arXiv subjects

Leonidas Palios

Publications and source records attributed to Leonidas Palios.

10 recordsLinked to original sources

arXiv2024

Optimizing generalized kernels of polygons

Let $\mathcal{O}$ be a set of $k$ orientations in the plane, and let $P$ be a simple polygon in the plane. Given two points $p,q$ inside $P$, we say that $p$ $\mathcal{O}$-\emph{sees} $q$ if there is an $\mathcal{O}$-\emph{staircase} contained in $P$ that connects $p$ and~$q$. The \emph{$\mathcal{O}$-Kernel} of the polygon $P$, denoted by $\mathcal{O}$-$\rm kernel(P)$, is the subset of points of $P$ which $\mathcal{O}$-see all the other points in $P$. This work initiates the study of the computation and maintenance of $\mathcal{O}$-$\rm kernel(P)$ as we rotate the set $\mathcal{O}$ by an angle $θ$, denoted by $\mathcal{O}$-$\rm kernel_θ(P)$. In particular, we consider the case when the set $\mathcal{O}$ is formed by either one or two orthogonal orientations, $\mathcal{O}=\{0^\circ\}$ or $\mathcal{O}=\{0^\circ,90^\circ\}$. For these cases and $P$ being a simple polygon, we design efficient algorithms for computing the $\mathcal{O}$-$\rm kernel_θ(P)$ while $θ$ varies in $[-\fracπ{2},\fracπ{2})$, obtaining: (i)~the intervals of angle~$θ$ where $\mathcal{O}$-$\rm kernel_θ(P)$ is not empty, (ii)~a value of angle~$θ$ where $\mathcal{O}$-$\rm kernel_θ(P)$ optimizes area or perimeter. Further, we show how the algorithms can be improved when $P$ is a simple orthogonal polygon. In addition, our results are extended to the case of a set $\mathcal{O}=\{α_1,\dots,α_k\}$.

cs.CG↗

arXiv2023

Adding a Tail in Classes of Perfect Graphs

Consider a graph $G$ which belongs to a graph class ${\cal C}$. We are interested in connecting a node $w \not\in V(G)$ to $G$ by a single edge $u w$ where $u \in V(G)$; we call such an edge a \emph{tail}. As the graph resulting from $G$ after the addition of the tail, denoted $G+uw$, need not belong to the class ${\cal C}$, we want to compute a minimum ${\cal C}$-completion of $G+w$, i.e., the minimum number of non-edges (excluding the tail $u w$) to be added to $G+uw$ so that the resulting graph belongs to ${\cal C}$. In this paper, we study this problem for the classes of split, quasi-threshold, threshold, and $P_4$-sparse graphs and we present linear-time algorithms by exploiting the structure of split graphs and the tree representation of quasi-threshold, threshold, and $P_4$-sparse graphs.

cs.DS↗

arXiv2023

Adding an Edge in a $P_4$-sparse Graph

The minimum completion (fill-in) problem is defined as follows: Given a graph family $\mathcal{F}$ (more generally, a property $Π$) and a graph $G$, the completion problem asks for the minimum number of non-edges needed to be added to $G$ so that the resulting graph belongs to the graph family $\mathcal{F}$ (or has property $Π$). This problem is NP-complete for many subclasses of perfect graphs and polynomial solutions are available only for minimal completion sets. We study the minimum completion problem of a $P_4$-sparse graph $G$ with an added edge. For any optimal solution of the problem, we prove that there is an optimal solution whose form is of one of a small number of possibilities. This along with the solution of the problem when the added edge connects two non-adjacent vertices of a spider or connects two vertices in different connected components of the graph enables us to present a polynomial-time algorithm for the problem.

cs.DS↗

arXiv2022

The Rectilinear Steiner Forest Arborescence problem

Let $r$ be a point in the first quadrant $Q_1$ of the plane $\mathbb{R}^2$ and let $P \subset Q_1$ be a set of points such that for any $p \in P$, its $x$- and $y$-coordinate is at least as that of $r$. A rectilinear Steiner arborescence for $P$ with the root $r$ is a rectilinear Steiner tree $T$ for $P \cup \{r\}$ such that for each point $p \in P$, the length of the (unique) path in $T$ from $p$ to the root $r$ equals $({\rm x}(p)-{\rm x}(r))+({\rm y}(p))-({\rm y}(r))$, where ${\rm x}(q)$ and ${\rm y}(q)$ denote the $x$- and $y$-coordinate, respectively, of point $q \in P \cup \{r\}$. Given two point sets $P$ and $R$ lying in the first quadrant $Q_1$ and such that $(0,0) \in R$, the Rectilinear Steiner Forest Arborescence (RSFA) problem is to find the minimum-length spanning forest $F$ such that each connected component $F$ is a rectilinear Steiner arborescence rooted at some root in $R$. The RSFA problem is a natural generalization of the Rectilinear Steiner Arborescence problem, where $R=\{(0,0)\}$, and thus it is NP-hard. In this paper, we provide a simple exact exponential time algorithm for the RSFA problem, design a polynomial time approximation scheme as well as a fixed-parameter algorithm.

