Genericity of Filling Elements

An element of a finitely generated non-Abelian free group F(X) is said to be filling if that element has positive translation length in every very small action of F(X) on an $\mathbb{R}$-tree. We give a proof that the set of filling elements of F(X) is exponentially F(X)-generic in the sense of Arzhantseva and Ol'shanskii. We also provide an algebraic sufficient condition for an element to be filling and show that there exists an exponentially F(X)-generic subset of filling elements whose membership problem is solvable in linear time.Comment: 9 page

Differences in Physical Activity and Built Environment Perceptions Between Younger and Older Adults Living in the Same Rural

This study examined associations between perceptions and physical activity of younger and older adults residing in the same rural/small town community. Thirty-two adults completed interviews about their community and their physical activity. Both groups perceived their neighborhood as pleasant and safe to be active. Fewer older adults perceived their neighborhood as having sidewalks. Younger adults reported being more vigorously active and they were more often utilizing private membership clubs for physical activity. Older adults utilized more public recreational facilities. These results suggest that higher perceived quality of the neighborhood built environment can enhance participation in moderate exercise in older adults

PERFORMANCE-BASED VOLUNTARY GROUP CONTRACTS FOR NONPOINT SOURCE POLLUTION

Pollution from nonpoint sources (NPS), and agriculture in particular, remains as one of the largest sources of water quality impairments in the United States. As is well known in the literature, there are many difficulties with designing regulations for reducing nonpoint source pollution (i.e., Tomasi, Segerson, and Braden, 1994). Uncertainty and asymmetric information are the key regulatory difficulties in the control of NPS. The main goal of this paper is to describe a potential incentive scheme that can be applied in limited information situations. The incentive scheme involves a contract written between a point source of pollution and a small group of other nonpoint polluters in the watershed to reduce a specific load of pollution. The contract allows the nonpoint sources to enter the contract voluntarily. To handle the incentive problems typical in many principal agent problems, it incorporates joint liability, and peer pressure/monitoring to induce the nonpoint sources of pollution to meet their contractual obligations. For this paper, we propose a group contract built upon the ideas of Stiglitz (1990) and Varian (1990), and originally applied to micro-lending arrangements in developing countries. As we hypothesize with nonpoint source pollution, joint liability contracts for micro-lending assume that individuals have more information about each other than the principal has about them, and they take advantage of joint liability and peer monitoring concepts to eliminate or reduce the moral hazard problem. Joint liability contracts have been shown to be successfully applied in practice in several situations (Ghatak and Guinnane, 1999; Van Tassel, 1999). The contract proposed for this paper assumes that a principal (a point source of pollution) offers a contract that specifies a price for each ton of pollution abated by individuals who participate. The contract is offered to individual farmers in a specified sub-watershed upstream from the discharge point. Farmers in the watershed decide whether or not to participate, and if they decide to participate, they bid into the contract the level of abatement services they will provide. The principal will form the group from these bidders, and will agree to pay the total amount if they meet the target. Given the fixed price for the contract, the bids determine the sharing rule for payments at the end of the season. With this basic idea, this paper explores the implications of moral hazard under several contractual arrangements. In particular, we are interested in the trade-offs between participation in the contract and shirking. At one extreme, existing voluntary incentive programs involve fixed payments with no requirements for performance. They will thus lead to high levels of participation, but likely to large levels of shirking as well. For example, in a typical year all of the money available for existing federal conservation programs is used by farmers, but there is little evidence available to prove that pollution declines as a result. At the other extreme, one could write a contract that specifies that payments will only be made if the target is met, following Holmstrom (1982) or Segerson (1988). Such an extreme, nonlinear contract such as this likely would eliminate most shirking, but it may lead to little participation among farmers. This paper thus explores the relationship between the participation constraint in principal agent problems and moral hazard with a simple theoretical model and numerical simulations. First, we propose a contract with a fixed initial payment and a bonus payment if the farmers meet the agreed upon target. The effects of the fixed payments upon the participation constraint and the consequent implications for shirking are shown theoretically and numerically. Fixed payments enhance the likelihood that individual farmers join the group, but fixed payments can also induce incentives to shirk. Second, we hypothesize that farmers can engage in other nonpecuniary penalties to punish farmers who shirk their responsibilities. For example, if someone in the group shirks, we assume that the group members can figure out whom, and take appropriate action to punish him or her. The numerical simulations show how the success of the proposed mechanism depends crucially on size of the fixed and bonus payments, group structure (i.e. ability and willingness to monitor each other) and group size. While not having any direct penalty in the contract, and having fixed payment plus bonus can increase the participation for the program, it can also increase free-riding problems in the group. Group size is also important, in that groups that are too large cannot take advantage of nonpecuniary penalties. Regions that have strong social ties are the most promising candidates for this kind of group contracts. They can be better able to apply social pressure on potential shirkers (Prescott, 1997). In addition to showing a theoretical model, and numerical simulations, we provide some results from recent focus groups providing data from actual farmers on their willingness to participate in the types of contracts proposed in this paper.Environmental Economics and Policy,

