Self-dual codes over exist for all even lengths. The smallest length for which the largest minimum weight among self-dual codes has not been determined
is 24, and the largest minimum weight is either 9 or 10. In this note, we show that there exists no self-dual [24, 12, 10]
code over , using the classification of 24-dimensional odd unimodular lattices due to Borcherds.
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Multivariate failure time data arise frequently in survival analysis. A commonly used technique is the working independence
estimator for marginal hazard models. Two natural questions are how to improve the efficiency of the working independence
estimator and how to identify the situations under which such an estimator has high statistical efficiency. In this paper,
three weighted estimators are proposed based on three different optimal criteria in terms of the asymptotic covariance of
weighted estimators. Simplified close-form solutions are found, which always outperform the working independence estimator.
We also prove that the working independence estimator has high statistical efficiency, when asymptotic covariance of derivatives
of partial log-likelihood functions is nearly exchangeable or diagonal. Simulations are conducted to compare the performance
of the weighted estimator and working independence estimator. A data set from Busselton population health surveys is analyzed
using the proposed estimators.
This work was supported by National Natural Science Foundation of China (Grant No. 10628104), Fan was also supported by National
Institutes of Health (Grant No. R01-GM072611) and Natural Science Foundation (Grant No. DMS-0714554), Zhou was supported by
National Natural Science Funds for Distinguisheel Young Scholar (Grant No. 70825004), National Natural Science Foundation
of China (Grant Nos. 10731010, 10628104), the National Basic Research Program (Grant No. 2007CB814902), Creative Research
Groups of China (Grant No. 10721101) and Leading Academic Disipline Program, the 10th five year plan of 211 Project for Shanghai University of Finance and Economics (the 3rd phase), Cai was supported by National Institutes of Health (Grant No. R01-HL57444) 相似文献
In this paper, the weighted boundedness of parametric Marcinkiewicz integral and its commutator with rough kernels are considered. In addition, the weak type norm inequalities for parametric Marcinkiewicz integral and its commutator with different weight functions and Dini kernel are also discussed. 相似文献
Summary. The anionic polymerization of four bis-functionalized styrene derivatives with α-alkylvinyl groups have been carried out in
THF at −78°C with the initiator prepared from oligo(α-methylstyryl)lithium and potassium tert-butoxide. The four monomers herein used are 4-isopropenylstyrene (4), 3-isopropenylstyrene (5), 2-isopropenylstyrene (6), and 4-(α-isopropylvinyl)styrene (7). It was found that under such polymerization conditions, the vinyl groups of both 4 and 7 are selectively polymerized and the isopropenyl and α-isopropylvinyl groups remain completely intact to afford stable living
anionic polymers. As expected, the resulting polymers possessed precisely controlled chain lengths and narrow molecular weight
distributions. More importantly, they also possessed the pendant isopropenyl and α-isopropylvinyl group in each monomer unit
possible for further modification. On the other hand, the anionic polymerization of either 5 or 6 proceeded more or less along with the unwanted side reactions leading to chain-branching, followed by cross-linking. The
positional effect of isopropenyl group on the polymerization and the cause of possible side reactions were discussed. 相似文献
Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (Mn) and d-lactide unit contents (Xd) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing Xd, while the k values did not depend on Mn. This means that the effects of Xd on the hydrolytic degradation rate of the films are higher than those of Mn. In contrast, in the period of 32-60 weeks neither Xd nor Mn was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks. 相似文献
This study investigated the relationship of growth conditions, host strains and molecular weights of poly[(R)-3-hydroxybutyrate] [P(3HB)] synthesized by genetically engineered Escherichia coli. Various PHA synthases belonging to types I-IV enzymes were expressed in E. coli JM109 under the same experimental conditions, and the molecular weights of the polymers were characterized by gel permeation chromatography. The results demonstrate that P(3HB) polymers have varied molecular weights and polydispersities dependent on the characteristics of the individual PHA synthase employed. P(3HB) with high number-average molecular weights (Mn) [(1.5-4.0) × 106] and narrow polydispersities (1.6-1.8) were synthesized by PHA synthases from Ralstonia eutropha (type I), Delftia acidovorans (type I) and Allochromatium vinosum (type III). Contrary to these, P(3HB) with relatively low Mn [(0.17-0.79) × 106] and broad polydispersities (2.2-9.0) were synthesized by PHA synthases from Aeromonas caviae (type I), Pseudomonas sp. 61-3 (type II) and Bacillus sp. INT005 (type IV). Furthermore, the molecular weights of P(3HB) synthesized under various culture conditions, in various hosts of E. coli and by mutants of PHA synthase were characterized. It was found that, in addition to culture pH [Kusaka et al. Appl Microbiol Biotechnol 1997;47:140], other variances such as culture temperature, host strain and use of mutants are effective in changing polymer molecular weight. 相似文献
Population balance equations in terms of generating functions (GF) are used to predict chain length distributions (CLD) of linear and non‐linear ideal controlled radical polymerization (CRP) systems. It is here shown that under simplified conditions analytical solutions for the CLD can be found and moreover the fundamental CLD derived by H. Tobita in 2006 is a limiting case of the more general solutions here presented. In order to deal with more complex CRP systems, solutions based upon the numerical inversion of GF are presented. These studies are also extended to the non‐linear CRP of vinyl/divinyl monomers where multimodal CLDs are predicted to occur.