The Dynamics of Cooperation in Group Lending - A Microfinance Experiment

We investigate the dynamics of borrower behavior in a microfinance experiment in which subjects are jointly responsible for credit repayment. Although cooperation levels are generally high, moral hazard problems persist among borrowers. Moreover, the path dependency of decisions mitigates the insurance effect of joint liability. We compare two conversion mechanisms from joint to individual liability. First, an active choice of the joint liability contract does not systematically increase cooperation. Second, conversion based on repayment success tends to have a detrimental impact on cooperation among the remaining joint liability borrowers.microfinance, group lending, individual lending, social preferences

Probing Higgs Boson - and Supersymmetry-Induced CP Violation

We consider, both for unpolarized and longitudinally polarized electron beams, the production of a top quark pair in e+e- collisions with subsequent semileptonic top and nonleptonic anti-top decay and vice versa and investigate optimized angular correlations which are sensitive to CP non-conservation in the top quark production vertex. We calculate these correlations for two-Higgs--doublet extensions and the minimal supersymmetric extension of the Standard Model (SM) with CP violation beyond the Kobayashi-Maskawa phase. While the sensitivity of the optimal correlation for tracing dispersive CP effects is enhanced with longitudinally polarized electron beams, we find that the sensitivity of the best correlation for probing absorptive CP effects is almost independent of the polarization degree.Comment: 12 pages LaTe

A newsvendor model with service and loss constraints

Actual performance measurement systems do not only consider financial measures like costs and profits but also non-financial indicators with respect customer service, quality and flexibility. Using the newsvendor model we explore the influence of possibly conflicting performance measures on important operations decisions like the order quantity and the selling price of a product. For price-independent as well as price-dependent demand distribution like in the classical newsvendor model the objective is to maximise the expected profit. But the optimal decisions are computed with respect to a service constraint - a lower bound for the level of product availability - and to a loss constraint - an upper bound for the probability of resulting in loss. For the price-independent model a condition for the existence of an optimal order quantity and its structure is presented. For the price-setting newsvendor the admissible region of the order quantity and the selling price is characterised for the additive and the multiplicative model. Furthermore, it is shown that higher variability of demand leads to a smaller admissible region of the decision variables thereby easing the computation of the optimal decisions.Constrained Newsvendor Model, Price-Setting Newsvendor

Lifshitz tails for spectra of Erd\H{o}s--R\'{e}nyi random graphs

We consider the discrete Laplace operator $\Delta^{(N)}$ on Erd\H{o}s--R\'{e}nyi random graphs with $N$ vertices and edge probability $p/N$. We are interested in the limiting spectral properties of $\Delta^{(N)}$ as $N\to\infty$ in the subcritical regime $0<p<1$ where no giant cluster emerges. We prove that in this limit the expectation value of the integrated density of states of $\Delta^{(N)}$ exhibits a Lifshitz-tail behavior at the lower spectral edge E=0.Comment: Published at http://dx.doi.org/10.1214/1050516000000719 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

Improving local search heuristics for some scheduling problems - I

Local search techniques like simulated annealing and tabu search are based on a neighborhood structure defined on a set of feasible solutions of a discrete optimization problem. For the scheduling problems $P2||C_{max}, 1|prec|\sum C_i$ and $1||\sum T_i$ we replace a simple neighborhood by a neighborhood on the set of all locally optimal solutions. This allows local search on the set of solutions that are locally optimal

Random Block Operators

We study fundamental spectral properties of random block operators that are common in the physical modelling of mesoscopic disordered systems such as dirty superconductors. Our results include ergodic properties, the location of the spectrum, existence and regularity of the integrated density of states, as well as Lifshits tails. Special attention is paid to the peculiarities arising from the block structure such as the occurrence of a robust gap in the middle of the spectrum. Without randomness in the off-diagonal blocks the density of states typically exhibits an inverse square-root singularity at the edges of the gap. In the presence of randomness we establish a Wegner estimate that is valid at all energies. It implies that the singularities are smeared out by randomness, and the density of states is bounded. We also show Lifshits tails at these band edges. Technically, one has to cope with a non-monotone dependence on the random couplings.Comment: 22 pages, 3 figure

Performance Measurement for Inventory Models with Risk Preferences

In financial economics in general the objective function expresses the risk preferences of the decision maker, see for example the mean variance approach in portfolio theory. Only recently in inventory management instead of maximizing expected profit or minimizing expected cost risk-averse objective functions have been used for determining the optimal order quantity. Examples are the exponential utility function and the conditional value at risk criterion. We use the well-known newsvendor model to determine the optimal performance measures for an objective function with two risk parameters, which can describe risk neutral, risk averse as well as risk taking behaviour of the inventory manager. We provide for this approach a complete characterization with respect to the performance measures expected profit and service level. We show that a risk averse inventory manager can not dominate a risk neutral or a risk taking inventory manager. Finally, we provide a managerial guideline for selecting the appropriate risk parameters of the objective function.Performance Measurement, Risk Preferences, Newsvendor Model