Group reputations: an experimental foray

Often information structures are such that while individual reputation building is impossible groups of agents would have the opportunity of building up a reputation. We experimentally examine whether groups of sellers in markets that suffer from moral hazard are able to build up reputations and, thus, avoid market breakdown. We contrast our findings with situations where sellers alternatively can build up an individual reputation or where there are no possibilities for reputation building at all. Our results offer a rather optimistic outlook on group reputations. Even though sellers only receive some of the reputation benefits of withstanding short-run incentives to exploit trust, they are able to overcome the dilemma and successfully exploit the information structure

Beliefs and actions in the trust game: creating instrumental variables to estimate the causal effect

In many economic contexts, an elusive variable of interest is the agent's expectation about relevant events, e.g. about other agents' behavior. Recent experimental studies as well as surveys have asked participants to state their beliefs explicitly, but little is known about the causal relation between beliefs and other behavioral variables. This paper discusses the possibility of creating exogenous instrumental variables for belief statements, by shifting the probabilities of the relevant events. We conduct trust game experiments where the amount sent back by the second player (trustee) is exogenously varied by a random process, in a way that informs only the �first player (trustor) about the realized variation. The procedure allows detecting causal links from beliefs to actions under plausible assumptions. The IV estimates indicate a signi�ficant causal effect, comparable to the connection between beliefs and actions that is suggested by OLS analyses

Pricing and trust

We experimentally examine the effects of flexible and fixed prices in markets for experience goods in which demand is driven by trust. With flexible prices, we observe low prices and high quality in competitive (oligopolistic) markets, and high prices coupled with low quality in non-competitive (monopolistic) markets. We then introduce a regulated intermediate price above the oligopoly price and below the monopoly price. In monopolies volume increases and so does quality, such that overall efficiency is raised by 50%. Somewhat surprisingly, the same pattern emerges in oligopolies. In fact, across all market forms transaction volume and traded quality are maximal in regulated oligopolies

Competition fosters trust

We study the effects of reputation and competition in a stylized market for experience goods. If interaction is anonymous, such markets perform poorly: sellers are not trustworthy, and buyers do not trust sellers. If sellers are identifiable and can, hence, build a reputation, efficiency quadruples but is still at only a third of the first best. Adding more information by granting buyers access to all sellers’ complete history has, somewhat surprisingly, no effect. On the other hand, we find that competition, coupled with some minimal information, eliminates the trust problem almost completely

Niveaux d'énergie de 28Si au voisinage de 13,245 MeV observés à l'aide des réactions 27Al(p, γ)28Si et 27Al(p, α) 24Mg

L'étude simultanée des caractéristiques des résonances de la réaction 27Al + p à une énergie Ep = 1 724 keV, pour différentes voies de sortie, a permis de conclure à l'existence de deux niveaux distincts (Jπ = 3-, Jπ = 5-) dans 28Si situés à 13 245 ± 2 keV

Dynamical Coulomb blockade and spin-entangled electrons

We consider the production of mobile and nonlocal pairwise spin-entangled electrons from tunneling of a BCS-superconductor (SC) to two normal Fermi liquid leads. The necessary mechanism to separate the two electrons coming from the same Cooper pair (spin-singlet) is achieved by coupling the SC to leads with a finite resistance. The resulting dynamical Coulomb blockade effect, which we describe phenomenologically in terms of an electromagnetic environment, is shown to be enhanced for tunneling of two spin-entangled electrons into the same lead compared to the process where the pair splits and each electron tunnels into a different lead. On the other hand in the pair-split process, the spatial correlation of a Cooper pair leads to a current suppression as a function of distance between the two tunnel junctions which is weaker for effectively lower dimensional SCs.Comment: 5 pages, 2 figure

Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid

The settling of heavy spherical particles in a column of quiescent fluid is investigated. The performed experiments cover a range of Galileo numbers ($110 \leq \text{Ga} \leq 310$) for a fixed density ratio of $\Gamma = \rho_p/\rho_f = 2.5$. In this regime the particles are known (M. Jenny, J. Du\v{s}ek and G. Bouchet, Journal of Fluid Mechanics 508, 201 (2004).) to show a variety of motions. It is known that the wake undergoes several transitions for increasing $\text{Ga}$ resulting in particle motions that are successively: vertical, oblique, oblique oscillating, and finally chaotic. Not only does this change the trajectory of single, isolated, settling particles, but it also changes the dynamics of a swarm of particles as collective effects become important even for dilute suspensions, with volume fraction up to $\Phi_V = \mathcal{O}\left(10^{-3}\right)$, which are investigated in this work. Multi-camera recordings of settling particles are recorded and tracked over time in 3 dimensions. A variety of analysis are performed and show a strong clustering behavior. The distribution of the cell areas of the Vorono\"i tessellation in the horizontal plane are compared to that of a random distribution of particles and shows clear clustering. Moreover, a negative correlation was found between the Vorono\"i area and the particle velocity; clustered particles fall faster. In addition, the angle between two adjacent particles and the vertical is calculated and compared to a homogeneous distribution of particles, clear evidence of vertical alignment of particles is found. The experimental findings are compared to simulations.Comment: 8 pages, 6 figure

Matrix Multiplication Verification Using Coding Theory

We study the Matrix Multiplication Verification Problem (MMV) where the goal is, given three $n \times n$ matrices $A$, $B$, and $C$ as input, to decide whether $AB = C$. A classic randomized algorithm by Freivalds (MFCS, 1979) solves MMV in $\widetilde{O}(n^2)$ time, and a longstanding challenge is to (partially) derandomize it while still running in faster than matrix multiplication time (i.e., in $o(n^{\omega})$ time). To that end, we give two algorithms for MMV in the case where $AB - C$ is sparse. Specifically, when $AB - C$ has at most $O(n^{\delta})$ non-zero entries for a constant $0 \leq \delta < 2$, we give (1) a deterministic $O(n^{\omega - \varepsilon})$-time algorithm for constant $\varepsilon = \varepsilon(\delta) > 0$, and (2) a randomized $\widetilde{O}(n^2)$-time algorithm using $\delta/2 \cdot \log_2 n + O(1)$ random bits. The former algorithm is faster than the deterministic algorithm of K\"{u}nnemann (ESA, 2018) when $\delta \geq 1.056$, and the latter algorithm uses fewer random bits than the algorithm of Kimbrel and Sinha (IPL, 1993), which runs in the same time and uses $\log_2 n + O(1)$ random bits (in turn fewer than Freivalds's algorithm). We additionally study the complexity of MMV. We first show that all algorithms in a natural class of deterministic linear algebraic algorithms for MMV (including ours) require $\Omega(n^{\omega})$ time. We also show a barrier to proving a super-quadratic running time lower bound for matrix multiplication (and hence MMV) under the Strong Exponential Time Hypothesis (SETH). Finally, we study relationships between natural variants and special cases of MMV (with respect to deterministic $\widetilde{O}(n^2)$-time reductions)

Broaching the brook : daylighting, community and the ‘stickiness’ of water

Over the last century, under the modern hydraulic model, waterways across the world have been heavily canalized and culverted, driven into underground pipes, drains and sewers. This hydraulic approach has hardwired an isolated water network into the urban fabric, fragmenting erstwhile patterns and dynamics of life, both human and nonhuman. Ecologically, it has been hugely damaging, reducing water quality and biotic diversity, but also socially, disconnecting citizens from the waterways that service and characterize the city. Consequently, since the 1990s, waterway restoration has become widespread as a design solution to degraded rivers and streams, reinstating compromised hydrological, geomorphological and ecological processes. Deculverting or ‘daylighting’, the focus of this paper, is a radical form of restoration, opening up subterranean, culverted waterways often forgotten by communities above ground. Yet, as this paper emphasizes, waterway restoration has tended to privilege ecological over social objectives, while public engagement in project conceptualization has been limited, conducted ‘downstream’ subsequent to planning and design stages. Restoration schemes have therefore tended to reflect the concerns of professionals rather than communities, overlooking their potential for social renewal and change. Drawing on workshop data collected through participatory mapping exercises, this paper explores the case for daylighting a culverted brook in Urmston, Greater Manchester, focusing in particular on the preferences, concerns and knowledge of local residents. The paper compares professional and community perspectives on the preferred scheme design and potential benefits of daylighting, drawing out differences and tensions between them, temporarily ‘unblackboxing’ the brook. It is ventured that daylighting can unleash the social ‘stickiness’ of water, its proclivity to draw and bind together, to revitalize the park, enhancing connection to wildness, attachment to place and sense of community. This is particularly crucial in the face of decreased local authority funding and related crises in park management