Latin transversals of rectangular arrays

Let m and n be integers, $2 \leq m \leq n$. An m by n array consists of mn cells, arranged in m rows and n columns, and each cell contains exactly one symbol. A transversal of an array consists of m cells, one from each row and no two from the same column. A latin transversal is a transversal in which no symbol appears more than once. We will establish a sufficient condition that a 3 by n array has a latin transversal.Comment: Theorem 4 has been added, which provides a lower bound on L(m,n

High temperature dispersion strengthening of NiAl

A potential high temperature strengthening mechanism for alloys based on the intermetallic compound NiAl was investigated. This study forms part of an overall program at NASA Lewis Research Center for exploring the potential of alloys based on NiAl for high temperature applications. An alloy containing 2.26 at% Nb and produced by hot extrusion of blended powders was examined in detail using optical and electron microscopy. Interdiffusion between the blended Nb and NiAl powders results in the formation of intermediate phases. A fine dispersion of precipitates of a hexagonal, ordered NiAlNb phases in a matrix of NiAl can be produced and this results in strengthening of the alloy by interfering with dislocation motion at high temperature. These precipitates are, however, found to coarsen during the high temperature (1300 K) deformation at slow strain rates and this may impose some limitatioins on the use of this strengthening mechanism

Effects of Oral Contraceptives on The Prevalence of Alveolar Osteitis After Mandibular Third Molar Surgery: A Retrospective Study

The objective of this study was to investigate the influence of oral contraceptives on the incidence rate of alveolar osteitis (AO) following the surgical extraction of both impacted mandibular third molars. This retrospective study reviewed the clinical records of patients who presented to the oral surgery clinic of a university school of dentistry for the extraction of impacted mandibular third molars. Using a database search, all patients were categorized by sex, age, occurrence of AO, and whether the females were taking oral contraceptives at the time of surgery. The patient was considered positive for AO if either one or both sockets developed AO. The incidence of AO among women taking oral contraceptives at the time of impacted mandibular third molar extraction differed significantly from that in the other patient groups. AO occurred in 37.9% (11/29) of females taking oral contraceptives, while only 8.9% (16/179) of females who were not taking oral contraceptives at the time of extraction developed AO. The total incidence of AO among females was 13.0% (27/208). The total incidence of AO among the 363 males and females presenting for mandibular third molar extractions was 13.8%. Females who are taking oral contraceptives at the time of impacted mandibular third molar extraction are at a higher risk of developing AO following extraction

Strain-induced insulator state in La_0.7Sr_0.3CoO_3

We report on the observation of a strain-induced insulator state in ferromagnetic La_0.7Sr_0.3CoO_3 films. Tensile strain above 1% is found to enhance the resistivity by several orders of magnitude. Reversible strain of 0.15% applied using a piezoelectric substrate triggers huge resistance modulations, including a change by a factor of 10 in the paramagnetic regime at 300 K. However, below the ferromagnetic ordering temperature, the magnetization data indicate weak dependence on strain for the spin state of the Co ions. We interpret the changes observed in the transport properties in terms of a strain-induced splitting of the Co e_g levels and reduced double exchange, combined with a percolation-type conduction in an electronic cluster state

The t-J model on a semi-infinite lattice

The hole spectral function of the t-J model on a two-dimensional semi-infinite lattice is calculated using the spin-wave and noncrossing approximations. In the case of small hole concentration and strong correlations, $t\gg J$, several near-boundary site rows appear to be depleted of holes. The reason for this depletion is a deformation of the magnon cloud, which surrounds the hole, near the boundary. The hole depletion in the boundary region leads to a more complicated spectral function in the boundary row in comparison with its bulk shape.Comment: 8 pages, 5 figure