Reference EPFL-ARTICLE-229257 URL: http: arxiv.org abs 0903.4856 Record created on 2017-06-21, modified on 2017-06-21
As @cite_8 point out, one drawback of their algorithm for the two-class SVM is that it does not work for singular kernel matrices, but requires that in the process of the algorithm, all occurring principal minors of the kernel matrix need to be invertible. The same is required by the other existing path algorithms mentioned above. However, large kernel matrices do often have very low numerical rank, even in those cases where radial base function kernels are used [Section 5.1] Hastie:2004p3702 , but of course also in the case of linear SVMs with sparse features, such as in the application to conjoint analysis discussed in this paper. The inability to deal with singular sub-matrices is probably one of the main reasons that none of the above mentioned algorithms could so far effectively be applied on medium larger scale problems @cite_8 @cite_11 . [Section 4.2] Rosset:2007p6880 report that their algorithm prematurely terminates on @math matrices due to this described problem.
@cite_11 @cite_8
2154462399 2031823405 172675048 2126326837
While classical kernel-based learning algorithms are based on a single kernel, in practice it is often desirable to use multiple kernels. (2004) considered conic combinations of kernel matrices for classification, leading to a convex quadratically constrained quadratic program. We show that it can be rewritten as a semi-infinite linear program that can be efficiently solved by recycling the standard SVM implementations. Moreover, we generalize the formulation and our method to a larger class of problems, including regression and one-class classification. Experimental results show that the proposed algorithm works for hundred thousands of examples or hundreds of kernels to be combined, and helps for automatic model selection, improving the interpretability of the learning result. In a second part we discuss general speed up mechanism for SVMs, especially when used with sparse feature maps as appear for string kernels, allowing us to train a string kernel SVM on a 10 million real-world splice data set from computational biology. We integrated multiple kernel learning in our machine learning toolbox SHOGUN for which the source code is publicly available at http: www.fml.tuebingen.mpg.de raetsch projects shogun . While classical kernel-based classifiers are based on a single kernel, in practice it is often desirable to base classifiers on combinations of multiple kernels. (2004) considered conic combinations of kernel matrices for the support vector machine (SVM), and showed that the optimization of the coefficients of such a combination reduces to a convex optimization problem known as a quadratically-constrained quadratic program (QCQP). Unfortunately, current convex optimization toolboxes can solve this problem only for a small number of kernels and a small number of data points; moreover, the sequential minimal optimization (SMO) techniques that are essential in large-scale implementations of the SVM cannot be applied because the cost function is non-differentiable. We propose a novel dual formulation of the QCQP as a second-order cone programming problem, and show how to exploit the technique of Moreau-Yosida regularization to yield a formulation to which SMO techniques can be applied. We present experimental results that show that our SMO-based algorithm is significantly more efficient than the general-purpose interior point methods available in current optimization toolboxes. Support vector machines (SVM) have been highly successful in many machine learning problems. Recently, it is also used for multi-instance (MI) learning by employing a kernel that is defined directly on the bags. As only the bags (but not the instances) have known labels, this MI kernel implicitly assumes all instances in the bag to be equally important. However, a fundamental property of MI learning is that not all instances in a positive bag necessarily belong to the positive class, and thus different instances in the same bag should have different contributions to the kernel. In this paper, we address this instance label ambiguity by using the method of marginalized kernels. It first assumes that all the instance labels are available and defines a label-dependent kernel on the instances. By integrating out the unknown instance labels, a marginalized kernel defined on the bags can then be obtained. A desirable property is that this kernel weights the instance pairs by the consistencies of their probabilistic instance labels. Experiments on both classification and regression data sets show that this marginalized MI kernel, when used in a standard SVM, performs consistently better than the original MI kernel. It also outperforms a number of traditional MI learning methods. Traditional classification approaches generalize poorly on image classification tasks, because of the high dimensionality of the feature space. This paper shows that support vector machines (SVM) can generalize well on difficult image classification problems where the only features are high dimensional histograms. Heavy-tailed RBF kernels of the form K(x, y)=e sup - spl rho spl Sigma sub i sup |xia-yia|b with a spl les 1 and b spl les 2 are evaluated on the classification of images extracted from the Corel stock photo collection and shown to far outperform traditional polynomial or Gaussian radial basis function (RBF) kernels. Moreover, we observed that a simple remapping of the input x sub i spl rarr x sub i sup a improves the performance of linear SVM to such an extend that it makes them, for this problem, a valid alternative to RBF kernels.