Combinatorics (math.CO)

Abstract: We express the number of anti-invariant subspaces for a linear operator on a finite vector space in terms of the number of its invariant subspaces. When the operator is diagonalizable with distinct eigenvalues, our formula gives a finite-field interpretation for the entries of the $q$-Hermite Catalan matrix. We also obtain an interesting new proof of Touchard's formula for these entries.

Abstract: For $n\leq 6$, we provide the number of $n$-variable unate and monotone Boolean functions under various restrictions. Additionally, we provide the number of balanced 7-variable monotone Boolean functions.

Abstract: We study the following natural arithmetic question regarding intersecting families: how large can a family of subsets of integers from $\{1, \ldots n\}$ be such that, for every pair of subsets in the family, the intersection contains a sum $x + y = z$? We conjecture that any such sum-intersecting family must have size at most $\frac14 \cdot 2^{n}$ (which would be tight if correct). Towards this conjecture, we show that every sum-intersecting family has at most $0.32 \cdot 2^n$ subsets.

Abstract: The study of existence of hamiltonian cycles and factors in graphs in terms of toughness was initiated by Chv\'atal in 1973 and has been a popular topic since then. The study of Berge cycles and factor in hypergraphs has attracted quite a bit attention in recent years and much progress has been made in terms of conditions such as degrees. In this article, we propose to study toughness conditions for a hypergraph to have a Berge $k$-factor. Our main result is that every $k$-tough hypergraph $H$ has a Berge $k$-factor if $k\cdot |V(H)|$ is even and $|V(H)|\ge k+1$ for integer $k\ge 1$. This extends a similar result on graphs from 1985 by Enomoto, Jackson, Keterinis, and Saito.

Abstract: A $d$-dimensional nowhere-zero $r$-flow on a graph $G$, an $(r,d)$-NZF from now on, is a flow where the value on each edge is an element of $\mathbb{R}^d$ whose (Euclidean) norm lies in the interval $[1,r-1]$. Such a notion is a natural generalization of the well-known concept of circular nowhere-zero $r$-flow (i.e.\ $d=1$). For every bridgeless graph $G$, the $5$-flow Conjecture claims that $\phi_1(G)\leq 5$, while a conjecture by Jain suggests that $\phi_d(G)=1$, for all $d \geq 3$. Here, we address the problem of finding a possible upper-bound also for the remaining case $d=2$. We show that, for all bridgeless graphs, $\phi_2(G) \le 1 + \sqrt{5}$ and that the oriented $5$-cycle double cover Conjecture implies $\phi_2(G)\leq \tau^2$, where $\tau$ is the Golden Ratio. Moreover, we propose a geometric method to describe an $(r,2)$-NZF of a cubic graph in a compact way, and we apply it in some instances. Our results and some computational evidence suggest that $\tau^2$ could be a promising upper bound for the parameter $\phi_2(G)$ for an arbitrary bridgeless graph $G$. We leave that as a relevant open problem which represents an analogous of the $5$-flow Conjecture in the $2$-dimensional case (i.e. complex case).

Abstract: We study the structure of the set of all possible affine hyperplane sections of a convex polytope. We present two different cell decompositions of this set, induced by hyperplane arrangements. Using our decomposition, we bound the number of possible combinatorial types of sections and craft algorithms that compute optimal sections of the polytope according to various combinatorial and metric criteria, including sections that maximize the number of $k$-dimensional faces, maximize the volume, and maximize the integral of a polynomial. Our optimization algorithms run in polynomial time in fixed dimension, but the same problems show hardness otherwise. Our tools can be extended to intersection with halfspaces and projections onto hyperplanes. Finally, we present several experiments illustrating our theorems and algorithms on famous polytopes.