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181.
We present a randomized procedure for rounding fractional perfect matchings to (integral) matchings. If the original fractional
matching satisfies any linear inequality, then with high probability, the new matching satisfies that linear inequality in
an approximate sense. This extends the well-known LP rounding procedure of Raghavan and Thompson, which is usually used to
round fractional solutions of linear programs.?We use our rounding procedure to design an additive approximation algorithm
to the Quadratic Assignment Problem. The approximation error of the algorithm is εn
2 and it runs in n
O
(log
n
/ε2) time.?We also describe Polynomial Time Approximation Schemes (PTASs) for dense subcases of many well-known NP-hard arrangement
problems, including MINIMUM LINEAR ARRANGEMENT, MINIMUM CUT LINEAR ARRANGEMENT, MAXIMUM ACYCLIC SUBGRAPH, and BETWEENNESS.
Received: December 12, 1999 / Accepted: October 25, 2001?Published online February 14, 2002 相似文献
182.
Sanjeev Arora 《Mathematical Programming》2003,97(1-2):43-69
Traveling Salesman, Steiner Tree, and many other famous geometric optimization problems are NP-hard. Since we do not expect
to design efficient algorithms that solve these problems optimally, researchers have tried to design approximation algorithms,
which can compute a provably near-optimal solution in polynomial time.
We survey such algorithms, in particular a new technique developed over the past few years that allows us to design approximation
schemes for many of these problems. For any fixed constant c> 0, the algorithm can compute a solution whose cost is at most (1 + c) times the optimum. (The running time is polynomial for every fixed c> 0, and in many cases is even nearly linear.) We describe how these schemes are designed, and survey the status of a large
number of problems.
Received: December 2, 2002 / Accepted: April 28, 2003
Published online: May 28, 2003
Supported by a David and Lucile Packard Fellowship, NSF grant CCR-0098180, NSF ITR grant CCR-0205594 相似文献
183.
S. Jha Mohamed Elfazani Sherif Yehia C. Mitros Sanjeev Kumar Glenn M. Julian R. A. Dunlap A. Vasquez J. G. M. DaCunha S. M. M. Ramos 《Hyperfine Interactions》1987,34(1-4):423-426
Hyperfine magnetic fields are measured at Cd-111 and Sn-119 in CdCr2Se4 and CuCr2Se4, as well as at Cd-111 in CdCr2S4 and CuCr2S4. Quadrupole frequencies are measured for Cd-111 in CdCr2S4 and CdCrvSe4 above Tc. 相似文献
184.
Ghosal P Sharma D Kumar B Meena S Sinha S Shaw AK 《Organic & biomolecular chemistry》2011,9(21):7372-7383
A stereoselective route for the total synthesis of anticancer marine natural product (+)-varitriol (1) is detailed herein. The impressive biological activity and interesting structural features of natural (+)-varitriol fuelled us to undertake the synthesis of some higher analogues (1a-j) of this molecule. The key features of the synthetic strategy include one-pot Wittig olefination followed by a highly diastereoselective oxa-Michael addition to assemble stereochemically pure tetrasubstituted THF moiety of the natural varitriol and olefin cross metathesis to couple the aromatic part with tetrasubstituted THF moiety. The total synthesis of title natural product is efficient with 21.8% overall yield for 9 linear steps from D-ribose and thus facilitates the more scaled-up practical route for the synthesis of 1 and its analogues as well. The synthetic (+)-varitriol (1) and its analogues were screened for their cytotoxicity. The present synthetic approach paves the way for preparation of numerous analogues of the title natural product for drug development. 相似文献
185.
Malviya N Srivastava M Diwakar SK Mishra SK 《Applied biochemistry and biotechnology》2011,165(2):397-405
Polyphenol oxidases (PPOs) are widely distributed enzymes among animals, plants, bacteria, and fungi. PPOs often have significant
role in many biologically essential functions including pigmentation, sclerotization, primary immune response, and host defense
mechanisms. In the present study, forty-seven full-length amino acid sequences of PPO from bacteria, fungi, and plants were
collected and subjected to multiple sequence alignment (MSA), domain identification, and phylogenetic tree construction. MSA
revealed that six histidine, two phenylalanine, two arginine, and two aspartic acid residues were highly conserved in all
the analyzed species, while a single cysteine residue was conserved in all the plant and fungal PPOs. Two major sequence clusters
were constructed by phylogenetic analysis. One cluster was of the plant origin, whereas the other one was of the fungal and
bacterial origin. Motif GGGMMGDVPTANDPIFWLHHCNVDRLWAVWQ was found in all the species of bacterial and fungus sources. In addition,
seven new motifs which were unique for their group were also identified. 相似文献
186.
