Composition Operators on the Dirichlet Space and Related Problems

In this paper we investigate the following problem: when a bounded analytic function $\phi$ on the unit disk $\mathbb{D}$, fixing 0, is such that $\{\phi^n : n = 0, 1, 2, . . . \}$ is orthogonal in $\mathbb{D}$?, and consider the problem of characterizing the univalent, full self-maps of $\mathbb{D}$ in terms of the norm of the composition operator induced. The first problem is analogous to a celebrated question asked by W. Rudin on the Hardy space setting that was answered recently ([3] and [15]). The second problem is analogous to a problem investigated by J. Shapiro in [14] about characterization of inner functions in the setting of $H^2$.Comment: 8 pages, 1 figure. See also http://webdelprofesor.ula.ve/nucleotachira/gchacon or http://webdelprofesor.ula.ve/humanidades/grchaco

When is Democracy an Equilibrium?: Theory and Evidence from Colombia's "La Violencia"

The conventional wisdom in political science is that for a democracy to be consolidated, all groups must have a chance to attain power. If they do not then they will subvert democracy and choose to fight for power. In this paper we show that this wisdom is seriously incomplete because it considers absolute, not relative payoffs. Although the probability of winning an election increases with the size of a group, so does the probability of winning a fight. Thus in a situation where all groups have a high chance of winning an election, they may also have a high chance of winning a fight. Indeed, in a natural model, we show that democracy may never be consolidated in such a situation. Rather, democracy may only be stable when one group is dominant. We provide a test of a key aspect of our model using data from "La Violencia", a political conflict in Colombia during the years 1946-1950 between the Liberal and Conservative parties. Consistent with our results, and contrary to the conventional wisdom, we show that fighting between the parties was more intense in municipalities where the support of the parties was more evenly balanced.

Some remarks on particle size effects on the abrasion of a range of Fe based alloys

The low-stress three body abrasion behaviour of a range of steels was investigated. The tests were carried out in a rubber wheel tester (according to ASTM G65-94, reapproved in 2000) at room temperature. The abrasive particles used were angular alumina particles of four different sizes. The results showed that, in general, the smaller particles (50 8m and 125 8m average size) caused more damage. With these particles, observations of surface morphology indicarted a more intense cutting and ploughing action, leading to more damage, whereas bigger particles i.e. larger 250 8m and 560 8m particles produced less damage, and their action involved more plastic deformation type wear. The 304 SS had a lower abrasion resistance than the 310 SS. For the austentic and ferritic steels the subsurface deformation was larger for impact with the coarser particles. Variations in substrate hardness had no effect on the abrasive behaviour observed. On the whole, the hardest steel (mild steel in martensitic condition) showed the higher extent of damage, irrespective of particle size

Mapping deuterated methanol toward L1544: I. Deuterium fraction and comparison with modeling

The study of deuteration in pre-stellar cores is important to understand the physical and chemical initial conditions in the process of star formation. In particular, observations toward pre-stellar cores of methanol and deuterated methanol, solely formed on the surface of dust grains, may provide useful insights on surface processes at low temperatures. Here we analyze maps of CO, methanol, formaldehyde and their deuterated isotopologues toward a well-known pre-stellar core. This study allows us to test current gas-dust chemical models. Single-dish observations of CH$_3$OH, CH$_2$DOH, H$_2$CO, H$_2\,^{13}$CO, HDCO, D$_2$CO and C$^{17}$O toward the prototypical pre-stellar core L1544 were performed at the IRAM 30 m telescope. We analyze their column densities, distributions, and compare these observations with gas-grain chemical models. The maximum deuterium fraction derived for methanol is [CH$_2$DOH]/[CH$_3$OH] $\sim$ 0.08$\pm$0.02, while the measured deuterium fractions of formaldehyde at the dust peak are [HDCO]/[H$_2$CO] $\sim$ 0.03$\pm$0.02, [D$_2$CO]/[H$_2$CO] $\sim$ 0.04$\pm$0.03 and [D$_2$CO]/[HDCO] $\sim$ 1.2$\pm$0.3. Observations differ significantly from the predictions of models, finding discrepancies between a factor of 10 and a factor of 100 in most cases. It is clear though that to efficiently produce methanol on the surface of dust grains, quantum tunneling diffusion of H atoms must be switched on. It also appears that the currently adopted reactive desorption efficiency of methanol is overestimated and/or that abstraction reactions play an important role. More laboratory work is needed to shed light on the chemistry of methanol, an important precursor of complex organic molecules in space.Comment: Accepted for publication in A&

Contribution of sustainable agricultural management practices to reduce the impacts of extreme weather events in Tropical America

One of the most accessible adaptation strategies for climate change and extreme weather events for smallholder farmers consists in the implementation of sustainable management practices. Drawing from a systematic review of 326 documents from peer-reviewed and grey literature our study analyses the potential contribution of 39 management practices to Ecosystem-based Adaptation (EBA) for two important agroecosystems (coffee and basic grains) for smallholders in Central America. Specifically, we examined which agricultural management practices confer adaptive benefits, what the specific mechanism were by which these practices conferred adaptation benefits, which of these practices can be considered to be 'EbA' and what key knowledge gaps remain. Our results suggest that many existing agricultural practices can reduce the negative impacts of extreme weather events (e.g., hurricanes and droughts) on smallholder coffee, maize, and bean agroecosystems. They do so by buffering temperature extremes, reducing wind speed within the agroecosystem, increasing the genetic tolerance to climatic stresses, improving soil biophysical and biological conditions, and regulating pest and disease cycles. Many practices (e.g., tree shade in coffee, or use of cover crops and conservation tillage for basic grains) not only help farmers adapt to climate change but also provide additional livelihood benefits (such as improved income or food security), however, some trade-offs still exists. For example, shaded coffee was the most promising and documented practice for adaptation in coffee agroecosystems because the use of trees increases the resilience of the coffee plantation to extreme weather events, diversifies farm production, and increases agroecosystem biomass. However, under certain conditions shading coffee can reduce short-term coffee yield, increase the severity of some pests and diseases, and even reduce soil water content. It is therefore important that policy makers designing adaptation strategies for agricultural landscapes have a clear understanding of the benefits and drawbacks of different adaptation measures in different landscape contexts and tailor their recommendations accordingly. (Texte intégral