Fixed Price Approximability of the Optimal Gain From Trade

Bilateral trade is a fundamental economic scenario comprising a strategically acting buyer and seller, each holding valuations for the item, drawn from publicly known distributions. A mechanism is supposed to facilitate trade between these agents, if such trade is beneficial. It was recently shown that the only mechanisms that are simultaneously DSIC, SBB, and ex-post IR, are fixed price mechanisms, i.e., mechanisms that are parametrised by a price p, and trade occurs if and only if the valuation of the buyer is at least p and the valuation of the seller is at most p. The gain from trade is the increase in welfare that results from applying a mechanism; here we study the gain from trade achievable by fixed price mechanisms. We explore this question for both the bilateral trade setting, and a double auction setting where there are multiple buyers and sellers. We first identify a fixed price mechanism that achieves a gain from trade of at least 2/r times the optimum, where r is the probability that the seller's valuation does not exceed the buyer's valuation. This extends a previous result by McAfee. Subsequently, we improve this approximation factor in an asymptotic sense, by showing that a more sophisticated rule for setting the fixed price results in an expected gain from trade within a factor O(log(1/r)) of the optimal gain from trade. This is asymptotically the best approximation factor possible. Lastly, we extend our study of fixed price mechanisms to the double auction setting defined by a set of multiple i.i.d. unit demand buyers, and i.i.d. unit supply sellers. We present a fixed price mechanism that achieves a gain from trade that achieves for all epsilon > 0 a gain from trade of at least (1-epsilon) times the expected optimal gain from trade with probability 1 - 2/e^{#T epsilon^2 /2}, where #T is the expected number of trades resulting from the double auction

Strong and weak coupling limits in optics of quantum well excitons

A transition between the strong (coherent) and weak (incoherent) coupling limits of resonant interaction between quantum well (QW) excitons and bulk photons is analyzed and quantified as a function of the incoherent damping rate caused by exciton-phonon and exciton-exciton scattering. For confined QW polaritons, a second, anomalous, damping-induced dispersion branch arises and develops with increasing damping. In this case, the strong-weak coupling transition is attributed to a critical damping rate, when the intersection of the normal and damping-induced dispersion branches occurs. For the radiative states of QW excitons, i.e., for radiative QW polaritons, the transition is described as a qualitative change of the photoluminescence spectrum at grazing angles along the QW structure. Furthermore, we show that the radiative corrections to the QW exciton states with in-plane wavevector approaching the photon cone are universally scaled by an energy parameter rather than diverge. The strong-weak coupling transition rates are also proportional to the same energy parameter. The numerical evaluations are given for a GaAs single quantum well with realistic parameters.Comment: Published in Physical Review B. 29 pages, 12 figure

Public welfare or sectarianism: A new challenge for planning

Introduction Theoretical background Methodology The Haredi in Jerusalem Kiryat-Ha'Yovel “Terrain of interests” between Haredi and secular in Kiryat-Ha'Yovel Organisation in housing Potential for influencing planning decisions Conclusions: failure of liberalism or all-you-can-grab planning? Full Article Figures & data References Citations Metrics Reprints & Permissions PDF Abstract This article examines the weaknesses of liberal planning institutions when dealing with organised group action. The case under review, the Kiryat-Ha'Yovel neighbourhood in Jerusalem, was considered as secular for many years. In 2000 the neighbourhood became attractive to the nearby Haredi (ultra-orthodox Jews) group of the “Kol-Torah’ community. Differences in lifestyle led to a collision between the group of “Kol-Torah”, who began “Haredification” processes to change the character of the area to be suitable to Haredim, and the veteran population, who tried to prevent it. Identifying the main engines of organised neighbourhood change and evaluating the difficulties of liberalism dealing with non-autonomous individuals in the housing market sheds light on similar processes occurring in other city centres with diverse population groups

A Discrete and Bounded Envy-free Cake Cutting Protocol for Four Agents

We consider the well-studied cake cutting problem in which the goal is to identify a fair allocation based on a minimal number of queries from the agents. The problem has attracted considerable attention within various branches of computer science, mathematics, and economics. Although, the elegant Selfridge-Conway envy-free protocol for three agents has been known since 1960, it has been a major open problem for the last fifty years to obtain a bounded envy-free protocol for more than three agents. We propose a discrete and bounded envy-free protocol for four agents

Correction to the Casimir force due to the anomalous skin effect

The surface impedance approach is discussed in connection with the precise calculation of the Casimir force between metallic plates. It allows to take into account the nonlocal connection between the current density and electric field inside of metals. In general, a material has to be described by two impedances $Z_{s}(\omega,q)$ and $Z_{p}(\omega,q)$ corresponding to two different polarization states. In contrast with the approximate Leontovich impedance they depend not only on frequency $\omega$ but also on the wave vector along the plate $q$. In this paper only the nonlocal effects happening at frequencies $\omega<\omega_{p}$ (plasma frequency) are analyzed. We refer to all of them as the anomalous skin effect. The impedances are calculated for the propagating and evanescent fields in the Boltzmann approximation. It is found that $Z_p$ significantly deviates from the local impedance as a result of the Thomas-Fermi screening. The nonlocal correction to the Casimir force is calculated at zero temperature. This correction is small but observable at small separations between bodies. The same theory can be used to find more significant nonlocal contribution at $\omega\sim\omega_p$ due to the plasmon excitation.Comment: 29 pages. To appear in Phys. Rev.

