41,966 research outputs found
List rankings and on-line list rankings of graphs
A -ranking of a graph is a labeling of its vertices from
such that any nontrivial path whose endpoints have the same
label contains a larger label. The least for which has a -ranking is
the ranking number of , also known as tree-depth. The list ranking number of
is the least such that if each vertex of is assigned a set of
potential labels, then can be ranked by labeling each vertex with a label
from its assigned list. Rankings model a certain parallel processing problem in
manufacturing, while the list ranking version adds scheduling constraints. We
compute the list ranking number of paths, cycles, and trees with many more
leaves than internal vertices. Some of these results follow from stronger
theorems we prove about on-line versions of list ranking, where each vertex
starts with an empty list having some fixed capacity, and potential labels are
presented one by one, at which time they are added to the lists of certain
vertices; the decision of which of these vertices are actually to be ranked
with that label must be made immediately.Comment: 16 pages, 3 figure
TiFi: Taxonomy Induction for Fictional Domains [Extended version]
Taxonomies are important building blocks of structured knowledge bases, and their construction from text sources and Wikipedia has received much attention. In this paper we focus on the construction of taxonomies for fictional domains, using noisy category systems from fan wikis or text extraction as input. Such fictional domains are archetypes of entity universes that are poorly covered by Wikipedia, such as also enterprise-specific knowledge bases or highly specialized verticals. Our fiction-targeted approach, called TiFi, consists of three phases: (i) category cleaning, by identifying candidate categories that truly represent classes in the domain of interest, (ii) edge cleaning, by selecting subcategory relationships that correspond to class subsumption, and (iii) top-level construction, by mapping classes onto a subset of high-level WordNet categories. A comprehensive evaluation shows that TiFi is able to construct taxonomies for a diverse range of fictional domains such as Lord of the Rings, The Simpsons or Greek Mythology with very high precision and that it outperforms state-of-the-art baselines for taxonomy induction by a substantial margin
Individual Security and Network Design with Malicious Nodes
Networks are beneficial to those being connected but can also be used as
carriers of contagious hostile attacks. These attacks are often facilitated by
exploiting corrupt network users. To protect against the attacks, users can
resort to costly defense. The decentralized nature of such protection is known
to be inefficient but the inefficiencies can be mitigated by a careful network
design. Is network design still effective when not all users can be trusted? We
propose a model of network design and defense with byzantine nodes to address
this question. We study the optimal defended networks in the case of
centralized defense and, for the case of decentralized defense, we show that
the inefficiencies due to decentralization can be fully mitigated, despite the
presence of the byzantine nodes.Comment: 19 pages, 3 figure
Lemmings is PSPACE-complete
Lemmings is a computer puzzle game developed by DMA Design and published by
Psygnosis in 1991, in which the player has to guide a tribe of lemming
creatures to safety through a hazardous landscape, by assigning them specific
skills that modify their behavior in different ways. In this paper we study the
optimization problem of saving the highest number of lemmings in a given
landscape with a given number of available skills.
We prove that the game is PSPACE-complete, even if there is only one lemming
to save, and only Builder and Basher skills are available. We thereby settle an
open problem posed by Cormode in 2004, and again by Forisek in 2010. However we
also prove that, if we restrict the game to levels in which the available
Builder skills are only polynomially many (and there is any number of other
skills), then the game is solvable in NP. Similarly, if the available Basher,
Miner, and Digger skills are polynomially many, the game is solvable in NP.
Furthermore, we show that saving the maximum number of lemmings is APX-hard,
even when only one type of skill is available, whatever this skill is. This
contrasts with the membership in P of the decision problem restricted to levels
with no "deadly areas" (such as water or traps) and only Climber and Floater
skills, as previously established by Cormode.Comment: 26 pages, 11 figure
A risk-security tradeoff in graphical coordination games
A system relying on the collective behavior of decision-makers can be
vulnerable to a variety of adversarial attacks. How well can a system operator
protect performance in the face of these risks? We frame this question in the
context of graphical coordination games, where the agents in a network choose
among two conventions and derive benefits from coordinating neighbors, and
system performance is measured in terms of the agents' welfare. In this paper,
we assess an operator's ability to mitigate two types of adversarial attacks -
1) broad attacks, where the adversary incentivizes all agents in the network
and 2) focused attacks, where the adversary can force a selected subset of the
agents to commit to a prescribed convention. As a mitigation strategy, the
system operator can implement a class of distributed algorithms that govern the
agents' decision-making process. Our main contribution characterizes the
operator's fundamental trade-off between security against worst-case broad
attacks and vulnerability from focused attacks. We show that this tradeoff
significantly improves when the operator selects a decision-making process at
random. Our work highlights the design challenges a system operator faces in
maintaining resilience of networked distributed systems.Comment: 13 pages, double column, 4 figures. Submitted for journal publicatio
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