503 research outputs found
On the Computability of Solomonoff Induction and Knowledge-Seeking
Solomonoff induction is held as a gold standard for learning, but it is known
to be incomputable. We quantify its incomputability by placing various flavors
of Solomonoff's prior M in the arithmetical hierarchy. We also derive
computability bounds for knowledge-seeking agents, and give a limit-computable
weakly asymptotically optimal reinforcement learning agent.Comment: ALT 201
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Changing Patterns of Sexual Risk Behavior Among London Gay Men: 1998-2008
Objectives: To examine changes in the sexual behavior of London gay men between 1998 and 2008.
Methods: Gay men using London gyms were surveyed annually between 1998 and 2005, and again in 2008 (n = 6064; range, 482â834 per year). Information was collected on human immunodeficiency virus (HIV) status of the respondent, unprotected anal intercourse (UAI) in the previous 3 months, type (main or casual) and HIV status of partner for UAI. Nonconcordant UAI (ncUAI) was defined as UAI with a partner of unknown or discordant HIV status. Concordant UAI (cUAI) was defined as UAI with a partner of the same HIV status (âserosortingâ).
Results: Between 1998 and 2008, the percentage of men reporting UAI increased from 24.3% to 36.6% (P = 0.07). This overall increase concealed important differences between nonconcordant and concordant UAI. While the percentage of men engaging in cUAI increased steadily between 1998 and 2008 (9.8%, 20.8%; P = 0.01), the percentage reporting ncUAI increased between 1998 and 2001 (14.5%, 23.7%; P < 0.001), decreased between 2001 and 2005 (23.7%, 15.6%; P < 0.001), and then leveled off between 2005 and 2008 (15.6%, 15.7%; P = 0.2). However, the percentage of men reporting ncUAI with a main partner increased between 2005 and 2008 for HIV-positive men (2.5%, 8.1%; P < 0.05) and HIV negative men (2.1%, 5.5%; P = 0.06). While the percentage of HIV negative men who reported cUAI with a main partner (i.e., serosorting) increased between 1998 and 2008 (12.4%, 21.1%; P < 0.05), less than half established seroconcordance by testing together.
Conclusions: The patterns of sexual behavior among London's gay men between 1998 and 2008 were dynamic and complex. Our data suggest that HIV risk with a main partner and HIV testing among couples should be given greater priority by health promotion programmes
Universal knowledge-seeking agents for stochastic environments
We define an optimal Bayesian knowledge-seeking agent, KL-KSA, designed for countable hypothesis classes of stochastic environments and whose goal is to gather as much information about the unknown world as possible. Although this agent works for arbitrary countable classes and priors, we focus on the especially interesting case where all stochastic computable environments are considered and the prior is based on Solomonoffâs universal prior. Among other properties, we show that KL-KSA learns the true environment in the sense that it learns to predict the consequences of actions it does not take. We show that it does not consider noise to be information and avoids taking actions leading to inescapable traps. We also present a variety of toy experiments demonstrating that KL-KSA behaves according to expectation
Treatment of obstructive sleep apnoea leads to improved microvascular endothelial function in the systemic circulation
Background: Obstructive sleep apnoea (OSA) is a common and potentially reversible cause of systemic hypertension. The mechanisms whereby OSA leads to hypertension and the effects of treatment on arterial function, however, are not well established. Microvascular arterial endothelial and smooth muscle function was assessed in subjects with OSA before and after treatment with continuous positive airways pressure (CPAP). Methods: Ten subjects of mean (SE) age 49 (8) years with at least moderately severe OSA had detailed forearm vascular reactivity studies before and after 3 months of CPAP treatment. The systemic circulation was assessed by measuring brachial artery pressure, flow and resistance responses to intra-arterial infusions of acetylcholine (ACh; an endothelium dependent vasodilator), sodium nitroprusside (SNP; an endothelium independent vasodilator), L-NMMA (a nitric oxide (NO) antagonist), and L-arginine (the substrate for NO). Results: Before CPAP, ACh and SNP infusions increased forearm blood flow in a dose dependent manner (p,0.01). After CPAP, endothelium dependent dilation to ACh was significantly increased (434 (23)% of baseline after CPAP v 278 (20)% before CPAP, p,0.001), whereas SNP induced dilation was unchanged. Resting NO production was higher after CPAP, evidenced by a significantly greater reduction in basal flow by L-NMMA (p = 0.05). L-Arginine reversed the effect of L-NMMA in all cases. Conclusion: In patients with OSA, treatment with CPAP improves baseline endothelial NO release and stimulates endothelium dependent vasorelaxation in the systemic circulation. This is a potential mechanism for improving systemic and vascular function in patients with OSA treated with CPAP
