RECREATIONAL ACTIVITIES AND NONMARKET VALUATION: THE CONCEPTUALIZATION ISSUE

Resource /Energy Economics and Policy,

Explicit isogeny descent on elliptic curves

In this note, we consider an l-isogeny descent on a pair of elliptic curves over Q. We assume that l > 3 is a prime. The main result expresses the relevant Selmer groups as kernels of simple explicit maps between finite- dimensional F_l-vector spaces defined in terms of the splitting fields of the kernels of the two isogenies. We give examples of proving the l-part of the Birch and Swinnerton-Dyer conjectural formula for certain curves of small conductor.Comment: 17 pages, accepted for publication in Mathematics of Computatio

Vertical shear instability in accretion disc models with radiation transport

The origin of turbulence in accretion discs is still not fully understood. While the magneto-rotational instability is considered to operate in sufficiently ionized discs, its role in the poorly ionized protoplanetary disc is questionable. Recently, the vertical shear instability (VSI) has been suggested as a possible alternative. Our goal is to study the characteristics of this instability and the efficiency of angular momentum transport, in extended discs, under the influence of radiative transport and irradiation from the central star. We use multi-dimensional hydrodynamic simulations to model a larger section of an accretion disc. First we study inviscid and weakly viscous discs using a fixed radial temperature profile in two and three spatial dimensions. The simulations are then extended to include radiative transport and irradiation from the central star. In agreement with previous studies we find for the isothermal disc a sustained unstable state with a weak positive angular momentum transport of the order of $\alpha \approx 10^{-4}$. Under the inclusion of radiative transport the disc cools off and the turbulence terminates. For discs irradiated from the central star we find again a persistent instability with a similar $\alpha$ value as for the isothermal case. We find that the VSI can indeed generate sustained turbulence in discs albeit at a relatively low level with $\alpha$ about few times $10^{-4}$Comment: 12 pages, 24 figures, accepted for publication in Astronomy & Astrophysic

On elliptic curves with an isogeny of degree 7

We show that if $E$ is an elliptic curve over $\mathbf{Q}$ with a $\mathbf{Q}$-rational isogeny of degree 7, then the image of the 7-adic Galois representation attached to $E$ is as large as allowed by the isogeny, except for the curves with complex multiplication by $\mathbf{Q}(\sqrt{-7})$. The analogous result with 7 replaced by a prime $p > 7$ was proved by the first author in [7]. The present case $p = 7$ has additional interesting complications. We show that any exceptions correspond to the rational points on a certain curve of genus 12. We then use the method of Chabauty to show that the exceptions are exactly the curves with complex multiplication. As a by-product of one of the key steps in our proof, we determine exactly when there exist elliptic curves over an arbitrary field $k$ of characteristic not 7 with a $k$-rational isogeny of degree 7 and a specified Galois action on the kernel of the isogeny, and we give a parametric description of such curves.Comment: The revision gives a complete answer to the question considered in Version 1. Version 3 will appear in the American Journal of Mathematic

A TEST OF CONTINGENT MARKET BID ELICITATION PROCEDURES FOR PIECEWISE VALUATION

Economists are frequently faced with the task of valuing commodity package components. The valuation of specific impacts of public policies is a case in point. Two contingent market bid elicitation procedures were tested for valuing changes in single components of multicomponent government program. Results of the test suggested that respondents provided more accurate component to piecewise valuation when a two-step bidding approach, rather than a one-step approach was used. Thus, there is evidence that a two-step approach which helps respondents to isolate valuations for package components is perhaps a preferable bid elicitation procedure for piecewise valuation.Research Methods/ Statistical Methods,

A mesocosm experiment investigating the effects of substratum quality and wave exposure on the survival of fish eggs

In a mesocosm experiment, the attachment of bream (Abramis brama) eggs to spawning substrata with and without periphytic biofilm coverage and their subsequent survival with and without low-intensity wave exposure were investigated. Egg attachment was reduced by 73% on spawning substrata with a natural periphytic biofilm, compared to clean substrata. Overall, this initial difference in egg numbers persisted until hatching. The difference in egg numbers was even increased in the wave treatment, while it was reduced in the no-wave control treatment. Exposure to a low-intensity wave regime affected egg development between the two biofilm treatments differently. Waves enhanced egg survival on substrata without a biofilm but reduced the survival of eggs on substrata with biofilm coverage. In the treatment combining biofilm-covered substrata and waves, no attached eggs survived until hatching. In all treatments, more than 75% of the eggs became detached from the spawning substrata during the egg incubation period, an

Optical waveguiding in proton-implanted GaAs

We have produced optical waveguides in n-type GaAs by implantation with 300-keV protons. The guiding is shown to be due to the elimination of charge carriers from the implanted region. Annealing of the waveguide leads to very large reductions in the 1.15-µ guided-wave absorption

The Induction of Self-cure and Protection with Special Reference to Experimental Vaccination Against Haemonchus

Talk given by Norman R. Stoll from the Rockefeller Institute for Medical Researc