Matrix H-theory approach to stock market fluctuations

We introduce matrix H theory, a framework for analyzing collective behavior arising from multivariate stochastic processes with hierarchical structure. The theory models the joint distribution of the multiple variables (the measured signal) as a compound of a large-scale multivariate distribution with the distribution of a slowly fluctuating background. The background is characterized by a hierarchical stochastic evolution of internal degrees of freedom, representing the correlations between stocks at different timescales. As in its univariate version, the matrix H-theory formalism also has two universality classes, Wishart and inverse Wishart, enabling a concise description of both the background and the signal probability distributions in terms of Meijer G functions with matrix argument. Empirical analysis of daily returns of stocks within the S&P 500 demonstrates the effectiveness of matrix H theory in describing fluctuations in stock markets. These findings contribute to a deeper understanding of multivariate hierarchical processes and offer potential for developing more informed portfolio strategies in financial markets.