Multi-Agent Only Knowing

Levesque introduced a notion of ``only knowing'', with the goal of capturing certain types of nonmonotonic reasoning. Levesque's logic dealt with only the case of a single agent. Recently, both Halpern and Lakemeyer independently attempted to extend Levesque's logic to the multi-agent case. Although there are a number of similarities in their approaches, there are some significant differences. In this paper, we reexamine the notion of only knowing, going back to first principles. In the process, we simplify Levesque's completeness proof, and point out some problems with the earlier definitions. This leads us to reconsider what the properties of only knowing ought to be. We provide an axiom system that captures our desiderata, and show that it has a semantics that corresponds to it. The axiom system has an added feature of interest: it includes a modal operator for satisfiability, and thus provides a complete axiomatization for satisfiability in the logic K45.Comment: To appear, Journal of Logic and Computatio

Baryonic Dark Halos: Machos and Cold Gas?

We consider the possibility that the dark matter in the halos of galaxies may be in the form of clusters of \macho s within which are embedded cold, dense gas clouds. Microlensing experiments have found evidence that the Galactic halo contains up to half of its mass in the form of low-mass \macho s. A number of observational and dynamical arguments point to the existence of hitherto unobserved cold gas around galaxies. We show that the cold gas can be stabilized by \macho\ clusters. Within the framework of a simple two-component model, we derive constraints on the \macho\ clusters and on the halo cold gas content. Typical cluster masses are \sim 10 \msun, typical \macho\ masses are \sim 0.01 \msun, and the gas content could be up to of order 50 \%. Various predictions are given for testing the hypothesis that such objects could constitute most of the mass in the dark halos of galaxies. If halos are indeed baryonic and contain significant amounts of cold gas, they are likely to play a much more active role in galaxy formation and evolution than is commonly supposed.Comment: 13 pages, submitted to ApJ. Uuencoded, gzip-compressed ps-file including 2 figures. Also available at http://www.astro.unibas.ch/~gerhard/papers/dmmn.ps.gz.u

Mosaics from arbitrary stereo video sequences

lthough mosaics are well established as a compact and non-redundant representation of image sequences, their application still suffers from restrictions of the camera motion or has to deal with parallax errors. We present an approach that allows construction of mosaics from arbitrary motion of a head-mounted camera pair. As there are no parallax errors when creating mosaics from planar objects, our approach first decomposes the scene into planar sub-scenes from stereo vision and creates a mosaic for each plane individually. The power of the presented mosaicing technique is evaluated in an office scenario, including the analysis of the parallax error

The dark matter problem in disc galaxies

In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centrally concentrated to host observed galactic discs. Moreover, discs are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic discs, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died dow

Time dependent dirac equation with relativistic mean field dynamics applied to heavy ion scattering

We treat the relativistic propagation of nucleons coupled to scalar- and vector-meson fields in a mean-field approximation. The time-dependent Dirac and mean-meson-field equations are solved numerically in three dimensions. Collisions of 16O(300, 600, and 1200 MeV/nucleon) + 16O are studied for various impact parameters. The results are compared to other recent theoretical approaches. The calculations predict spallation, large transverse-momentum transfer, and positive-angle sidewards flow, in qualitative agreement with the data in this energy regime

Cybersex Use and Problematic Cybersex Use Among Young Swiss Men: Associations with Sociodemographic, Sexual, and Psychological Factors

Background and aims: Cybersex use (CU) is highly prevalent in Switzerland’s population, particularly among young men. CU may have negative consequences if it gets out of control. This study estimated prevalence of CU, frequency of CU (FCU), and problematic CU (PCU) and their correlates. Methods: A non-selective sample of young Swiss men (N =5,332, mean age = 25.45) completed a questionnaire assessing FCU and PCU, sociodemographics (age, linguistic region, and education), sexuality (being in a relationship, number of sexual partners, and sexual orientation), dysfunctional coping (denial, self-distraction, behavioral disengagement, and self-blame), and personality traits (aggression/hostility, sociability, anxiety/neuroticism, and sensation seeking). Associations were tested using hurdle and negative binomial regression models. Results: At least monthly CU was reported by 78.6% of participants. CU was associated positively with post-secondary schooling (vs. primary schooling), German-speaking (vs. French-speaking), homosexuality, bisexuality (vs. heterosexuality), more than one sexual partner (vs. one), dysfunctional coping (except denial), and all personality traits except sociability, but negatively with being in a relationship (vs. not), age, and sociability. FCU was associated positively with homosexuality, bisexuality, no or more than one sexual partner, dysfunctional coping (except denial), and all personality traits except sociability, but negatively with age, being in a relationship, and sociability. PCU was associated positively with bisexuality, four or more sexual partners, dysfunctional coping, and all personality traits except sociability, but negatively with German-speaking and sociability. Discussion and conclusions: CU should be viewed in light of its associations with sociodemographic, sexual, and psychological factors. Healthcare professionals should consider these aspects to adapt their interventions to patients’ needs

Prevalence and factors associated with use of smokeless tobacco in young Swiss men

Background: Smokeless tobacco is of increasing interest to public health researchers and policy makers. This study aims to measure prevalence of smokeless tobacco use (nasal dry snuff, snus and chewing tobacco) among young Swiss men, and to describe its correlates. Methods: We invited 13 245 young men to participate in this survey on socio-economic and substance use data. Response rate was 45.2%. We included 5720 participants. Descriptive statistics and multivariable-adjusted logistic regression were performed. Results: Mean age of participants was 19.5 years. Self-reported use once a month or more often was 8% for nasal dry snuff, 3% for snus and negligible for chewing tobacco. In multivariable-adjusted logistic regression, the odds for nasal dry snuff use increased in non daily smokers [odds ratio (OR) 2.41, 95% confidence interval (CI) 1.90-3.05], compared with non smokers, participants reporting risky weekly drinking volume (OR 3.93, 95% CI 1.86-8.32), compared with abstinents, and binge drinking once a month or more often (OR 7.41, 95% CI 4.11-13.38), compared with never binge drinking. Nasal dry snuff use was positively associated with higher BMI, average or above family income and German language, compared with French, and negatively associated with academic higher education, compared with non higher education, and occasional cannabis use, compared with no cannabis use. Correlates of snus were similar to those of nasal dry snuff. Conclusion: One in 12 young Swiss men use nasal dry snuff and 3% use snus. Consumption of smokeless tobacco is associated with a cluster of other risky behaviours, especially binge drinkin