Riemannian manifolds with structure groupG 2

Summary Riemannian manifolds with structure group G ₂ are 7-dimensional and have a distinguished 3-form. In this paper such manifolds are treated as analogues of almost Hermitian manifolds. Thus S ⁷ has structure group G ₂ just as S ⁶ is an almost Hermitian manifold. We study the covariant derivative of the fundamental 3-form as was done in [GH]for almost Hermitian manifolds.     

Production of hyperons and hyperon resonances inΞ − N interactions at 102 and 135 GeV/c*

Ξ ^? interactions in hydrogen and deuterium are studied close to the forward direction using the CERN charged hyperon beam. The inclusive production of ∑^*?(1385),Ξ ^?,Ξ ^*0(1530),Ξ ^*?(1700),Ξ ^*?(1830), and Ω^? is observed, as well as an enhancement in theΞ ^?π⁺ channel at 1940 MeV/c². The momentum distributions and the production cross sections are measured for ∑^*?(1385),Ξ ^?,Ξ ^*0(1530), and Ω^?.     

Stereochemical substructure codes for 13C spectral analysis

Computer programs for encoding canonical representations of stereochemical substructural environments are presented. Applications of these substructure codes in the study of relationships between molecular structure and chemical shifts observed in ¹³C NMR spectra are described, using natural products as examples. The utility of the codes for the detection of erroneous spectral assignment is illustrated.     

A simple high-sensitivity interferometric position sensor for test mass control on an advanced LIGO interferometer

A small Michelson interferometer has been configured as a tracking mirror displacement sensor in order to achieve both large dynamic range (2.1 mm) and excellent sensitivity across a broad frequency range (6Hz–3 kHz). The interferometer is illuminated by a simple LED, uses broadband, non-polarising beamsplitters and contains no lensing optics. A DC-coupled balanced detector provides an error signal that is used to position the tracking mirror of the Michelson interferometer so as to maintain an interferometer operating position close to the centre of a particular fringe. The total interferometric sensor provides a small, simple and cost-effective means of achieving high-resolution displacement measurements.     

Ventricular fibrillation and atrial fibrillation are two different beasts

Although the mechanisms of fibrillation are no doubt multi-faceted, the geometry of the heart may play a major role in the dynamics of wave propagation during fibrillation [A. T. Winfree, Science 266, 1003-1006 (1994)]. The ventricles are thick chambers made up of sheets of parallel muscle fibers with the direction of fibers rotating across the ventricular walls (rotational anisotropy). The thick walls of the ventricles allow reentry to develop transmurally, provided the wavelength is sufficiently small. Depending on the kinetics of heart cells, the dynamics of rotating waves in three dimensions may be fundamentally different than in two dimensions, leading to destabilization of reentry and ventricular fibrillation (VF) in thick ventricles. The atria have an intricate geometry comprised of a thin sheet of cardiac tissue attached to a very complex network of pectinate muscles. The branching geometry of the pectinate muscles may lead to destabilization of two-dimensional reentry via "long-distance" electrical connections giving rise to atrial fibrillation (AF). Therefore, although fibrillation occurs via complex three-dimensional wave propagation in the ventricles and the atria, the underlying mechanisms and factors that sustain VF and AF are probably different.(c) 1998 American Institute of Physics.