Gilibili RR Kandaswamy M Sharma K Giri S Rajagopal S Mullangi R 《Biomedical chromatography : BMC》2011,25(12):1352-1359
A highly sensitive and specific LC‐MS/MS method was developed for simultaneous estimation of acetyl co‐enzyme A (ACoA) and malonyl co‐enzyme A (MCoA) in surrogate matrix using n‐propionyl co‐enzyme A as an internal standard (IS). LC‐MS/MS was operated under the multiple reaction‐monitoring mode using the electrospray ionization technique. Simple acidification followed by dilution using an assay buffer process was used to extract ACoA, MCoA and IS from surrogate matrix and tissue samples. The total run time was 3 min and the elution of both analytes (ACoA, MCoA) and IS occurred at 1.28 min; this was achieved with a mobile phase consisting of 5 mM ammonium formate (pH 7.5)–acetonitrile (30:70, v/v) delivered at a flow rate of 1 mL/min on a monolithic RP‐18e column. A linear response function was established for the range of concentrations 1.09–2187 and 1.09–2193 ng/mL for ACoA and MCoA, respectively. The intra‐ and inter‐day precision values for ACoA and MCoA met the acceptance as per FDA guidelines. ACoA and MCoA were stable in a battery of stability studies viz. bench‐top, auto‐sampler and long‐term. The developed assay was used to quantitate ACoA and MCoA levels in various tissues of rat. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
187.
Using a hybrid method based on fermionic diagonalization and classical Monte Carlo techniques, we investigate the interplay between itinerant and localized spins, with competing double- and superexchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized ground state is stable that has macroscopic degeneracy. The presence of these states on a nonfrustrated honeycomb lattice highlights novel phenomena in this itinerant electron system: emergent geometrical frustration and degeneracy related to a symmetry intermediate between local and global. 相似文献
188.
Mondal M Kumar S Chand M Kamlapure A Saraswat G Seibold G Benfatto L Raychaudhuri P 《Physical review letters》2011,107(21):217003
We analyze the occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below T(BKT) and of the resistivity above T(BKT). By direct comparison between the experimental data and the theory, we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor. 相似文献
189.
Kumar S Singh S Bajwa BS Sabharwal AD 《Isotopes in environmental and health studies》2011,47(4):446-455
Radon concentration levels in water and soil gas from 36 locations pertaining to some areas of Malwa region of Punjab have been measured on an in situ basis using a continuous active radon detector (AlphaGuard, Model - PQ 2000 PRO, Genitron instruments, Germany). Exhalation rate measurements have also been carried out at these places, using a closed-circuit technique. The radon concentrations in soil and water varied from 1.9 to 16.4?kBq?m(-3) and 5.01 to 11.6?kBq?m(-3), respectively. The exhalation rate (E (Rn)) ranged between 7.48 and 35.88?mBq?m(-2)?s(-1) with an average value of 18.17?mBq?m(-2)?s(-1). Annual dose rates have been calculated for water radon concentrations. The minimum to maximum values of dose rates were found to be 13.42-31.08?μSv?y(-1). The recorded values of radon concentration in water are within the safe limit of 11?Bq?l(-1) recommended by the US Environment Protection Agency [National Research Council, Risk Assessment of Radon in Drinking Water (Academy Press, Washington, DC, USA, 1999)]. All measurements were made in similar climatic and environmental conditions to ensure minimal variations in meteorological parameters. An intermediate correlation coefficient (0.5) was observed between radon exhalation rates and soil gas values. 相似文献
190.
Comparative evaluation of magnetization transfer MR imaging and in-vivo proton MR spectroscopy in brain tuberculomas 总被引:3,自引:0,他引:3
Gupta RK Husain M Vatsal DK Kumar R Chawla S Husain N 《Magnetic resonance imaging》2002,20(5):375-381
We have compared and analyzed the value of in vivo proton MR spectroscopy (PMRS) and T1 weighted magnetization transfer (MT) MR imaging in tissue characterization of brain tuberculomas. We studied 33 cases of proven intracranial tuberculomas with in vivo PMRS and T1 weighted MT MR imaging. MT ratios from the rim and core of the tuberculomas were calculated and compared with metabolites seen on PMRS. Final diagnosis of tuberculoma was based on histopathology (n = 26) and/or associated tuberculous meningitis (n = 7) in all the cases. Out of the 33 patients who underwent both PMRS and T1 weighted MT MR imaging, spectroscopy showed only lipids at 0.9 ppm, 1.3 ppm, 2.0 ppm, and 2.80 ppm in 26 cases while lipids at 0.9 ppm, 1.3 ppm, 2.0 ppm and 2.80 ppm along with choline at 3.22 ppm was seen in remaining 7 patients. MT ratios from the core or solid necrosis varied from 21-29% while from the rim or cellular region varied from 16-24%. MT ratios from all the 33 lesions were consistent with tuberculomas while PMRS showed choline along with lipids in 7 predominantly cellular lesions simulating a neoplasm. We conclude that T1 weighted MT MR imaging appears to be more consistent in the tissue characterization of brain tuberculomas. 相似文献