919 research outputs found
Perfectly secure data aggregation via shifted projections
We study a general scenario where confidential information is distributed
among a group of agents who wish to share it in such a way that the data
becomes common knowledge among them but an eavesdropper intercepting their
communications would be unable to obtain any of said data. The information is
modelled as a deck of cards dealt among the agents, so that after the
information is exchanged, all of the communicating agents must know the entire
deal, but the eavesdropper must remain ignorant about who holds each card.
Valentin Goranko and the author previously set up this scenario as the secure
aggregation of distributed information problem and constructed weakly safe
protocols, where given any card , the eavesdropper does not know with
certainty which agent holds . Here we present a perfectly safe protocol,
which does not alter the eavesdropper's perceived probability that any given
agent holds . In our protocol, one of the communicating agents holds a
larger portion of the cards than the rest, but we show how for infinitely many
values of , the number of cards may be chosen so that each of the agents
holds more than cards and less than
The intuitionistic temporal logic of dynamical systems
A dynamical system is a pair , where is a topological space and
is continuous. Kremer observed that the language of
propositional linear temporal logic can be interpreted over the class of
dynamical systems, giving rise to a natural intuitionistic temporal logic. We
introduce a variant of Kremer's logic, which we denote , and show
that it is decidable. We also show that minimality and Poincar\'e recurrence
are both expressible in the language of , thus providing a
decidable logic expressive enough to reason about non-trivial asymptotic
behavior in dynamical systems
Succinctness in subsystems of the spatial mu-calculus
In this paper we systematically explore questions of succinctness in modal
logics employed in spatial reasoning. We show that the closure operator,
despite being less expressive, is exponentially more succinct than the
limit-point operator, and that the -calculus is exponentially more
succinct than the equally-expressive tangled limit operator. These results hold
for any class of spaces containing at least one crowded metric space or
containing all spaces based on ordinals below , with the usual
limit operator. We also show that these results continue to hold even if we
enrich the less succinct language with the universal modality
Secure aggregation of distributed information: How a team of agents can safely share secrets in front of a spy
We consider the generic problem of Secure Aggregation of Distributed
Information (SADI), where several agents acting as a team have information
distributed among them, modeled by means of a publicly known deck of cards
distributed among the agents, so that each of them knows only her cards. The
agents have to exchange and aggregate the information about how the cards are
distributed among them by means of public announcements over insecure
communication channels, intercepted by an adversary "eavesdropper", in such a
way that the adversary does not learn who holds any of the cards. We present a
combinatorial construction of protocols that provides a direct solution of a
class of SADI problems and develop a technique of iterated reduction of SADI
problems to smaller ones which are eventually solvable directly. We show that
our methods provide a solution to a large class of SADI problems, including all
SADI problems with sufficiently large size and sufficiently balanced card
distributions
Well-orders in the transfinite Japaridze algebra
This paper studies the transfinite propositional provability logics
\glp_\Lambda and their corresponding algebras. These logics have for each
ordinal a modality \la \alpha \ra. We will focus on the closed
fragment of \glp_\Lambda (i.e., where no propositional variables occur) and
\emph{worms} therein. Worms are iterated consistency expressions of the form
\la \xi_n\ra \ldots \la \xi_1 \ra \top. Beklemishev has defined
well-orderings on worms whose modalities are all at least and
presented a calculus to compute the respective order-types.
In the current paper we present a generalization of the original
orderings and provide a calculus for the corresponding generalized order-types
. Our calculus is based on so-called {\em hyperations} which are
transfinite iterations of normal functions.
Finally, we give two different characterizations of those sequences of
ordinals which are of the form \la {\formerOmega}_\xi (A) \ra_{\xi \in \ord}
for some worm . One of these characterizations is in terms of a second kind
of transfinite iteration called {\em cohyperation.}Comment: Corrected a minor but confusing omission in the relation between
Veblen progressions and hyperation
Hyperations, Veblen progressions and transfinite iterations of ordinal functions
In this paper we introduce hyperations and cohyperations, which are forms of
transfinite iteration of ordinal functions.
Hyperations are iterations of normal functions. Unlike iteration by pointwise
convergence, hyperation preserves normality. The hyperation of a normal
function f is a sequence of normal functions so that f^0= id, f^1 = f and for
all ordinals \alpha, \beta we have that f^(\alpha + \beta) = f^\alpha f^\beta.
These conditions do not determine f^\alpha uniquely; in addition, we require
that the functions be minimal in an appropriate sense. We study hyperations
systematically and show that they are a natural refinement of Veblen
progressions.
Next, we define cohyperations, very similar to hyperations except that they
are left-additive: given \alpha, \beta, f^(\alpha + \beta)= f^\beta f^\alpha.
