PhD Thesis Proposal Defence
Title: "Optimization with a Low-Rank Regularization"
Mr. Quanming YAO
Low-rank modeling is important for machine learning, which has already been
widely used in many other areas. Examples are recommender systems and social
network analysis in data mining, image and video processing in computer vision.
As direct rank minimization is NP-hard, the nuclear norm, which is the tightest
convex envelope to the rank function, is often used for optimization. We first
show a state-of-the-art matrix completion algorithm, i.e., Soft-Impute, can be
accelerated. Its efficiency lies in utilizing the special "sparse plus
low-rank" structure of the matrix iterates, which allows efficient SVD in each
iteration. Though Soft-Impute is a proximal algorithm, it is generally believed
that acceleration destroys the special structure and is thus not useful. We
show that Soft-Impute can indeed be accelerated without comprising this
structure. To further reduce the iteration time complexity, we propose an
approximate singular value thresholding scheme based on the power method.
Theoretical analysis shows that the proposed algorithm still enjoys the fast
convergence rate of accelerated proximal algorithms.
While the matrix rank is often approximated by the convex nuclear norm, the use
of nonconvex low-rank regularizers has demonstrated better empirical
performance. However, the resulting optimization problem is much more
challenging. Recent state-of-the-art requires an expensive full SVD in each
iteration. We then show that for many commonly-used nonconvex low-rank
regularizers, a cutoff can be derived to automatically threshold the singular
values obtained from the proximal operator. This allows such operator being
efficiently approximated by power method. Based on it, we develop a proximal
algorithm (and its accelerated variant) with inexact proximal splitting and
prove its convergence to some critical points.
Finally, extensive experiments on both synthetic and real datasets demonstrate
the efficiency of developed algorithms.
Date: Tuesday, 21 November 2017
Time: 1:00pm - 3:00pm
Venue: Room 3494
Committee Members: Prof. James Kwok (Supervisor)
Dr. Yangqiu Song (Chairperson)
Prof. Dit-Yan Yeung
Prof. Nevin Zhang
**** ALL are Welcome ****