Single stage experimental evaluation of compressor blading with slots and vortex generators. Part 2 - Data and performance for stage 5 without slots or vortex generators

Comparison performance data of compressor blading without slots or vortex generator

Impact of stressing a pen mate on physiological responses of growing pigs

Crossbred barrows and gilts (n = 36), weighing 16.59 ± 2.1 kg, were used to test the effects of stressing a pen mate on the physiological responses of growing pigs. Pigs were randomly allotted to 6 groups after stratifying according to gender, litter origin, and body weight. Dominance order was determined within each group, and 1 to 3 d prior to the stress treatment the most- and leastdominant pigs within a group were fitted with indwelling catheters in their vena cavas. Over 3 d, groups were either: 1) isolated from audile and visual contact with stressed pigs in a separate room (non-stressed control); 2) separated by a curtain from visual contact with stressed pigs; or 3) allowed to maintain audile and visual contact with stressed pigs. Blood samples were collected 30, 15, and 0 min before exposure to the stressor (snout-snare) treatment and again at 1, 2, 3, 4, 5, 7.5, 10, 15, 20, 25, and 30 min after stressor application. Serum cortisol and plasma glucose, lactate, and nonesterified fatty acids (NEFA) concentrations were measured. There were no treatment × sampling-time interactions (P \u3e 0.17) for concentrations of cortisol, glucose, lactate or NEFA, nor were these metabolites affected by stressor treatment (P \u3e 0.42). Humoral measures of the stress response were not affected by visual and/or audile contact with pen mates undergoing a stressful event

Raman scattering in correlated thin films as a probe of chargeless surface states

Several powerful techniques exist to detect topologically protected surface states of weakly-interacting electronic systems. In contrast, surface modes of strongly interacting systems which do not carry electric charge are much harder to detect. We propose resonant light scattering as a means of probing the chargeless surface modes of interacting quantum spin systems, and illustrate its efficacy by a concrete calculation for the 3D hyperhoneycomb Kitaev quantum spin liquid phase. We show that resonant scattering is required to efficiently couple to this model's sublattice polarized surface modes, comprised of emergent Majorana fermions that result from spin fractionalization. We demonstrate that the low-energy response is dominated by the surface contribution for thin films, allowing identification and characterization of emergent topological band structures.Comment: 7 pages, 4 figures; added supplemental materia

The Cosmological Mean Density and its Local Variations Probed by Peculiar Velocities

Peculiar velocities thoughout the region of the local supercluster are reconstructed by two different orbit-retracing methods. The requirement of the optimal correlation between the radial components of reconstructed velocities and the observed peculiar velocities derived from our extensive new catalog of distances puts stringent constraints on the values of the cosmological parameters. Our constraints intersect those from studies of microwave background fluctuations and statistical properties of galaxy clustering: the ensemble of constraints are consistent with Omega_m=0.22\pm 0.02. While motions throughout the Local Supercluster provide a measure of the mean ratio of mass to light, there can be large local fluctuations. Our reconstruction of the infall velocities in the immediate vicinity of the Virgo Cluster shows that there is a mass-to-light anomaly of a factor of 3 to 6 between groups in the general field environment and the heavily populated Virgo Cluster.Comment: 4 pages, 2 figures, version to appear in Astrophysical Journal Letter

Theory of Raman response in three-dimensional Kitaev spin liquids: application to β−\beta- and γ−\gamma-Li2_2IrO3_3 compounds

We calculate the Raman response for the Kitaev spin model on the $\mathcal{H}$-$0$, $\mathcal{H}$-$1$, and $\mathcal{H}$-$\infty$ harmonic honeycomb lattices. We identify several quantitative features in the Raman spectrum that are characteristic of the spin liquid phase. Unlike the dynamical structure factor, which probes both the Majorana spinons and flux excitations that emerge from spin fractionalization, the Raman spectrum in the Kitaev models directly probes a density of states of pairs of fractional, dispersing Majorana spinons. As a consequence, the Raman spectrum in all these models is gapless for sufficiently isotropic couplings, with a low-energy power law that results from the Fermi lines (or points) of the dispersing Majorana spinons. We show that the polarization dependence of the Raman spectrum contains crucial information about the symmetry of the ground state. We also discuss to what extent the features of the Raman response that we find reflect generic properties of the spin liquid phase, and comment on their possible relevance to $\alpha-$, $\beta-$ and $\gamma-$Li$_2$IrO$_3$ compounds.Comment: 19 pages, 10 figures. VERSION 2: Corrected Figure 5 and fixed inconsistencies between A and B chain-labelings. Also- a few typos and two new ref