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支持向量机特征选择中的L_p正则化方法研究
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摘要
特征选择是机器学习领域中一个重要的研究课题.特征选择可以剔除数据集中冗余和噪声特征,得到一个精简且判别能力更强的特征子集,从而避免学习过程中的“过拟合”问题,提高模型的泛化能力和可解释性,减少数据的采集量和存储量,节省训练和预测时间.
     岛正则化方法在特征选择中具有重要地位,已成为当前研究的热点课题.在标准的支持向量机中所使用的L2范数不具备特征选择的能力.为了能在学习分类模型的同时实现特征选择,常采用L0范数或L1范数正则化方法.但Lo-SVM是一个难以求解的组合优化问题,而Li-SVM存在欠稀疏的缺点,因此介于两者之间的LP-SVM(0     1.针对LP-SVM(0     2.大量计算研究表明:L1/2正则化可作为Lp(0收敛性.人工数据实验结果表明,与L0-SVM和L1-SVM相比,L1/2-SVM能够更准确的找到相关且非冗余的特征.真实数据实验表明,L1/2-SVM可获得比L0-SVM更精确的分类结果,以及比L1-SVM更稀疏的特征选择结果.
     3.本文研究求解L1/2-SVM的惩罚序列线性规划算法(PSLP)该算法利用线性规划逼近最优解,适用于变量和约束都很多的大规模问题.我们将PSLP算法应用于具有高维小样本、高噪声、高冗余等特点的基因表达谱数据集.数值实验结果表明,PSLP算法的准确性高于求解Lo-SVM的FSV算法.与L1-SVM相比,PSLP算法不仅能找到比L1-SVM更少的特征基因,而且可获得比L1-SVM更好或相当的分类结果.我们统计得出各数据集中频繁被选择的前十位基因,为生物学的进一步研究提供参考.
     4.本文对Lp正则化支持向量机在特征选择方面的能力进行理论分析.我们首先分析对特定数据进行特征选择的可能性,研究表明支持向量机实现特征选择不仅与目标函数采用的范数有关,还与数据本身有关.然后推导出一个用于度量支持向量机特征选择能力的概率计算公式,并应用该公式计算LP-SVM在p不同取值时的特征选择概率.计算结果表明,较小的正则化阶数p有助于提升LP-SVM的特征选择能力.
Feature selection plays an important role in machine learning. Feature s-election is to eliminate the redundant or irrelevant features, which results in a simplified but more information feature subset. The advantages of feature selec-tion include:preventing overfitting and improving the generalization performance, improving data interpretability and visualization, reducing the computation cost, reducing data measurement and storage requirements.
     We study feature selection strategies for support vector machine. Recently, the so called LP-SVM (0     1. We reformulate the LP-SVM into an optimization model with linear ob-jective function and smooth constraints (LOSC-SVM), so that it can be solved by efficient numerical methods for smooth constrained optimization. We establish the equivalence between LOSC-SVM and LP-SVM, and show some good properties of LOSC-SVM. This efficient reformulation would open a new path to develop nu-merical methods for Lp-SVM. Numerical experiments on artificial datasets show that the LOSC-SVM (0     2. Recent numerical researches have shown that the L1/2regularization prob-lem can be regarded as a representative in the Lp (0     3. We introduce a penalty successive linear programming algorithm (PSLP) to solve the L1/2-SVM. The PSLP can be applied to as large a problem as the LP code can handle, and is especially efficient on highly constrained problems. Under mild conditions, we establish the convergence of the PSLP algorithm. We explore the application of L1/2-SVM to gene selection for cancer classification. The characters of gene expression data are high dimensionality, huge redundancy and noise and highly imbalanced. Numerical results on three well-known gene expres-sion data sets support the effectiveness of the proposed method compared to other commonly-used regularization SVMs.
     4. To our knowledge, there is still no formal proof or comprehensive mathe-matical understanding on how Lp regularization can bring feature selection. We analyze theoretically the feature selection ability of the LP-SVM. We first discuss the possibilities of feature selection for specified data. The results show that fea-ture selection depends not only on the parameter p but also on the data itself. Then we provide a formula for computing the probabilities which measure the fea-ture selection ability. Based on this formula, we compute the probabilities for some popular p. Computations show that, small p really helps to enhance the feature selection ability of Lp-SVMs.
引文
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