22 research outputs found
On the matrix square root via geometric optimization
This paper is triggered by the preprint "\emph{Computing Matrix Squareroot
via Non Convex Local Search}" by Jain et al.
(\textit{\textcolor{blue}{arXiv:1507.05854}}), which analyzes gradient-descent
for computing the square root of a positive definite matrix. Contrary to claims
of~\citet{jain2015}, our experiments reveal that Newton-like methods compute
matrix square roots rapidly and reliably, even for highly ill-conditioned
matrices and without requiring commutativity. We observe that gradient-descent
converges very slowly primarily due to tiny step-sizes and ill-conditioning. We
derive an alternative first-order method based on geodesic convexity: our
method admits a transparent convergence analysis ( page), attains linear
rate, and displays reliable convergence even for rank deficient problems.
Though superior to gradient-descent, ultimately our method is also outperformed
by a well-known scaled Newton method. Nevertheless, the primary value of our
work is its conceptual value: it shows that for deriving gradient based methods
for the matrix square root, \emph{the manifold geometric view of positive
definite matrices can be much more advantageous than the Euclidean view}.Comment: 8 pages, 12 plots, this version contains several more references and
more words about the rank-deficient cas
Riemannian kernel based Nystr\"om method for approximate infinite-dimensional covariance descriptors with application to image set classification
In the domain of pattern recognition, using the CovDs (Covariance
Descriptors) to represent data and taking the metrics of the resulting
Riemannian manifold into account have been widely adopted for the task of image
set classification. Recently, it has been proven that infinite-dimensional
CovDs are more discriminative than their low-dimensional counterparts. However,
the form of infinite-dimensional CovDs is implicit and the computational load
is high. We propose a novel framework for representing image sets by
approximating infinite-dimensional CovDs in the paradigm of the Nystr\"om
method based on a Riemannian kernel. We start by modeling the images via CovDs,
which lie on the Riemannian manifold spanned by SPD (Symmetric Positive
Definite) matrices. We then extend the Nystr\"om method to the SPD manifold and
obtain the approximations of CovDs in RKHS (Reproducing Kernel Hilbert Space).
Finally, we approximate infinite-dimensional CovDs via these approximations.
Empirically, we apply our framework to the task of image set classification.
The experimental results obtained on three benchmark datasets show that our
proposed approximate infinite-dimensional CovDs outperform the original CovDs.Comment: 6 pages, 3 figures, International Conference on Pattern Recognition
201
Riemannian Optimization via Frank-Wolfe Methods
We study projection-free methods for constrained Riemannian optimization. In
particular, we propose the Riemannian Frank-Wolfe (RFW) method. We analyze
non-asymptotic convergence rates of RFW to an optimum for (geodesically) convex
problems, and to a critical point for nonconvex objectives. We also present a
practical setting under which RFW can attain a linear convergence rate. As a
concrete example, we specialize Rfw to the manifold of positive definite
matrices and apply it to two tasks: (i) computing the matrix geometric mean
(Riemannian centroid); and (ii) computing the Bures-Wasserstein barycenter.
Both tasks involve geodesically convex interval constraints, for which we show
that the Riemannian "linear oracle" required by RFW admits a closed-form
solution; this result may be of independent interest. We further specialize RFW
to the special orthogonal group and show that here too, the Riemannian "linear
oracle" can be solved in closed form. Here, we describe an application to the
synchronization of data matrices (Procrustes problem). We complement our
theoretical results with an empirical comparison of Rfw against
state-of-the-art Riemannian optimization methods and observe that RFW performs
competitively on the task of computing Riemannian centroids.Comment: Under Review. Largely revised version, including an extended
experimental section and an application to the special orthogonal group and
the Procrustes proble
Component SPD Matrices: A lower-dimensional discriminative data descriptor for image set classification
In the domain of pattern recognition, using the SPD (Symmetric Positive
Definite) matrices to represent data and taking the metrics of resulting
Riemannian manifold into account have been widely used for the task of image
set classification. In this paper, we propose a new data representation
framework for image sets named CSPD (Component Symmetric Positive Definite).
Firstly, we obtain sub-image sets by dividing the image set into square blocks
with the same size, and use traditional SPD model to describe them. Then, we
use the results of the Riemannian kernel on SPD matrices as similarities of
corresponding sub-image sets. Finally, the CSPD matrix appears in the form of
the kernel matrix for all the sub-image sets, and CSPDi,j denotes the
similarity between i-th sub-image set and j-th sub-image set. Here, the
Riemannian kernel is shown to satisfy the Mercer's theorem, so our proposed
CSPD matrix is symmetric and positive definite and also lies on a Riemannian
manifold. On three benchmark datasets, experimental results show that CSPD is a
lower-dimensional and more discriminative data descriptor for the task of image
set classification.Comment: 8 pages,5 figures, Computational Visual Media, 201
A Riemannian Primal-dual Algorithm Based on Proximal Operator and its Application in Metric Learning
In this paper, we consider optimizing a smooth, convex, lower semicontinuous
function in Riemannian space with constraints. To solve the problem, we first
convert it to a dual problem and then propose a general primal-dual algorithm
to optimize the primal and dual variables iteratively. In each optimization
iteration, we employ a proximal operator to search optimal solution in the
primal space. We prove convergence of the proposed algorithm and show its
non-asymptotic convergence rate. By utilizing the proposed primal-dual
optimization technique, we propose a novel metric learning algorithm which
learns an optimal feature transformation matrix in the Riemannian space of
positive definite matrices. Preliminary experimental results on an optimal fund
selection problem in fund of funds (FOF) management for quantitative investment
showed its efficacy.Comment: 8 pages, 2 figures, published as a conference paper in 2019
International Joint Conference on Neural Networks (IJCNN