These functions are a fundamental tool in many multivariate statistical methods, such as hypothesis testing, principal component analysis, canonical correlation analysis, multivariate analysis of variance, etc. (Muirhead, 1982). This type of analysis is an integral part of many practical applications where multiple signal sources/receivers are present and thus random covariance matrices naturally occur. Applications include telecommunications and wireless networks, image and signal processing, military applications (automatic 3D target recognition and classification), etc.
FIGURE: Blue = Analytical, Red = Empirical
LEFT: p.d.f. of the trace
of the 3-by-3 Wishart ensemble with 4 degrees of freedom and covariance
matrix diag(0.25,0.4,1).
RIGHT: c.d.f. of the largest eigenvalue of the 3-by-3 complex
Wishart
ensemble with 4 degrees of freedom and covariance matrix diag(1.1,1.2,1.2).
Many explicit formulas for the distributions of the eigenvalues of the classical random matrix ensembles have been known for over 40 years (Constantine 1963, James 1964, Muirhead 1982). Unfortunately, most of these formulas are in terms of the hypergeometric function of a matrix argument - an extremely slowly converging series of Jack functions.
The hypergeometric function of a matrix argument has been incredibly difficult to compute even in the simplest cases (matrix argument of size 3 or 4); the development of efficient algorithms for its computation has been identified as a central open research problem in a large number of recent publications. (In about 50% of the papers returned by a search at http://ieeexplore.ieee.org for "hypergeometric function" and "matrix argument").
Recently, I developed the first practical algorithm (see also our paper) for computing the hypergeometric function of a matrix argument. This new algorithm is very efficient for matrix arguments of size up to 10 (takes at most a few seconds time) and is exponentially faster than the previous best algorithm of Gurierrez, Rodriguez and Saez (ETNA, 2000): on the same 5-by-5 example our algorithm takes less than 1/100 of a second as opposed to 8 days.
Jointly with Ioana Dumitriu, I also established formulas for the distributions of the extreme eigenvalues of the Jacobi random matrix ensemble.
My algorithms for computing the hypergeometric function of a matrix argument were used to produce the figures above. It can also be used to observe the convergence of the largest eigenvalue of a Wishart matrix to the Tracy - Widom limit of order 1, below (see Tracy and Widom, 2000). Such limits apply much more generally (Soshnikov, 2002).
Figure: Convergence of the density of the largest eigenvalue of the m-by-m
Wishart matrix
with 2m degrees of freedom to the Tracy-Widom law of order 1.