Universal homogeneous causal sets

Causal sets are particular partially ordered sets which have been proposed as a basic model for discrete space-time in quantum gravity. We show that the class C of all countable past-finite causal sets contains a unique causal set (U,<) which is universal (i.e., any member of C can be embedded into (U,<)) and homogeneous (i.e., (U,<) has maximal degree of symmetry). Moreover, (U,<) can be constructed both probabilistically and explicitly. In contrast, the larger class of all countable causal sets does not contain a universal object.Comment: 14 page

Molecular and serological evidence of flea-associated typhus group and spotted fever group rickettsial infections in Madagascar

This research was supported by the Wellcome Trust (RCDF and Senior Fellowship to ST, #081705 and #095171), the Institut Pasteur de Madagascar, and the Global Emerging Infections Surveillance and Response System, a Division of the Armed Forces Health Surveillance Center [847705.82000.25GB.A0074].Peer reviewedPublisher PD

Grade Recovery, Value, and Return-To-Log for the Production of NZ Visual Grades (Cuttings and Framing) and Australian Machine Stress Grades

The objective of this study is to link radiata pine tree characteristics to the quality and value of boards in New Zealand (NZ) Cuttings, NZ Visual Framing, and Australian Machine Stress Grades (MSG) from both clonal and standing tree perspectives. Specifically, this paper presents an analysis of clonal variation in the quality and value of 2 X 4s, establishes the relationships between the tree and products characteristics, and documents the broad sense heritability of the tree variables associated with products value.Ten clones were selected to cover a broad range of radiata pine representative of the forest being harvested in New Zealand in the coming years. Two trees were harvested for each clone. The trees were pruned up to 4 m. The stems were cut into logs, and four logs were cross-cut to be sawn: the pruned butt log and three unpruned. The yield analysis was performed separately for pruned and unpruned logs. Tree quality assessed included DBH, Branch Index, Internode Index, bulk density, outer wood density (from increment cores), ring width, microfibril angle, spiral grain, tracheid length, and compression wood. On the lumber pieces, knot area ratio was also assessed.The value of boards in NZ Cuttings from pruned butt logs averaged 310 $/m3 as compared to 204$/m3 for unpruned upper logs. These were significant differences between clones for pruned butt logs and for the boards from unpruned upper logs. Regression analysis confirmed that for NZ Cuttings, small trees with lots of small branches perform badly when compared to large trees with larger branches. Regression analysis also showed that for boards from unpruned upper logs, the longer the internode length, the better the yield in NZ Cuttings.The value of boards in NZ Visual Framing from pruned butt logs averaged 333 $/m3 as compared with 227$/m3 for unpruned upper logs. There was no significant difference between clones for boards from the pruned butt logs (P = 0.12), but there were highly significant differences between boards from unpruned upper logs. Regression analysis showed that best performing clones among the unpruned upper logs were the ones with small branches