Groups and semigroups with a one-counter word problem

We prove that a finitely generated semigroup whose word problem is a one-counter language has a linear growth function. This provides us with a very strong restriction on the structure of such a semigroup, which, in particular, yields an elementary proof of a result of Herbst, that a group with a one-counter word problem is virtually cyclic. We prove also that the word problem of a group is an intersection of finitely many one-counter languages if and only if the group is virtually abelian

Registration of retinal images from Public Health by minimising an error between vessels using an affine model with radial distortions

In order to estimate a registration model of eye fundus images made of an affinity and two radial distortions, we introduce an estimation criterion based on an error between the vessels. In [1], we estimated this model by minimising the error between characteristics points. In this paper, the detected vessels are selected using the circle and ellipse equations of the overlap area boundaries deduced from our model. Our method successfully registers 96 % of the 271 pairs in a Public Health dataset acquired mostly with different cameras. This is better than our previous method [1] and better than three other state-of-the-art methods. On a publicly available dataset, ours still better register the images than the reference method

A microgravity isolation mount

The design and preliminary testing of a system for isolating microgravity sensitive payloads from spacecraft vibrational and impulsive disturbances is discussed. The Microgravity Isolation Mount (MGIM) concept consists of a platform which floats almost freely within a limited volume inside the spacecraft, but which is constrained to follow the spacecraft in the long term by means of very weak springs. The springs are realized magnetically and form part of a six degree of freedom active magnetic suspension system. The latter operates without any physical contact between the spacecraft and the platform itself. Power and data transfer is also performed by contactless means. Specifications are given for the expected level of input disturbances and the tolerable level of platform acceleration. The structural configuration of the mount is discussed and the design of the principal elements, i.e., actuators, sensors, control loops and power/data transfer devices are described. Finally, the construction of a hardware model that is being used to verify the predicted performance of the MGIM is described

Evidence for a Molecular Cloud Origin for Gamma-Ray Bursts: Implications for the Nature of Star Formation in the Universe

It appears that the majority of rapidly-, well-localized gamma-ray bursts with undetected, or dark, optical afterglows, or `dark bursts' for short, occur in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun}, where L is the isotropic-equivalent peak luminosity of the optical flash. We show that clouds of this size and mass cannot be modeled as a gas that is bound by pressure equilibrium with a warm or hot phase of the interstellar medium (i.e., a diffuse cloud): Such a cloud would be unstable to gravitational collapse, resulting in the collapse and fragmentation of the cloud until a burst of star formation re-establishes pressure equilibrium within the fragments, and the fragments are bound by self-gravity (i.e., a molecular cloud). Consequently, dark bursts probably occur in molecular clouds, in which case dark bursts are probably a byproduct of this burst of star formation if the molecular cloud formed recently, and/or the result of lingering or latter generation star formation if the molecular cloud formed some time ago. We then show that if bursts occur in Galactic-like molecular clouds, the column densities of which might be universal, the number of dark bursts can be comparable to the number of bursts with detected optical afterglows: This is what is observed, which suggests that the bursts with detected optical afterglows might also occur in molecular clouds. We confirm this by modeling and constraining the distribution of column densities, measured from absorption of the X-ray afterglow, of the bursts with detected optical afterglows: We find that this distribution is consistent with the expectation for bursts that occur in molecular clouds, and is not consistent with the expectation for bursts that occur in diffuse clouds. More...Comment: Accepted to The Astrophysical Journal, 22 pages, 6 figures, LaTe

Alfalfa Cultivar Yield Test for South Dakota: 2000 Report

The South Dakota Alfalfa Cultivar Yield Test reports relative forage production characteristic for available cultivars at several locations in South Dakota. Cultivar are entered in the test by seed companies and public breeders at their own discretion. A list of cultivar and companies is in Table 8 at the end of this circula

Combination of self-harm methods and fatal and non-fatal repetition: A cohort study.

BACKGROUND: Assessment and aftercare for people who self-harm needs to be related to an understanding of risks of adverse outcomes. We aimed to determine whether self-harm by a combination of methods and its early repetition are associated with adverse outcomes - especially non-fatal repetition and suicide. METHOD: 10,829 consecutive general hospital attendances due to self-harm in one large English city were monitored, through scrutiny of Emergency Department attendances, over three years and followed up to determine the incidence of non-fatal repetition. Subsequent deaths, by any cause and by suicide, were determined from national statistical records. RESULTS: 6155 patients accounted for the 10,829 episodes: 72% by self-poisoning, 21% self-injury, and 746 episodes (7%) due to a combination of methods. After a combined-methods index episode, non-fatal repetition (P=0.001) and suicide (P=0.002) occurred sooner and more frequently than it did among those who had self-poisoned. Further hospital attendance due to self-harm within a month was associated with a 3.7-fold (95% CI 2.1-6.4) risk of subsequent suicide. LIMITATIONS: The data exclude self-harm episodes that do not result in a hospital attendance. Index episodes in the study are not generally life-time first episodes so follow-up data are based on an arbitrary start-point. Both of these limitations are common to all studies of this kind. CONCLUSIONS: At psychosocial assessment and the making of aftercare arrangements, combined methods of self-harm or another recent episode should be considered 'red-flag' indicators for attention to care

Cross Pixel Optical Flow Similarity for Self-Supervised Learning

We propose a novel method for learning convolutional neural image representations without manual supervision. We use motion cues in the form of optical flow, to supervise representations of static images. The obvious approach of training a network to predict flow from a single image can be needlessly difficult due to intrinsic ambiguities in this prediction task. We instead propose a much simpler learning goal: embed pixels such that the similarity between their embeddings matches that between their optical flow vectors. At test time, the learned deep network can be used without access to video or flow information and transferred to tasks such as image classification, detection, and segmentation. Our method, which significantly simplifies previous attempts at using motion for self-supervision, achieves state-of-the-art results in self-supervision using motion cues, competitive results for self-supervision in general, and is overall state of the art in self-supervised pretraining for semantic image segmentation, as demonstrated on standard benchmarks

Alfalfa Cultivar Yield Test for South Dakota: 1998 Report

The South Dakota Alfalfa Cultivar Yield Test reports relative forage production characteristics for available cultivars at several locations in South Dakota. Alfalfas are entered in the test by seed companies and public breeders at their own discretion. A list of the cultivars and the companies marketing them is at the end of this bulletin