Consensus Acceleration in Multiagent Systems with the Chebyshev Semi-Iterative Method

We consider the fundamental problem of reaching consensus in multiagent systems; an operation required in many applications such as, among others, vehicle formation and coordination, shape formation in modular robotics, distributed target tracking, and environmental modeling. To date, the consensus problem (the problem where agents have to agree on their reported values) has been typically solved with iterative decentralized algorithms based on graph Laplacians. However, the convergence of these existing consensus algorithms is often too slow for many important multiagent applications, and thus they are increasingly being combined with acceleration methods. Unfortunately, state-of-the-art acceleration techniques require parameters that can be optimally selected only if complete information about the network topology is available, which is rarely the case in practice. We address this limitation by deriving two novel acceleration methods that can deliver good performance even if little information about the network is available. The first proposed algorithm is based on the Chebyshev semi-iterative method and is optimal in a well defined sense; it maximizes the worst-case convergence speed (in the mean sense) given that only rough bounds on the extremal eigenvalues of the network matrix are available. It can be applied to systems where agents use unreliable communication links, and its computational complexity is similar to those of simple Laplacian-based methods. This algorithm requires synchronization among agents, so we also propose an asynchronous version that approximates the output of the synchronous algorithm. Mathematical analysis and numerical simulations show that the convergence speed of the proposed acceleration methods decrease gracefully in scenarios where the sole use of Laplacian-based methods is known to be impractical

Entangled granular media

We study the geometrically induced cohesion of ensembles of granular "u-particles" which mechanically entangle through particle interpenetration. We vary the length-to-width ratio $l/w$ of the u-particles and form them into free-standing vertical columns. In laboratory experiment we monitor the response of the columns to sinusoidal vibration (frequency $f$, peak acceleration $\Gamma$). Column collapse occurs in a characteristic time, $\tau$, which follows the relation $\tau = f^{-1} \exp(\Delta / \Gamma)$. $\Delta$ resembles an activation energy and is maximal at intermediate $l/w$. Simulation reveals that optimal strength results from competition between packing and entanglement.Comment: 4 pages, 5 figure

From ‘As Good as Gold’ to ‘Gold Diggers’: Farming Women and the Survival of British Family Farming

The survival of family farming in British agriculture has long been a topic of interest for rural researchers and is undergoing something of a current renewal of interest. However, insights from feminist approaches remain underutilised despite the crucial role farming women continue to play in family farming. This article addresses the unity of farm, family and business by interpreting it as a patriarchal way of life. An ethnographically informed repeated life history methodology is employed to study in detail the family members of seven farms in rural mid-Wales. Findings show that the recent survival of the family farms investigated has been heavily dependent upon compliance with a patriarchal ideology that demands that women be ‘as good as gold’. However, it is discovered that a new view of women is emerging in the world of British family farming, that of ‘gold digger’. Women entering relationships with farming men are increasingly being considered a threat to farm survival by virtue of their entitlements if the relationship breaks down. The necessity to study the intricacies of personal relationships in family farming has important implications for most future research into this form of agricultural business arrangement

Controlling broad-leaved dock (Rumex obtusifolius) in grass clover mixtures

This article describes three experiments on the control of broad-leaved dock. Experiment 1: Dock seeds were ensiled in grass silages of different dry matter percentages; 23, 34 and 60% respectively. All silages showed a decline of seed vitality in time. Grass clover with dock seeds should be ensiled at a low dry matter percentage or remain in the silage bin for a longer period than 8 weeks. Experiment 2: In a potassium fertilisation trial on grass clover the development of dock was followed. After two years of potassium fertilisation, the number of dock and the root mass was not significant different between the fertilised and the unfertilised plots. It is concluded that potassium fertilisation at a low potassium status does not positively influence the dock development. Experiment 3: In a resown grass clover, dock seedlings were cut at three frequencies; 2, 4 and 6 weeks. After 3 months the number of seedlings had decreased the same in all treatments. However the root biomass of the seedlings was significantly affected. It is concluded that frequent cutting has a negative effect on root biomass but should be practised for a longer period than 12 weeks to have an effect on seedling numbers

Higher Education Collaboratives for Community Engagement and Improvement

Our society is in a period of dramatic change with the transition from an industrial-based to a knowledge-based economy, as well as technological advances, fiscal challenges of higher education, and cultural shifts in society as a whole. Increasing collaborations between communities and universities in order to influence the public good becomes paramount during this time of dramatic change. As frustratingly slow as the movement to strengthen the relationship between higher education and society sometimes seems to be, few social institutions are better situated than colleges and universities to stimulate significant community improvement. Individually and collectively, institutions of higher education possess considerable resources—human, fiscal, organizational, and intellectual— which are critical to addressing significant social issues. Additionally, these institutions are physically rooted in their communities. Therefore, investing in the betterment of their immediate environments is good for both the community and the institution. However, it is recognized that higher education institutions often fall short of making a real impact in their home communities. Therefore, a conference was convened to examine the current and evolving role of higher education institutions, particularly those operating within the context of coalitions, consortia and state systems, to catalyze change on issues affecting communities and society as a whole. Specifically, the focus of the conference was to develop and strengthen an understanding of how higher education might work more effectively with communities and we believe that consortia or collaboratives of higher education institutions, along with community partners, can learn from one another. Cooperation between efforts is important. The papers offered in this monograph are the result of the Wingspread Conference on Higher Education Collaboratives for Community Engagement and Improvement, conducted on October 27-29, 2004, in Racine, Wisconsin. The conference, the second of a three-part series, was sponsored by the National Forum on Higher Education for the Public Good at the University of Michigan School of Education, the Johnson Foundation, Atlantic Philanthropies, and the W.K. Kellogg Foundation. The first conference in the series held in October 2003 and titled Public Understanding, Public Support and Public Policy focused on higher education’s role in society and the concluding conference will be held in the fall of 2005