For a finite set A ⊂ ℝ², a map φ: A → ℝ² is orientation preserving if for every non-collinear triple u,v,w ∈ A the orientation of the triangle u,v,w is the same as that of the triangle φ(u),φ(v),φ(w). We prove that for every n ∈ ℕ and for every ε > 0 there is N = N(n,ε) ∈ ℕ such that the following holds. Assume that φ:G(N) → ℝ² is an orientation preserving map where G(N) is the grid {(i,j) ∈ ℤ²: -N ≤ i,j ≤ N}. Then there is an affine transformation ψ :ℝ² → ℝ² and a ∈ ℤ² such that a+G(n) ⊂ G(N) and ‖ψ∘φ (z)-z‖ < ε for every z ∈ a+G(n). This result was previously proved in a completely different way by Nešetřil and Valtr, without obtaining any bound on N. Our proof gives N(n,ε) = O(n⁴ε^{-2}).

A semiorder is a model of preference relations where each element $x$ is associated with a utility value $α(x)$, and there is a threshold $t$ such that $y$ is preferred to $x$ iff $α(y) > α(x)+t$. These are motivated by the notion that there is some uncertainty in the utility values we assign an object or that a subject may be unable to distinguish a preference between objects whose values are close. However, they fail to model the well-known phenomenon that preferences are not always transitive. Also, if we are uncertain of the utility values, it is not logical that preference is determined absolutely by a comparison of them with an exact threshold. We propose a new model in which there are two thresholds, $t_1$ and $t_2$; if the difference $α(y) - α(x)$ less than $t_1$, then $y$ is not preferred to $x$; if the difference is greater than $t_2$ then $y$ is preferred to $x$; if it is between $t_1$ and $t_2$, then then $y$ may or may not be preferred to $x$. We call such a relation a double-threshold semiorder, and the corresponding directed graph $G = (V,E)$ a double threshold digraph. Every directed acyclic graph is a double threshold graph; bounds on $t_2/t_1$ give a nested hierarchy of subclasses of the directed acyclic graphs. In this paper we characterize the subclasses in terms of forbidden subgraphs, and give algorithms for finding an assignment of of utility values that explains the relation in terms of a given $(t_1,t_2)$ or else produces a forbidden subgraph, and finding the minimum value $λ$ of $t_2/t_1$ that is satisfiable for a given directed acyclic graph. We show that $λ$ gives a measure of the complexity of a directed acyclic graph with respect to several optimization problems that are NP-hard on arbitrary directed acyclic graphs.

By a curve in Rd we mean a continuous map γ: I → Rd, where I ⊂ R is a closed interval. We call a curve γ in Rd (≤ k)-crossing if it intersects every hyperplane at most k times (counted with multiplicity). The (≤ d)-crossing curves in Rd are often called convex curves and they form an important class; a primary example is the moment curve {(t, t2, …, td): t ∈ [0, 1]}. They are also closely related to Chebyshev systems, which is a notion of considerable importance, e.g., in approximation theory. Our main result is that for every d there is M = M(d) such that every (≤ d + 1)-crossing curve in Rd can be subdivided into at most M(≤ d)-crossing curve segments. As a consequence, based on the work of Eliáš, Roldán, Safernová, and the second author, we obtain an essentially tight lower bound for a geometric Ramsey-type problem in Rd concerning order-type homogeneous sequences of points, investigated in several previous papers.

Giving a (partial) solution to a problem of Fekete [Geometry and the Traveling Salesman Problem, Ph.D. thesis, University of Waterloo, Waterloo, ON, Canada, 1992] and Fekete and Woeginger [Comput. Geom., 8 (1997), pp. 195–218], we show that given a finite set X of points in the plane, it is possible to find a polygonal path with $|X|-1$ segments and with vertex set X so that every angle on the polygonal path is at least $\pi/9$. According to a conjecture of Fekete and Woeginger, $\pi/9$ can be replaced by $\pi/6$. Previously, the result has not been known with any positive constant. We show further that the same result holds, with an angle smaller than $\pi/9$, in higher dimensions.

The Kneser graph $K_{n:k}$ for positive integers $n\ge k$ has as its vertex set the k-element subsets of some n-set, with disjoint sets being adjacent. Every finite simple graph can be found as an induced subgraph of some Kneser graph; this article explores some questions arising from that fact.

In 1981, Bermond and Thomassen conjectured that every digraph with minimum out-degree at least $2k-1$ contains k disjoint cycles. This conjecture is trivial for $k=1$, and was established for $k=2$ by Thomassen in 1983. We verify it for the next case, proving that every digraph with minimum out-degree at least five contains three disjoint cycles. To show this, we improve Thomassen's result by proving that every digraph whose vertices have out-degree at least three, except at most two with out-degree two, indeed contains two disjoint cycles.