Twistors and induced representations of SU(2,2)

This is the published version, also available here: http://dx.doi.org/10.1063/1.523494.We give an explicit realization of a series of representations of SU(2,2) induced by R+⊕SL(2,C). Vectors in these representation spaces are homogeneous spinor‐valued functions of two twistor variables. They may also be realized, in a frame‐dependent way, either as conformally invariant fields in Minkowski space or as homogeneous spinor‐valued functions on the O(2,4) null cone. The conformal invariance of the massless free fields is discussed from this point of view, and the twistor version of the field equations is derived. Finally, irreducible twistors are shown to correspond to conformally invariant fields satisfying the generalized twistor equation

A Genetic Locus Regulates the Expression of Tissue-Specific mRNAs from Multiple Transcription Units

129 GIX- mice, unlike animals of the congeneic partner strain GIX+, do not express significant amounts of the retroviral antigens gp70 and p30. Evidence is presented indicating that the GIX phenotype is specified by a distinct regulatory gene acting on multiple transcription units to control the levels of accumulation of specific mRNA species. The steady-state levels of retroviral-homologous mRNA from the tissues of GIX+ and GIX- mice were examined by blot hybridization using as probes DNA fragments from cloned murine leukemia viruses. RNA potentially encoding viral antigens was reduced or absent in GIX- mice, even though no differences in integrated viral genomes were detected between these congeneic strains by DNA blotting. Tissue-specific patterns of accumulation of these RNA species were detected in brain, epididymis, liver, spleen, and thymus, and several distinct RNA species were found to be coordinately regulated with the GIX phenotype. Measurements of RNA synthesis suggest a major role for transcriptional control in the regulation of some retroviral messages

Complex line bundles in relativity

This is the published version, also available here: http://dx.doi.org/10.1063/1.523750.We exhibit the complexified spin and conformally weighted functions as sections of holomorphic line bundles over P 1(C) ×P 1(C). As an example of a nontrivial bundle, we discuss the complex null cone in some detail

Weak gravitational fields

This is the published version, also available here: http://dx.doi.org/10.1063/1.1666824.We consider the set of Ck bounded tensor fields of type (r,s) on R 4 in the topology of uniform Ck convergence. For each k≥2, the map sending a metric to its curvature tensor is shown to be analytic at the Minkowski metric. The same is true of the map sending a metric to its Einstein tensor. The well‐known linearized theory of gravitation amounts to studying the directional derivatives of these maps. An iterative method for solving the full field equations along an analytic curve of Einstein tensors passing through zero is proposed

Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap

The superfluid phase transition of bosons in a two-dimensional (2D) system with disorder and an external parabolic potential is studied. The theory is applied to experiments on indirect excitons in coupled quantum wells. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The slope of the external parabolic trap is assumed to change slowly enough to apply the local density approximation (LDA) for the superfluid density, which allows us to calculate the Kosterlitz-Thouless temperature $T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid phase occurs around the center of the trap ($\mathbf{r}=0$) with the normal phase outside this area. As temperature increases, the superfluid area shrinks and disappears at temperature $T_{c}(n(r=0))$. Disorder acts to deplete the condensate; the minimal total number of excitons for which superfluidity exists increases with disorder at fixed temperature. If the disorder is large enough, it can destroy the superfluid entirely. The effect of magnetic field is also calculated for the case of indirect excitons. In a strong magnetic field $H$, the superfluid component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure

On Conformal Infinity and Compactifications of the Minkowski Space

Using the standard Cayley transform and elementary tools it is reiterated that the conformal compactification of the Minkowski space involves not only the "cone at infinity" but also the 2-sphere that is at the base of this cone. We represent this 2-sphere by two additionally marked points on the Penrose diagram for the compactified Minkowski space. Lacks and omissions in the existing literature are described, Penrose diagrams are derived for both, simple compactification and its double covering space, which is discussed in some detail using both the U(2) approach and the exterior and Clifford algebra methods. Using the Hodge * operator twistors (i.e. vectors of the pseudo-Hermitian space H_{2,2}) are realized as spinors (i.e., vectors of a faithful irreducible representation of the even Clifford algebra) for the conformal group SO(4,2)/Z_2. Killing vector fields corresponding to the left action of U(2) on itself are explicitly calculated. Isotropic cones and corresponding projective quadrics in H_{p,q} are also discussed. Applications to flat conformal structures, including the normal Cartan connection and conformal development has been discussed in some detail.Comment: 38 pages, 8 figures, late

Using temporal averaging to decouple annual and nonannual information in AVHRR NDVI time series

As regularly spaced time series imagery becomes more prevalent in the remote sensing community, monitoring these data for temporal consistency will become an increasingly important problem. Long-term trends must be identified, and it must be determined if such trends correspond to true changes in reflectance characteristics of the study area (natural), or if their source is a signal collection and/or processing artifact that can be identified and corrected in the data (artificial). Spectrally invariant targets (SITS) are typically used for sensor calibration and data consistency checks. Unfortunately, such targets are not always available in study regions. The temporal averaging technique described in this research can be used to determine the presence of artificial interannual value drift in any region possessing multiyear regularly sampled time series remotely sensed imagery. Further, this approach is objective and does not require the prior identification of a SIT within the region of study. Using biweekly Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data from 1990 to 2001 covering the conterminous United States, an interannual trend present in the entire scene was identified using the proposed technique and found to correspond extremely well with interannual trends identified using two SITS within the region

A General Circulation Model Study of Atmospheric Carbon Monoxide

The carbon monoxide cycle is studied by incorporating the known and hypothetical sources and sinks in a tracer model that uses the winds generated by a general circulation model. Photochemical production and loss terms, which depend on OH radical concentrations, are calculated in an interactive fashion. The computed global distribution and seasonal variations of CO are compared with observations to obtain constraints on the distribution and magnitude of the sources and sinks of CO, and on the tropospheric abundance of OH. The simplest model that accounts for available observations requires a low latitude plant source of about 1.3×10^(15) g yr^(−1), in addition to sources from incomplete combustion of fossil fuels and oxidation of methane. The globally averaged OH concentration calculated in the model is 7×10^5 cm^(−3). Models that calculate globally averaged OH concentrations much lower than our nominal value are not consistent with the observed variability of CO. Such models are also inconsistent with measurements of CO isotopic abundances, which imply the existence of plant sources

Development of the research lifecycle model for library services

Can the niche services of individual librarians across multiple libraries be developed into a suite of standard services available to all scientists that support the entire research lifecycle

Toward a Socialism for the Future, in the Wake of the Demise of the Socialism of the Past

http://deepblue.lib.umich.edu/bitstream/2027.42/51230/1/464.pd