cs.CG↗

arXiv2020

Shortest Watchman Tours in Simple Polygons under Rotated Monotone Visibility

We present an $O(nrG)$ time algorithm for computing and maintaining a minimum length shortest watchman tour that sees a simple polygon under monotone visibility in direction $θ$, while $θ$ varies in $[0,180^{\circ})$, obtaining the directions for the tour to be the shortest one over all tours, where $n$ is the number of vertices, $r$ is the number of reflex vertices, and $G\leq r$ is the maximum number of gates of the polygon used at any time in the algorithm.

cs.CG↗

arXiv2018

Characterizing Watermark Numbers encoded as Reducible Permutation Graphs against Malicious Attacks

In the domain of software watermarking, we have proposed several graph theoretic watermarking codec systems for encoding watermark numbers $w$ as reducible permutation flow-graphs $F[π^*]$ through the use of self-inverting permutations $π^*$. Following up on our proposed methods, we theoretically study the oldest one, which we call W-RPG, in order to investigate and prove its resilience to edge-modification attacks on the flow-graphs $F[π^*]$. In particular, we characterize the integer $w\equivπ^*$ as strong or weak watermark through the structure of self-inverting permutations $π^*$ which encodes it. To this end, for any integer watermark $w \in R_n=[2^{n-1}, 2^n-1]$, where $n$ is the length of the binary representation $b(w)$ of $w$, we compute the minimum number of 01-modifications needed to be applied on $b(w)$ so that the resulting $b(w')$ represents the valid watermark number $w'$; note that a number $w'$ is called valid (or, true-incorrect watermark number) if $w'$ can be produced by the W-RPG codec system and, thus, it incorporates all the structural properties of $π^* \equiv w$.

cs.DM↗

arXiv2018

Capturing points with a rotating polygon (and a 3D extension)

We study the problem of rotating a simple polygon to contain the maximum number of elements from a given point set in the plane. We consider variations of this problem where the rotation center is a given point or lies on a line segment, a line, or a polygonal chain. We also solve an extension to 3D where we rotate a polyhedron around a given point to contain the maximum number of elements from a set of points in the space.

cs.CG↗

arXiv2017

Encoding Watermark Numbers as Reducible Permutation Graphs using Self-inverting Permutations

Several graph theoretic watermark methods have been proposed to encode numbers as graph structures in software watermarking environments. In this paper, we propose an efficient and easily implementable codec system for encoding watermark numbers as reducible permutation flow-graphs and, thus, we extend the class of graphs used in such a watermarking environment. More precisely, we present an algorithm for encoding a watermark number $w$ as a self-inverting permutation $π^*$, an algorithm for encoding the self-inverting permutation $π^*$ into a reducible permutation graph $F[π^*]$ whose structure resembles the structure of real program graphs, as well as decoding algorithms which extract the permutation $π^*$ from the reducible permutation graph $F[π^*]$ and the number $w$ from $π^*$. Both the encoding and the decoding process takes time and space linear in the length of the binary representation of $w$. The two main components of our proposed codec system, i.e., the self-inverting permutation $π^*$ and the reducible permutation graph $F[π^*]$, incorporate the binary representation of the watermark~$w$ in their structure and possess important structural properties, which make our system resilient to attacks; to this end, we experimentally evaluated our system under edge modification attacks on the graph $F[π^*]$ and the results show that we can detect such attacks with high probability.

cs.DS↗

arXiv2010

Join-Reachability Problems in Directed Graphs

For a given collection G of directed graphs we define the join-reachability graph of G, denoted by J(G), as the directed graph that, for any pair of vertices a and b, contains a path from a to b if and only if such a path exists in all graphs of G. Our goal is to compute an efficient representation of J(G). In particular, we consider two versions of this problem. In the explicit version we wish to construct the smallest join-reachability graph for G. In the implicit version we wish to build an efficient data structure (in terms of space and query time) such that we can report fast the set of vertices that reach a query vertex in all graphs of G. This problem is related to the well-studied reachability problem and is motivated by emerging applications of graph-structured databases and graph algorithms. We consider the construction of join-reachability structures for two graphs and develop techniques that can be applied to both the explicit and the implicit problem. First we present optimal and near-optimal structures for paths and trees. Then, based on these results, we provide efficient structures for planar graphs and general directed graphs.

cs.DS↗

arXiv2008

Computing the Newton polygon of the implicit equation

We consider polynomially and rationally parameterized curves, where the polynomials in the parameterization have fixed supports and generic coefficients. We apply sparse (or toric) elimination theory in order to determine the vertex representation of its implicit polygon, i.e. of the implicit equation's Newton polygon. In particular, we consider mixed subdivisions of the input Newton polygons and regular triangulations of point sets defined by Cayley's trick. We distinguish polynomial and rational parameterizations, where the latter may have the same or different denominators; the implicit polygon is shown to have, respectively, up to 4, 5, or 6 vertices.

math.AG↗