The Power of Priority Algorithms for Facility Location and Set Cover

We apply and extend the priority algorithm framework introduced by Borodin, Nielsen, and Rackoff to define greedy-like algorithms for the (uncapacitated) facility location problems and set cover problems. These problems have been the focus of extensive research from the point of view of approximation algorithms and for both problems greedy-like algorithms have been proposed and analyzed. The priority algorithm definitions are general enough to capture a broad class of algorithms that can be characterized as greedy-like while still possible to derive non-trivial lower bounds on the approximability of the problems by algorithms in such a class. Our results are orthogonal to complexity considerations, and hence apply to algorithms that are not necessarily polynomial time.     

Dual source fourier transform polarization modulation spectroscopy: an improved method for the measurement of circular and linear dichroism

It is shown that the use of two sources in a four-port interferometer equipped with cube-corner mirrors leads to increased signal-to-noise ratios in Fourier transform (FT-IR) circular and linear dichroism spectra. The output beam to the sample is a superposition of two interferograms, one from each source, having opposite Fourier phases. These two interferograms cancel one another to the degree that the two sources are matched in intensity. If the radiation from each of the two sources is first polarized orthogonally with respect to the other and passed through a polarization modulator before reaching the sample, the resulting polarization-modulation interferograms are out of Fourier phase and out of polarization-modulation phase. As a result, the polarization-modulation interferograms, due to circular or linear dichroism in the sample, from the two sources combine positively rather than negatively. An improvement in signal-to-noise ratio of up to two (or a factor of four in scan-time reduction for the same signal-to-noise ratio) compared to single source operation can be realized, while at the same time, the potential for saturation of the detector signal is significantly reduced due to the reduction in magnitude of the combined ordinary infrared transmission interferogram. Absorption and circular dichroism spectra from a dual-source FT-IR spectrometer are presented and analyzed.     

Standardization of the chromium-51 release, cell-mediated cytotoxicity assay: cryopreservation of mouse effector and target cells

Utilizing controlled cryopreservation techniques, we were able to standardize the 51Cr release cytotoxicity assay and thereby ensured reliable comparisons between results obtained on different days. Optimal conditions for freezing of both effector and target cells were quite similar. Dimethyl sulfoxide (DMSO) at a concentration of 7.5-10.0% was employed as the cryoprotective agent and cells were frozen at the rate of -1 degrees C/minute. The handling procedures for the cells before and after freezing were important. Factors affecting recovery of functional reactivity were related to toxicity of DMSO for the cells, the osmotic stress placed upon the cells as the DMSO was being removed after thawing, the handling temperature of the freshly thawed cells, and the susceptibility of cells to mechanical damage immediately after thawing. The recovery of lymphocytes after freezing was about 70%; the recovery of cytotoxicity was around 85%. Syngeneic cytotoxic reactivity induced by inoculation with the Moloney strain of murine sarcoma virus was cryopreserved, as were allogeneic cytotoxicity and natural cytotoxic reactivity. Multiple tests employing effector cells from the same frozen pool gave reproducible results; the standard error of the mean percent cytotoxicity was less than 1.5%. Cryopreserved target cells gave decreased day-to-day variability in susceptibility to lysis, since the same population of cells could be employed in each assay. These results demonstrated conclusively that we can now have a constant source of effector cells and target cells, which can be used from assay to assay as an internal standard.     

Sudden incapacitation: USAF experience, 1970-80

During the period 1970-80, there were reported 146 cases of in-flight sudden incapacitation in the USAF. Of these, 62 involved pilots, 14 were navigators, and 70 were student pilots. The etiologies of sudden incapacitation included illness without loss of consciousness, loss of consciousness, spatial disorientation, and improper M-1 maneuver. Each of these categories is analyzed with emphasis upon prevention, for example, not flying with symptomatic preexisting disease, continued emphasis upon spatial disorientation training, and correct performance of the M-1 maneuver. Based upon the data, conclusions and recommendations are suggested to minimize the risk of episodes of in-flight sudden incapacitation.     

Adaptive resynthesis of O6-methylguanine-accepting protein can explain the differences between mammalian cells proficient and deficient in methyl excision repair

Mammalian cells have been classified as proficient (Mer(+)) or deficient (Mer(-)) in methyl excision repair in terms of their cytotoxic reactions to agents that form O(6)-alkylguanine and their abilities to reactivate alkylated adenoviruses. O(6)-Methylguanine (O(6)MeGua) is considered to be a lethal, mutagenic, and carcinogenic lesion. We measured the abilities of cell extracts to transfer the methyl group from an exogenous DNA containing O(6)MeGua to acceptor protein. The constitutive level of acceptor activity was independent of the Mer phenotype and was approximately 100,000 acceptor sites per cell. Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) results in a dose-dependent decrease in the acceptor activity in extracts because the rapid reaction between endogenous O(6)MeGua and acceptor protein makes the latter unavailable for further reaction. Treatment of cells with 1 muM MNNG for 15 min or 2 muM for approximately 2 min uses up >95% of the constitutive activity. However, Mer(+) cells, which are resistant to MNNG, rapidly resynthesize new acceptor protein, and the activity returns to the basal level in approximately 90 min. In Mer(-) tumor cells and Chinese hamster cells, which are sensitive to MNNG, resynthesis is not detectable in 90 min. Mer(-) simian virus 40-transformed fibroblasts, known to have an intermediate sensitivity to MNNG, have an intermediate resynthesis rate. Treatment of cells with multiple low doses of MNNG results in the enhanced production of O(6)MeGua-accepting protein in levels 2.5-fold above the constitutive values for Mer(+) tumor cells and to approximately 1.5-fold for Mer(+) fibroblasts or Mer(-) simian virus 40-transformed cells. Such treatments reduce the activities in Mer(-) tumor cells and Chinese hamster cells. We conclude: (i) estimates of O(6)MeGua in cellular DNA shortly after treatment may be seriously in error because of the rapid repair of this lesion, and (ii) the adaptive resynthesis of acceptor protein, not its constitutive level, is the important correlate of cell resistance to methylating agents.     

Eye movements of monkeys during discrimination learning: Role of visual scanning.

In Exps I and II with 7 stumptailed monkeys (Macaca arctoides), Ss were given a series of 2-choice, dot-pattern discrimination reversal problems. Results indicate that the rate of reversal learning was markedly retarded by the experimental condition. The magnitude of the effect was unrelated to the similarity of the uninformative stimulus to the discriminative stimuli. In Exp III, 5 experienced Ss were given a series of discrimination problems without reversals, during half of which the experimental condition was in effect. The results were similar to those of the 1st 2 experiments. In Exp IV (3 experienced Ss), each trial began with uninformative stimuli, which were replaced by the discriminative stimuli when visual scanning occurred. The uninformative stimuli had no clear-cut effect on discrimination learning. Findings indicate that the information provided by above-minimum levels of scanning is not necessary for discrimination learning per se, but it does appear necessary for efficent discrimination learning. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)