2 Contribution Number 1328-E from the Agricultural Research Organization, The Volcani Center

ABSTRACT We studied the balance of Na + , K + , Cl -, and water in six high yielding (&gt;39 kg/d of milk) cows between wk 2 to 1 prepartum and at 2 and 7 wk postpartum during winter in Israel. Cows were fed complete diets; Na + and Cl -contents exceeded dietary recommendations, and K + content was equal to dietary recommendations. Milk yield was related positively and significantly to retention of Cl -and K + , indicating that ions that are the main constituents of sweat can limit the ability of cows to express full genetic potential. The highest ion retention was recorded for cows that had the highest dry matter intake and, hence, the highest ion intake. Retention of Cl -was highest for cows that were most efficient in retaining Cl -in the kidney. In hot climates, increasing the concentrations of ions in the diet of early lactation cows according to the actual dry matter intake could prevent or reduce the severity of ion deficiencies. Water turnover rate of the cows was dependent on dry matter intake, milk yield, and respiratory-cutaneous water loss. The milk-free water balance (water turnover rate minus water secreted in milk) could be very efficiently predicted for lactating and nonlactating cows by the following equation: milk-free water balance (kilograms per day) = digestible energy intake (megacalories per day) × 0.58 + respiratory-cutaneous loss (kilograms per day) × 0.97 ( n = 18; R 2 = 0.97). This formula provides a tool to assess the evaporative-cutaneous water loss from feed and water intake measurements to evaluate the severity of heat stress. ( Key words: water, ions, metabolism, lactation) Abbreviation key: FWI = free water intake, MFWB = milk-free water balance, RCW = respiratorycutaneous water, WTO = water turnover

The Hash Function Fugue

We describe Fugue, a hash function supporting inputs of length upto 2^{64}-1 bits and hash outputs of length upto 512 bits. Notably, Fugue is not based on a compression function. Rather, it is directly a hash function that supports variable-length inputs. The starting point for Fugue is the hash function Grindahl, but it extends that design to protect against the kind of attacks that were developed for Grindahl, as well as earlier hash functions like SHA-1. A key enhancement is the design of a much stronger round function which replaces the AES round function of Grindahl, using better codes (over longer words) than the AES 4 X 4 MDS matrix. Also, Fugue makes judicious use of this new round function on a much larger internal state. The design of Fugue is proof-oriented: the various components are designed in such a way as to allow proofs of security, and yet be efficient to implement. As a result, we can prove that current attack methods cannot find collisions in Fugue any faster than the trivial birthday attack. Although the proof is computer assisted, the assistance is limited to computing ranks of various matrices

Theoretical analysis of the focusing of acoustic waves by two-dimensional sonic crystals

Motivated by a recent experiment on acoustic lenses, we perform numerical calculations based on a multiple scattering technique to investigate the focusing of acoustic waves with sonic crystals formed by rigid cylinders in air. The focusing effects for crystals of various shapes are examined. The dependance of the focusing length on the filling factor is also studied. It is observed that both the shape and filling factor play a crucial role in controlling the focusing. Furthermore, the robustness of the focusing against disorders is studied. The results show that the sensitivity of the focusing behavior depends on the strength of positional disorders. The theoretical results compare favorably with the experimental observations, reported by Cervera, et al. (Phys. Rev. Lett. 88, 023902 (2002)).Comment: 8 figure

The weak password problem: chaos, criticality, and encrypted p-CAPTCHAs

Vulnerabilities related to weak passwords are a pressing global economic and security issue. We report a novel, simple, and effective approach to address the weak password problem. Building upon chaotic dynamics, criticality at phase transitions, CAPTCHA recognition, and computational round-off errors we design an algorithm that strengthens security of passwords. The core idea of our method is to split a long and secure password into two components. The first component is memorized by the user. The second component is transformed into a CAPTCHA image and then protected using evolution of a two-dimensional dynamical system close to a phase transition, in such a way that standard brute-force attacks become ineffective. We expect our approach to have wide applications for authentication and encryption technologies.Comment: 5 pages, 6 figer