29 research outputs found
The interval constrained 3-coloring problem
In this paper, we settle the open complexity status of interval constrained
coloring with a fixed number of colors. We prove that the problem is already
NP-complete if the number of different colors is 3. Previously, it has only
been known that it is NP-complete, if the number of colors is part of the input
and that the problem is solvable in polynomial time, if the number of colors is
at most 2. We also show that it is hard to satisfy almost all of the
constraints for a feasible instance.Comment: minor revisio
An Exact Algorithm for Robust Network Design
Modern life heavily relies on communication networks that operate efficiently. A crucial issue for the design of communication networks is robustness with respect to traffic fluctuations, since they often lead to congestion and traffic bottlenecks. In this paper, we address an NP-hard single commodity robust network design problem, where the traffic demands change over time. For k different times of the day, we are given for each node the amount of single-commodity flow it wants to send or to receive. The task is to determine the minimum-cost edge capacities such that the flow can be routed integrally through the net at all times. We present an exact branch-and-cut algorithm, based on a decomposition into biconnected network components, a clever primal heuristic for generating feasible solutions from the linear-programming relaxation, and a general cutting-plane separation routine that is based on projection and lifting. By presenting extensive experimental results on realistic instances from the literature, we show that a suitable combination of these algorithmic components can solve most of these instances to optimality. Furthermore, cutting-plane separation considerably improves the algorithmic performance
Single-Sink Fractionally Subadditive Network Design
We study a generalization of the Steiner tree problem, where we are given a weighted network G together with a collection of k subsets of its vertices and a root r. We wish to construct a minimum cost network such that the network supports one unit of flow to the root from every node in a subset simultaneously. The network constructed does not need to support flows from all the subsets simultaneously.
We settle an open question regarding the complexity of this problem for k=2, and give a 3/2-approximation algorithm that improves over a (trivial) known 2-approximation. Furthermore, we prove some structural results that prevent many well-known techniques from doing better than the known O(log n)-approximation. Despite these obstacles, we conjecture that this problem should have an O(1)-approximation. We also give an approximation result for a variant of the problem where the solution is required to be a path
Node Connectivity Augmentation of Highly Connected Graphs
Node-connectivity augmentation is a fundamental network design problem. We
are given a -node connected graph together with an additional set of
links, and the goal is to add a cheap subset of links to to make it
-node connected.
In this work, we characterize completely the computational complexity status
of the problem, by showing hardness for all values of which were not
addressed previously in the literature.
We then focus on -node connectivity augmentation for , which
corresponds to the highest value of for which the problem is NP-hard. We
improve over the previously best known approximation bounds for this problem,
by developing a -approximation algorithm for the weighted setting,
and a -approximation algorithm for the unweighted setting
The phytochelatin synthase from Nitella mucronata (Charophyta) plays a role in the homeostatic control of iron(II)/(III)
Although some charophytes (sister group to land plants) have been shown to synthesize phytochelatins (PCs) in
response to cadmium (Cd), the functional characterization of their phytochelatin synthase (PCS) is still completely
lacking. To investigate the metal response and the presence of PCS in charophytes, we focused on the
species Nitella mucronata. A 40 kDa immunoreactive PCS band was revealed in mono-dimensional western blot
by using a polyclonal antibody against Arabidopsis thaliana PCS1. In two-dimensional western blot, the putative
PCS showed various spots with acidic isoelectric points, presumably originated by post-translational modifications.
Given the PCS constitutive expression in N. mucronata, we tested its possible involvement in the homeostasis
of metallic micronutrients, using physiological concentrations of iron (Fe) and zinc (Zn), and verified its
role in the detoxification of a non-essential metal, such as Cd. Neither in vivo nor in vitro exposure to Zn resulted
in PCS activation and PC significant biosynthesis, while Fe(II)/(III) and Cd were able to activate the PCS in vitro,
as well as to induce PC accumulation in vivo. While Cd toxicity was evident from electron microscopy observations,
the normal morphology of cells and organelles following Fe treatments was preserved. The overall
results support a function of PCS and PCs in managing Fe homeostasis in the carophyte N. mucronata
Approximating Weighted Tree Augmentation via Chvatal-Gomory Cuts
The weighted tree augmentation problem (\WTAP) is a fundamental network design problem. We are given an undirected tree with nodes, an additional set of edges called \emph{links} and a cost vector . The goal is to choose a minimum cost subset such that is -edge-connected. In the unweighted case, that is, when we have for all , the problem is called the tree augmentation problem (\TAP).
Both problems are known to be \APX-hard, and the best known
approximation factors are for \WTAP by (Frederickson and J\'aJ\'a,
'81) and for \TAP due to (Kortsarz and Nutov, TALG
'16). Adjashvili (SODA~'17) recently presented an
\approx 1.96418+\eps-approximation algorithm for \WTAP\ for the case
where all link costs are bounded by a constant. This is the first
approximation with a better guarantee than that does not require
restrictions on the structure of the tree or the links.
In this paper, we improve Adjiashvili's approximation to a \tfrac{3}{2}+\eps-approximation for \WTAP under the bounded cost assumption. We achieve this by introducing a strong \LP that combines \zhcgcuts for the standard \LP for the problem with bundle constraints from Adjiashvili. We show that our \LP can be solved efficiently and that it is exact for some instances that arise at the core of Adjiashvili's approach. This results in the improved performance guarantee of \tfrac{3}{2}+\eps, which is asymptotically on par with the result by Kortsarz and Nutov. Our result also is the best-known \LP-relative approximation algorithm for \TAP.Non UBCUnreviewedAuthor affiliation: University of WaterlooResearche
Integer programming and combinatorial optimization : 23rd International Conference, IPCO 2022, Eindhoven, The Netherlands, June 27-29, 2022, Proceedings
Exponentiality of the exchange algorithm for finding another room-partitioning
Let T be a triangulated surface given by the list of vertex-triples of its triangles, called rooms. A room-partitioning for T is a subset R of the rooms such that each vertex of T is in exactly one room in R. Given a room-partitioning R for T, the exchange algorithm walks from room to room until it finds a second different room-partitioning R'. In fact, this algorithm generalizes the Lemke-Howson algorithm for finding a Nash equilibrium for two-person games. In this paper, we show that the running time of the exchange algorithm is not polynomial relative to the number of rooms, by constructing a sequence of (planar) instances, in which the algorithm walks from room to room an exponential number of times. We also show a similar result for the problem of finding a second perfect matching in Eulerian graphs. (C) 2012 Elsevier B.V. All rights reserved