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Background  Positive semi-definiteness is a critical property in kernel methods for Support Vector Machine (SVM) by which efficient solutions can be guaranteed through convex quadratic programming. However, a lot of similarity functions in applications do not produce positive semi-definite kernels.    Methods  We propose projection method by constructing projection matrix on indefinite kernels. As a generalization of the spectrum method (denoising method and flipping method), the projection method shows better or comparable performance comparing to the corresponding indefinite kernel methods on a number of real world data sets. Under the Bregman matrix divergence theory, we can find suggested optimal  λ  in projection method using unconstrained optimization in kernel learning. In this paper we focus on optimal  λ  determination, in the pursuit of precise optimal  λ  determination method in unconstrained optimization framework. We developed a perturbed von-Neumann divergence to measure kernel relationships.    Results  We compared optimal  λ  determination with Logdet Divergence and perturbed von-Neumann Divergence, aiming at finding better  λ  in projection method. Results on a number of real world data sets show that projection method with optimal  λ  by Logdet divergence demonstrate near optimal performance. And the perturbed von-Neumann Divergence can help determine a relatively better optimal projection method.    Conclusions  Projection method ia easy to use for dealing with indefinite kernels. And the parameter embedded in the method can be determined through unconstrained optimization under Bregman matrix divergence theory. This may provide a new way in kernel SVMs for varied objectives.    Electronic supplementary material  The online version of this article (doi:10.1186/s12918-017-0479-0) contains supplementary material, which is available to authorized users.

Optimal projection method determination by Logdet Divergence and perturbed von-Neumann Divergence