correlation methods integral equations matrix algebra statistical analysis Hawkes kernel matrix Wiener-Hopf integral equations circular dependence correlation matrix earthquakes occurrence dynamics estimation error financial markets first-order statistics characterization high-frequency trading events monovariate processes multivariate Hawkes process nonparametric estimation procedure nonpositive kernels numerical inversion power-law second-order statistics characterization three-variate processes Correlation Kernel Mathematical model Shape Stochastic processes covariance matrices discrete-event systems earthquakes estimation inverse problems microstructure multivariate point processes