Cohyperations iterate initial functions which are functions that map initial
segments to initial segments. We systematically study cohyperations and see how
they can be employed to define left inverses to hyperations.
Hyperations provide an alternative presentation of Veblen progressions and
can be useful where a more fine-grained analysis of such sequences is called
for. They are very amenable to algebraic manipulation and hence are convenient
to work with. Cohyperations, meanwhile, give a novel way to describe slowly
increasing functions as often appear, for example, in proof theory
The Evolution of Primate Societies - Chapter 3
Compared with other primates, New World monkeys display relatively limited ecological variability. New World monkey anatomy and social systems, however, are extremely diverse. Several unique morphological features (e.g., claws, prehensile tails) and uncommon patterns of social organization (e.g., paternal care, cooperative breeding, female dispersal) have evolved in some platyrrhine species. Social organization and mating patterns include typical harem- like structures where mating is largely polygynous, and large multimale, multifemale groups with promiscuous mating and fi ssion- fusion societies. In addition, some species are socially monogamous and polyandrous. Even closely related species may exhibit strikingly different social organizations, as the example of the squirrel monkeys demonstrates (Mitchell et al. 1991; Boinski et al. 2005b). New World monkey behavior varies within species as well as between them. While the behavior of many species is known from only one study site, intriguing patterns of intraspecific variation are beginning to emerge from observations of populations that sometimes live in close proximity. For example, spider monkeys are often described as showing sex- segregated ranging behavior. Several studies show that males range farther, travel faster, and use larger areas than females, who tend to restrict their habitual ranging to smaller core areas within a groupβs large territory (Symington 1988; Chapman 1990; Shimooka 2005). In at least one well- studied population in YasunΓ National Park, Ecuador, however, males and females both travel over the entire community home range, and different females within the community show little evidence of occupying distinct core areas (Spehar et al. 2010). Similarly, in most well- studied populations of spider monkeys, females disperse and the resident males within a group are presumed to be close relativesβa suggestion corroborated by genetic data for one local population of spider monkeys in YasunΓ. Still, in a different local population, males are not closely related to one another, an unexpected pattern if signifi cant male philopatry were common (Di Fiore 2009; Di Fiore et al. 2009). While the causes of this local variation in group genetic structure are not clear, it may be signifi cant that the groups examined likely had different histories of contact with humans. For longlived animals who occupy relatively small social groups, the loss of even a handful of individuals to hunting, or to any other demographic disturbance, can have a dramatic impact on a groupβs genetic structure. Intragroup social relationships, in turn, are likely to be infl uenced by patterns of intragroup relatedness and by the relative availability of social partners of different age or sex class (chapter 21, this volume). Thus, historical and demographic contingencies are likely to create conditions where considerable local, intrapopulation variation in social systems exists. Slight changes in ecological conditions may also contribute to variation in the behavior of individuals living in a single population over time. For example, some authors have hypothesized that howler monkey populations may undergo dramatic fluctuations in size and composition in response to several ecological factors, including resource abundance, parasite and predation pressure, and climate (Milton 1982; Crockett & Eisenberg 1986; Crockett 1996; Milton 1996; Rudran & Fernandez- Duque 2003). This variability, not only among populations, but also within populations across time highlights the need for long- term studies. In sum, our understanding of the behavior of New World monkeys has increased dramatically over the past 25 years. This understanding highlights how their behavior varies within populations over time and among populations or species across space. As our knowledge of platyrrhine behavior continues to unfold and is enriched via additional long- term studies, a central challenge will be to explain how these variations arise. It will be important to entertain adaptive explanations while acknowledging that some differences may emerge via stochastic changes in demography (Struhsaker 2008) or nongenetic, relatively short- term, nonadaptive responses to sudden ecological change
Evidence and plausibility in neighborhood structures
The intuitive notion of evidence has both semantic and syntactic features. In
this paper, we develop an {\em evidence logic} for epistemic agents faced with
possibly contradictory evidence from different sources. The logic is based on a
neighborhood semantics, where a neighborhood indicates that the agent has
reason to believe that the true state of the world lies in . Further notions
of relative plausibility between worlds and beliefs based on the latter
ordering are then defined in terms of this evidence structure, yielding our
intended models for evidence-based beliefs. In addition, we also consider a
second more general flavor, where belief and plausibility are modeled using
additional primitive relations, and we prove a representation theorem showing
that each such general model is a -morphic image of an intended one. This
semantics invites a number of natural special cases, depending on how uniform
we make the evidence sets, and how coherent their total structure. We give a
structural study of the resulting `uniform' and `flat' models. Our main result
are sound and complete axiomatizations for the logics of all four major model
classes with respect to the modal language of evidence, belief and safe belief.
We conclude with an outlook toward logics for the dynamics of changing
evidence, and the resulting language extensions and connections with logics of
plausibility change
- β¦