimizing the number of integer points inparallel translations of a rational convex polytope in $\mathbb{R}^6$ is NP-hard. We apply this result to show that given a rational convex polytope $P \subset \mathbb{R}^6$, finding the largest integer $t$ s.t. the expansion $tP$ contains fewer than $k$ integer points is also NP-hard. We conclude that the Ehrhart quasi-polynomials of rational polytopes can have arbitrary fluctuations.
math.CO↗
Hook formulas for skew shapes I. $q$-analogues and bijections
The celebrated hook-length formula gives a product formula for the number of standard Young tableaux of a straight shape. In 2014, Naruse announced a more general formula for the number of standard Young tableaux of skew shapes as a positive sum over excited diagrams of products of hook-lengths. We give an algebraic and a combinatorial proof of Naruse's formula, by using factorial Schur functions and a generalization of the Hillman--Grassl correspondence, respectively. The main new results are two different $q$-analogues of Naruse's formula: for the skew Schur functions, and for counting reverse plane partitions of skew shapes. We establish explicit bijections between these objects and families of integer arrays with certain nonzero entries, which also proves the second formula.
math.CO↗
Log smoothness and polystability over valuation rings
Let $\mathcal{O}$ be a valuation ring of height one of residual characteristic exponent $p$ and with algebraically closed field of fractions. Our main result provides a best possible resolution of the monoidal structure $M_X$ of a log variety $X$ over $\calO$ with a vertical log structure: there exists a log modification $Y\to X$ such that the monoidal structure of $Y$ is polystable. In particular, if $X$ is log smooth over $\mathcal{O}$, then $Y$ is polystable with a smooth generic fiber. As a corollary we deduce that any variety over $\mathcal{O}$ possesses a polystable alteration of degreee $p^n$. The core of our proof is a subdivision result for polyhedral complexes satisfying certain rationality conditions.
math.AG↗
Combinatorial inequalities
This is an expanded version of the Notices of the AMS column with the same title. The text is unchanged, but we added acknowledgements and a large number of endnotes which provide the context and the references.
math.CO↗
Hook inequalities
We give an elementary proof of the recent hook inequality given in [MPP3]: $\prod_{u\in λ} h(u) \, \le \, \prod_{u\in λ} h^\ast(u),$ where $h(u)$ is the usual hook in Young diagram $λ$, and $h^\ast(i,j)=i+j-1$. We then obtain a large variety of similar inequalities and their high-dimensional generalizations.
math.CO↗
Kirszbraun-type Theorems For Graphs
The classical Kirszbraun theorem says that all $1$-Lipschitz functions $f:A\longrightarrow \mathbb{R}^n$, $A\subset \mathbb{R}^n$, with the Euclidean metric have a $1$-Lipschitz extension to $\mathbb{R}^n$. For metric spaces $X,Y$ we say that $Y$ is $X$-Kirszbraun if all $1$-Lipschitz functions $f:A\longrightarrow Y$, $A\subset X$, have a $1$-Lipschitz extension to~$X$. We analyze the case when $X$ and $Y$ are graphs with the usual path metric. We prove that $\mathbb{Z}^d$-Kirszbraun graphs are exactly graphs that satisfies a certain Helly property. We also consider complexity aspects of these properties.
math.CO↗
Asymptotics of principal evaluations of Schubert polynomials for layered permutations
Denote by $u(n)$ the largest principal specialization of the Schubert polynomial: $ u(n) := \max_{w \in S_n} \mathfrak{S}_w(1,\ldots,1) $ Stanley conjectured in [arXiv:1704.00851] that there is a limit $\lim_{n\to \infty} \, \frac{1}{n^2} \log u(n), $ and asked for a limiting description of permutations achieving the maximum $u(n)$. Merzon and Smirnov conjectured in [arXiv:1410.6857] that this maximum is achieved on layered permutations. We resolve both Stanley's problems restricted to layered permutations.
math.CO↗
On the largest Kronecker and Littlewood--Richardson coefficients
We give new bounds and asymptotic estimates for Kronecker and Littlewood--Richardson coefficients. Notably, we resolve Stanley's questions on the shape of partitions attaining the largest Kronecker and Littlewood--Richardson coefficients. We apply the results to asymptotics of the number of standard Young tableaux of skew shapes.
math.CO↗
Bounds on the largest Kronecker and induced multiplicities of finite groups
We give new bounds and asymptotic estimates on the largest Kronecker and induced multiplicities of finite groups. The results apply to large simple groups of Lie type and other groups with few conjugacy classes.
math.GR↗
Complexity problems in enumerative combinatorics
We give a broad survey of recent results in Enumerative Combinatorics and their complexity aspects.
math.CO↗
Enumerating projections of integer points in unbounded polyhedra
We extend the Barvinok-Woods algorithm for enumerating projections of integer points in polytopes to unbounded polyhedra. For this, we obtain a new structural result on projections of semilinear subsets of the integer lattice. We extend the results to general formulas in Presburger Arithmetic. We also give an application to the k-Frobenius problem.
math.CO↗
Counting linear extensions of restricted posets
The classical 1991 result by Brightwell and Winkler states that the number of linear extensions of a poset is #P-complete. We extend this result to posets with certain restrictions. First, we prove that the number of linear extension for posets of height two is #P-complete. Furthermore, we prove that this holds for incidence posets of graphs. Finally, we prove that the number of linear extensions for posets of dimension two is #P-complete.
math.CO↗
Short Presburger arithmetic is hard
We study the computational complexity of short sentences in Presburger arithmetic (Short-PA). Here by "short" we mean sentences with a bounded number of variables, quantifiers, inequalities and Boolean operations; the input consists only of the integer coefficients involved in the linear inequalities. We prove that satisfiability of Short-PA sentences with $m+2$ alternating quantifiers is $Σ_{P}^m$-complete or $Π_{P}^m$-complete, when the first quantifier is $\exists$ or $\forall$, respectively. Counting versions and restricted systems are also analyzed. Further application are given to hardness of two natural problems in Integer Optimizations.
math.CO↗
Complexity of short generating functions
We give complexity analysis of the class of short generating functions (GF). Assuming $\#P \not\subseteq FP/poly$, we show that this class is not closed under taking many intersections, unions or projections of GFs, in the sense that these operations can increase the bitlength of coefficients of GFs by a super-polynomial factor. We also prove that truncated theta functions are hard in this class.
math.CO↗
VC-dimension of short Presburger formulas
We study VC-dimension of short formulas in Presburger Arithmetic, defined to have a bounded number of variables, quantifiers and atoms. We give both lower and upper bounds, which are tight up to a polynomial factor in the bit length of the formula.
math.LO↗