Concentration of kernel matrices with application to kernel spectral clustering

We study the concentration of random kernel matrices around their mean. We derive nonasymptotic exponential concentration inequalities for Lipschitz kernels assuming that the data points are independent draws from a class of multivariate distributions on $\mathbb{R}^d$, including the strongly log-concave distributions under affine transformations. A feature of our result is that the data points need not have identical distributions or have zero mean, which is key in certain applications such as clustering. For comparison, we also derive the companion result for the Euclidean (inner product) kernel under a slightly modified set of distributional assumptions, more precisely, a class of sub-Gaussian vectors. A notable difference between the two cases is that, in contrast to the Euclidean kernel, in the Lipschitz case, the concentration inequality does not depend on the mean of the underlying vectors. As an application of these inequalities, we derive a bound on the misclassification rate of a kernel spectral clustering (KSC) algorithm, under a perturbed nonparametric mixture model. We show an example where this bound establishes the high-dimensional consistency (as $d \to \infty$) of the KSC, when applied with a Gaussian kernel, to a signal consisting of nested nonlinear manifolds (e.g., spheres) plus noise.