Real-Time Planning with Primitives for Dynamic Walking over Uneven Terrain

We present an algorithm for receding-horizon motion planning using a finite family of motion primitives for underactuated dynamic walking over uneven terrain. The motion primitives are defined as virtual holonomic constraints, and the special structure of underactuated mechanical systems operating subject to virtual constraints is used to construct closed-form solutions and a special binary search tree that dramatically speed up motion planning. We propose a greedy depth-first search and discuss improvement using energy-based heuristics. The resulting algorithm can plan several footsteps ahead in a fraction of a second for both the compass-gait walker and a planar 7-Degree-of-freedom/five-link walker.Comment: Conference submissio

Liquidity, Volatility, and Equity Trading Costs Across Countries and Over Time

Actual investment performance reflects the underlying strategy of the portfolio manager and the execution costs incurred in realizing those objectives. Execution costs, especially in illiquid markets, can dramatically reduce the notional return to an investment strategy. This paper examines the interactions between cost, liquidity, and volatility, and analyzes their determinants using panel-data for 42 countries from September 1996 to December 1998. We document wide variation in trading costs across countries; emerging markets in particular have significantly higher trading costs even after correcting for factors affecting costs such as market capitalization and volatility. We analyze the inter-relationships between turnover, equity trading costs, and volatility, and investigate the impact of these variables on equity returns. In particular, we show that increased volatility, acting through costs, reduces a portfolio's expected return. However, higher volatility reduces turnover also, mitigating the actual impact of higher costs on returns. Further, turnover is inversely related to trading costs, providing a possible explanation for the increase in turnover in recent years. The results demonstrate that the composition of global efficient portfolios can change dramatically when cost and turnover are taken into account.privatization, government priorities, auctions, revenue maximization, probit analysis, selection bias.

Liquidity, Volatility, and Equity Trading Costs Across Countries and Over Time

Actual investment performance reflects the underlying strategy of the portfolio manager and the execution costs incurred in realizing those objectives. Execution costs, especially in illiquid markets, can dramatically reduce the notional return to an investment strategy. This paper examines the interactions between cost, liquidity, and volatility, and analyzes their determinants using panel-data for 42 countries from September 1996 to December 1998. We document wide variation in trading costs across countries; emerging markets in particular have significantly higher trading costs even after correcting for factors affecting costs such as market capitalization and volatility. We analyze the inter-relationships between turnover, equity trading costs, and volatility, and investigate the impact of these variables on equity returns. In particular, we show that increased volatility, acting through costs, reduces a portfolio's expected return. However, higher volatility reduces turnover also, mitigating the actual impact of higher costs on returns. Further, turnover is inversely related to trading costs, providing a possible explanation for the increase in turnover in recent years. The results demonstrate that the composition of global efficient portfolios can change dramatically when cost and turnover are taken into account.http://deepblue.lib.umich.edu/bitstream/2027.42/39706/3/wp322.pd

Dynamical phase transitions in one-dimensional hard-particle systems

We analyse a one-dimensional model of hard particles, within ensembles of trajectories that are conditioned (or biased) to atypical values of the time-averaged dynamical activity. We analyse two phenomena that are associated with these large deviations of the activity: phase separation (at low activity) and the formation of hyperuniform states (at high activity). We consider a version of the model which operates at constant volume, and a version at constant pressure. In these non-equilibrium systems, differences arise between the two ensembles, because of the extra freedom available to the constant-pressure system, which can change its total density. We discuss the relationships between different ensembles, mechanical equilibrium, and the probability cost of rare density fluctuations.Comment: 11 pages, 11 figure

In Memoriam Maureen Byrne

A tribute to Maureen Byrne

Why Take Both Boxes?

The crucial premise of the standard argument for two‐boxing in Newcomb's problem, a causal dominance principle, is false. We present some counterexamples. We then offer a metaethical explanation for why the counterexamples arise. Our explanation reveals a new and superior argument for two‐boxing, one that eschews the causal dominance principle in favor of a principle linking rational choice to guidance and actual value maximization

Day-length is central to maintaining consistent seasonal diversity in marine bacterioplankton

Marine bacterial diversity is vast, but seasonal variation in diversity is poorly understood. Here we present the longest bacterial diversity time series consisting of monthly (72) samples from the western English Channel over a 6 year period (2003-2008) using 747,494 16SrDNA-V6 amplicon-pyrosequences. Although there were characteristic cycles for each phylum, the overall community cycle was remarkably stable year after year. The majority of taxa were not abundant, although on occasion these rare bacteria could dominate the assemblage. Bacterial diversity peaked at the winter solstice and showed remarkable synchronicity with day-length, which had the best explanatory power compared to a combination of other variables (including temperature and nutrient concentrations). Day-length has not previously been recognised as a major force in structuring microbial communities

Bias in culture-independent assessments of microbial biodiversity in the global ocean

On the basis of 16S rRNA gene sequencing, the SAR11 clade of marine bacteria has almost universal distribution, being detected as abundant sequences in all marine provinces. Yet SAR11 sequences are rarely detected in fosmid libraries, suggesting that the widespread abundance may be an artefact of PCR cloning and that SAR 11 has a relatively low abundance. Here the relative abundance of SAR11 is explored in both a fosmid library and a metagenomic sequence data set from the same biological community taken from fjord surface water from Bergen, Norway. Pyrosequenced data and 16S clone data confirmed an 11-15% relative abundance of SAR11 within the community. In contrast not a single SAR11 fosmid was identified in a pooled shotgun sequenced data set of 100 fosmid clones. This under-representation was evidenced by comparative abundances of SAR11 sequences assessed by taxonomic annotation; functional metabolic profiling and fragment recruitment. Analysis revealed a similar under-representation of low-GC Flavobacteriaceae. We speculate that the fosmid bias may be due to DNA fragmentation during preparation due to the low GC content of SAR11 sequences and other underrepresented taxa. This study suggests that while fosmid libraries can be extremely useful, caution must be used when directly inferring community composition from metagenomic fosmid libraries