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At least 55 records · Page 3Linked to original sources
arXiv2025

The Sunflower-Free Process

An $r$-sunflower is a collection of $r$ sets such that the intersection of any two sets in the collection is identical. We analyze a random process which constructs a $w$-uniform $r$-sunflower free family starting with an empty family and at each step adding a set chosen uniformly at random from all choices that could be added without creating an $r$-sunflower with the previously chosen sets. To analyze this process, we extend results of the first author and Bohman arXiv:1308.3732v5 [math.CO], who analyzed a general random process which adds one object at a time chosen uniformly at random from all objects that can be added without creating certain forbidden subsets.

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arXiv2026

Colouring ($P_2\cup P_4$, diamond)-free graphs with $ω$ colours

In this paper, we establish an optimal $χ$-binding function for $(P_2\cup P_4,\text{ diamond})$-free graphs. We prove that for any graph $G$ in this class, $χ(G)\le 4$ when $ω(G)=2$, $χ(G)\le 6$ when $ω(G)=3$, and $χ(G)=ω(G)$ when $ω(G)\ge 4$, where $χ(G)$ and $ω(G)$ denote the chromatic number and clique number of $G$, respectively. This result extends the known chromatic bounds for $(P_2\cup P_3,\text{ diamond})$-free graphs by showing that $(P_2\cup P_4,\text{ diamond})$-free graphs admit the same $χ$-binding function. It also refines the chromatic bound obtained by Angeliya, Karthick and Huang [arXiv:2501.02543v3 [math.CO], 2025] for $(P_2\cup P_4,\text{ diamond})$-free graphs.

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arXiv2026

Small $q$-kernels in digraphs with minimum in-degree $δ$

For a digraph $D$, a subset $Q\subseteq V(D)$ is called a $q$-kernel if $Q$ is an independent set and all vertices in $V(D)$ are reachable from $Q$ via a directed path of length at most $q$. Given integers $q\geq 2$ and $δ\geq 1$, Spiro arXiv:2404.07305 [math.CO] posed the question: what is the smallest constant $c_{δ,q}$ such that every digraph $D$ with minimum in-degree $δ$ has a $q$-kernel of size at most $c_{δ,q}|V(D)|$? We show the constants $c_{δ,q}$ are monotone in both $δ$ and $q$, and we improve upon the known upper bounds for $c_{δ,q}$. Our main results show $\frac{1}{δ+1} \leq c_{δ,q}\leq \frac{1}{\lfloor\sqrt{δ+1}\rfloor+1}$ for all $q \geq 3$ and $δ\geq 1$, and $ c_{δ,q}=\frac{1}{δ+1}$ whenever $δ\geq 1$ and $q \geq \left\lceil\frac{3δ}{2}\right\rceil + 1$.

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arXiv2000

Une identité remarquable en théorie des partitions

We prove an identity about partitions, previously conjectured in the study of shifted Jack polynomials (math.CO/9903020). The proof given is using $λ$-ring techniques. It would be interesting to obtain a bijective proof.

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arXiv2000

Higher Nimbers in pawn endgames on large chessboards

We answer a question posed in [Elkies 1996] (math.CO/9905198) by constructing a class of pawn endgames on m-by-n boards that show the Nimbers *k for large k. We do this by modifying and generalizing T.R. Dawson's ``pawns game'' [Berlekamp et al. 1982] (Winning Ways I). Our construction works for m>8 and n sufficiently large; on the basis of computational evidence we conjecture, but cannot yet prove, that the construction yields *k for all integers k.

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arXiv2001

Generating functions for intersection numbers on moduli spaces of curves

Using the connection between intersection theory on the Deligne-Mumford spaces and the edge scaling of the GUE matrix model (see math.CO/9903176, math.AG/0101147), we express the n-point functions for the intersection numbers as n-dimensional error-function-type integrals and also give a derivation of Witten's KdV equations using the higher Fay identities of Adler, Shiota, and van Moerbeke.

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arXiv2001

The Floyd-Warshall Algorithm, the AP and the TSP, Part II

In math.CO/0111309, we used admissible permutations and a variant of the Floyd-Warshall Algorithm to obtain an optimal solution to the Assignment Problem and an approximate solution to the Traveling Salesman Problem. Here we give a large, detailed illustration of how the algorithms are applied.