Iterative Budgeted Exponential Search
We tackle two long-standing problems related to re-expansions in heuristic search algorithms. For graph search, A* can require Ω(2^n) expansions, where n is the number of states within the final f bound. Existing algorithms that address this problem like B and B' improve this bound to Ω(n^2). For tree search, IDA* can also require Ω(n^2) expansions. We describe a new algorithmic framework that iteratively controls an expansion budget and solution cost limit, giving rise to new graph and tree search algorithms for which the number of expansions is O(n log C*), where C* is the optimal solution cost. Our experiments show that the new algorithms are robust in scenarios where existing algorithms fail. In the case of tree search, our new algorithms have no overhead over IDA* in scenarios to which IDA* is well suited and can therefore be recommended as a general replacement for IDA*
Bayesian reinforcement learning with exploration
We consider a general reinforcement learning problem and
show that carefully combining the Bayesian optimal policy and an exploring
policy leads to minimax sample-complexity bounds in a very general
class of (history-based) environments. We also prove lower bounds
and show that the new algorithm displays adaptive behaviour when the
environment is easier than worst-case
Setting up an enhanced surveillance of newly acquired hepatitis C infection in men who have sex with men: a pilot in London and South East region of England.
Preliminary findings suggest ongoing HCV transmission among MSM infected with human immunodeficiency virus (HIV) and that enhanced surveillance for newly acquired HCV in MSM is feasible
The diverse nature of island isolation and its effect on land bridge insular faunas
Aim:
Isolation is a key factor in island biology. It is usually defined as the distance to the geographically nearest mainland, but many other definitions exist. We explored how testing different isolation indices affects the inference of impacts of isolation on faunal characteristics. We focused on land bridge islands and compared the relationships of many spatial and temporal (i.e., through time) isolation indices with communityâ, populationâ and individualâlevel characteristics (species richness, population density and body size, respectively).
Location:
Aegean Sea islands, Greece.
Time period:
Current.
Taxon:
Many animal taxa.
Methods:
We estimated 21 isolation indices for 205 islands and recorded species richness data for 15 taxa (invertebrates and vertebrates). We obtained body size data for seven lizard species and population density data for three. We explored how well indices predict each characteristic, in each taxon, by conducting a series of ordinary least squares regressions (controlling for island area when needed) and a metaâanalysis.
Results:
Isolation was significantly (and negatively) associated with species richness in 10 of 15 taxa. It was significantly (and positively) associated with body size in only one of seven species and was not associated with population density. The effect of isolation on species richness was much weaker than that of island area, regardless of the index tested. Spatial indices generally outâperformed temporal indices, and indices directly related to the mainland outâperformed those related mainly to neighbouring islands. No index was universally superior to others, including the distance to the geographically nearest mainland.
Main conclusions:
The choice of index can alter our perception of the impacts of isolation on biological patterns. The nearly automatic, ubiquitous use of distance to the geographically nearest mainland misrepresents the complexity of the effects of isolation. We recommend the simultaneous testing of several indices that represent different aspects of isolation, in order to produce more constructive and thorough investigations and avoid imprecise inference
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