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2—甲基—1,4—萘醌对DNA光敏损伤的诱导作用 总被引:3,自引:0,他引:3
选择波长337nm的激光作为激励光源,借助凝胶电泳研究了2-甲基-1,4-萘醌诱导的DNA光敏损伤。结果表明:在氧气饱和、脱氧条件下光敏损伤显著,DNA损伤主要与光子剂量、核酸与萘醌浓度比及DNA存在形式有关。 相似文献
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Parallel matrix factorization for recommender systems 总被引:1,自引:0,他引:1
Hsiang-Fu Yu Cho-Jui Hsieh Si Si Inderjit S. Dhillon 《Knowledge and Information Systems》2014,41(3):793-819
Matrix factorization, when the matrix has missing values, has become one of the leading techniques for recommender systems. To handle web-scale datasets with millions of users and billions of ratings, scalability becomes an important issue. Alternating least squares (ALS) and stochastic gradient descent (SGD) are two popular approaches to compute matrix factorization, and there has been a recent flurry of activity to parallelize these algorithms. However, due to the cubic time complexity in the target rank, ALS is not scalable to large-scale datasets. On the other hand, SGD conducts efficient updates but usually suffers from slow convergence that is sensitive to the parameters. Coordinate descent, a classical optimization approach, has been used for many other large-scale problems, but its application to matrix factorization for recommender systems has not been thoroughly explored. In this paper, we show that coordinate descent-based methods have a more efficient update rule compared to ALS and have faster and more stable convergence than SGD. We study different update sequences and propose the CCD++ algorithm, which updates rank-one factors one by one. In addition, CCD++ can be easily parallelized on both multi-core and distributed systems. We empirically show that CCD++ is much faster than ALS and SGD in both settings. As an example, with a synthetic dataset containing 14.6 billion ratings, on a distributed memory cluster with 64 processors, to deliver the desired test RMSE, CCD++ is 49 times faster than SGD and 20 times faster than ALS. When the number of processors is increased to 256, CCD++ takes only 16 s and is still 40 times faster than SGD and 20 times faster than ALS. 相似文献
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The thermal and thermomechanical properties of two series of poly(ethylene oxide) networks (NPEOs) were investigated as a function of the chain length between crosslink sites (Mc) and the concentration of LiClO4 (CL) in the NPEOs. The two series of networks were produced with silica and organic crosslinking agents and, therefore, had crosslink sites of different natures: one was an inorganic silicate network (silica NPEO), and the other was an organic polar group (organic NPEO). The crosslink sites in both series of networks were commonly covalently bonded to the poly(ethylene oxide) (PEO) phase through a urethane group in the NPEOs. The glass‐transition temperatures (Tg's) of the PEO phases in the NPEOs, according to differential scanning calorimetry, increased with a decrease in Mc and were higher in the silica NPEOs than in the organic NPEOs under the same Mc conditions. The difference in Tg between the two series of networks with the same Mc values increased with decreasing Mc. These results suggested that the interaction of crosslink sites with the PEO phase was stronger in the silica NPEOs than in the organic NPEOs. The addition of LiClO4 to the NPEOs resulted in Tg of the PEO phase in the NPEOs being elevated and increased according to the increase in CL. The increase of Tg of the PEO phase according to the increase of CL in the NPEOs was retarded or saturated at high values of CL, and this indicated that the limit of solubility of the salt in the polymer was attained. The retardation or saturation of the increase of Tg was also observed in dynamic mechanical analyses. The curves of the loss factor tan δ and temperatures from the dynamic mechanical analyses for the NPEOs with high values of CL showed shoulders or double peaks indicating the existence of the second phase in the polymer networks. In the curves of tan δ for salt‐complexed NPEOs with high values of CL, silica NPEOs showed a shoulder of low intensity, but organic NPEOs showed a distinguished second peak becoming stronger with increasing CL. The results of the Tg behavior and tan δ curves suggested that the salt solubility in the NPEOs was limited and that the salt solubility of PEO in the silica NPEOs was higher than that in the organic NPEOs. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 270–277, 2003 相似文献
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直接凝固注模成型Si_3N_4及SiC陶瓷──基本原理及工艺过程 总被引:10,自引:3,他引:10
直接凝固注模成型(directcoagulationcasting,DCC)是一种崭新的(准)净尺寸陶瓷成型方法。本文报道了采用此法成型Si_3N_4及SiC陶瓷的基本原理和工艺过程。DCC成型工艺过程为把高固相含量低粘度的陶瓷浆料浇注到无孔模具中,事先加入到浆料中的生物酶及化学物质通过改变浆料的pH或电解质浓度来改变浆料的胶体化学行为,从而使浆料原位凝固,得到有足够脱模强度的陶瓷坯体。DCC成型的特点为坯体密度高(理论密度的55%~70%),坯体均匀,不用或只需少量的有机添加剂(少于1%),可成型大尺寸、复杂形状、高可靠性的陶瓷部件。 相似文献