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arXiv2002

The Poincaré series of the algebra of rational functions which are regular outside hyperplanes

Let $Δ$ be a finite set of nonzero linear forms in several variables with coefficients in a field $\mathbf K$ of characteristic zero. Consider the $\mathbf K$-algebra $R(Δ)$ of rational functions on V which are regular outside $\bigcup_{α\inΔ} \kerα$. Then the ring $R(Δ)$ is naturally doubly filtered by the degrees of denominators and of numerators. In this paper we give an explicit combinatorial formula for the Poincaré series in two variables of the associated bigraded vector space $\bar{R}(Δ)$. This generalizes the main theorem of Terao, H.: Algebras generated by reciprocals of linear forms, to appear in J.Algebra (arXiv:math.CO/0105095).

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arXiv2002

Differential Operators and Weighted Isobaric Polynomials

We characterize those sequences of weighted isobaric polynomials as defined in math.CO/0106213 which belong to the kernel of the linear operator $D_{11} - \sum_{j=1}^k a_j t_j D_{2j} - mD_2$, and we characterize those linear operators of this form in terms of the coefficients $a_j$ which have a non-zero kernel.

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arXiv2002

Lagrangian Matroids: Representations of Type $B_n$

We introduce the concept of orientation for Lagrangian matroids represented in the flag variety of maximal isotropic subspaces of dimension N in the real vector space of dimension 2N+1. The paper continues the study started in math.CO/0209100.

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arXiv2003

H-admissible permutations and the HCP

This version is similar to math.CO/0210113. We've changed Conjectures 1.1 and 1.2 so that they cover arbitrary graphs(digraphs). Let G be an arbitrary graph(digraph). Then - in polynomial time - either an algorithm obtains a hamilton circuit(cycle)or else the algorithm points to at least one vertex that cannot belong to any hamilton circuit(cycle) of G. We give criteria for determining which vertices should be examined.

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arXiv2002

Obtaining hamilton cicuits in graphs and digraphs

This paper improves algorithms given in math.CO/0012036. Although the graph (digraph) becomes non-random as the algorithm proceeds, the probability for success stays the same. We also give examples.

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arXiv2004

Recent developments in algebraic combinatorics

A survey of three recent developments in algebraic combinatorics: (1) the Laurent phenomenon, (2) Gromov-Witten invariants and toric Schur functions, and (3) toric h-vectors and intersection cohomology. This paper is a continuation of "Recent progress in algebraic combinatorics" (math.CO/0010218), which dealt with three other topics.

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arXiv2003

The volume of the 10th Birkhoff polytope

The n'th Birkhoff polytope $B_n$ is the set of all doubly stochastic $n \times n$ matrices, that is, those matrices with nonnegative real coefficients in which every row and column sums to one. A long-standing open problem is the determination of the relative volume of $B_n$. In [arXiv:math.CO/0202267] we introduced a method of calculating this volume and used it to compute $\vol B_9$. This note is an update on our progress: with the same program (but much longer computing time), we have now derived $\vol B_{10}$.

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arXiv2003

A Schubert calculus recurrence from the noncomplex W-action on G/B

In this paper, as in our previous "Descent-cycling in Schubert calculus" math.CO/0009112, we study the structure constants in equivariant cohomology of flag manifolds G/B. In this one we give a recurrence (which is frequently, but alas not always, positive) to compute these one by one, using the non-complex action of the Weyl group on G/B. Probably the most noteworthy feature of this recurrence is that to compute a particular structure constant c_{lambda,mu}^nu, one does not have to compute the whole product S_lambda * S_mu.

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arXiv2003

Freely braided elements in Coxeter groups, II

We continue the study of freely braided elements of simply laced Coxeter groups, which we introduced in a previous work (math.CO/0301104). A known upper bound for the number of commutation classes of reduced expressions for an element of a simply laced Coxeter group is shown to be achieved only when the element is freely braided; this establishes the converse direction of a previous result. It is also shown that a simply laced Coxeter group has finitely many freely braided elements if and only if it has finitely many fully commutative elements.

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arXiv2003

Large-determinant sign matrices of order 4k+1

The Hadamard maximal determinant problem asks for the largest n-by-n determinant with entries in {+1,-1}. When n is congruent to 1 (mod 4), the maximal excess construction of Farmakis & Kounias has been the most successful general method for constructing large (though seldom maximal) determinants. For certain small n, however, still larger determinants have been known; several new records were recently reported in ArXiv preprint math.CO/0304410 . Here, we define ``3-normalized'' n-by-n Hadamard matrices, and construct large-determinant matrices of order n+1 from them. Our constructions account for most of the previous ``small n'' records, and set new records when n=37, 49, 65, 73, 77, 85, 93, and 97, most of which are beyond the reach of the maximal excess technique. We conjecture that our n=37 determinant, 72 x 9^{17} x 2^{36}, achieves the global maximum.

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