735 research outputs found
Imposing early and asymptotic constraints on LiGME with application to nonconvex enhancement of fused lasso models
For the constrained LiGME model, a nonconvexly regularized least squares
estimation model, under its overall convexity condition, we newly present an
iterative algorithm of guaranteed convergence to its globally optimal solution.
The proposed algorithm can deal with two different types of constraints
simultaneously. The first type called the asymptotic constraint requires for
the limit point of the produced sequence by the proposed algorithm to achieve
asymptotically. The second type called the early constraint requires for the
whole sequence by the proposed algorithm to satisfy. We also propose a
nonconvex and constraint enhancement of fused lasso models for sparse piecewise
constant signal estimations, possibly under nonzero baseline assumptions, to
which the proposed enhancement with two types of constraints can achieve
robustness against possible model mismatch as well as higher estimation
accuracy compared with conventional fused lasso type models.Comment: 5 pages, 7 figure
Adaptive Localized Cayley Parametrization for Optimization over Stiefel Manifold
We present an adaptive parametrization strategy for optimization problems
over the Stiefel manifold by using generalized Cayley transforms to utilize
powerful Euclidean optimization algorithms efficiently. The generalized Cayley
transform can translate an open dense subset of the Stiefel manifold into a
vector space, and the open dense subset is determined according to a tunable
parameter called a center point. With the generalized Cayley transform, we
recently proposed the naive Cayley parametrization, which reformulates the
optimization problem over the Stiefel manifold as that over the vector space.
Although this reformulation enables us to transplant powerful Euclidean
optimization algorithms, their convergences may become slow by a poor choice of
center points. To avoid such a slow convergence, in this paper, we propose to
estimate adaptively 'good' center points so that the reformulated problem can
be solved faster. We also present a unified convergence analysis, regarding the
gradient, in cases where fairly standard Euclidean optimization algorithms are
employed in the proposed adaptive parametrization strategy. Numerical
experiments demonstrate that (i) the proposed strategy succeeds in escaping
from the slow convergence observed in the naive Cayley parametrization
strategy; (ii) the proposed strategy outperforms the standard strategy which
employs a retraction.Comment: 29 pages, 4 figures, 4 table
A Unified Framework for Solving a General Class of Nonconvexly Regularized Convex Models
Recently, several nonconvex sparse regularizers which can preserve the
convexity of the cost function have received increasing attention. This paper
proposes a general class of such convexity-preserving (CP) regularizers, termed
partially smoothed difference-of-convex (pSDC) regularizer. The pSDC
regularizer is formulated as a structured difference-of-convex (DC) function,
where the landscape of the subtrahend function can be adjusted by a
parameterized smoothing function so as to attain overall-convexity. Assigned
with proper building blocks, the pSDC regularizer reproduces existing CP
regularizers and opens the way to a large number of promising new ones.
With respect to the resultant nonconvexly regularized convex (NRC) model, we
derive a series of overall-convexity conditions which naturally embrace the
conditions in previous works. Moreover, we develop a unified framework based on
DC programming for solving the NRC model. Compared to previously reported
proximal splitting type approaches, the proposed framework makes less stringent
assumptions. We establish the convergence of the proposed framework to a global
minimizer. Numerical experiments demonstrate the power of the pSDC regularizers
and the efficiency of the proposed DC algorithm.Comment: 15 pages, 6 figures, submitted to journa
Robust Reduced-Rank Adaptive Processing Based on Parallel Subgradient Projection and Krylov Subspace Techniques
In this paper, we propose a novel reduced-rank adaptive filtering algorithm
by blending the idea of the Krylov subspace methods with the set-theoretic
adaptive filtering framework. Unlike the existing Krylov-subspace-based
reduced-rank methods, the proposed algorithm tracks the optimal point in the
sense of minimizing the \sinq{true} mean square error (MSE) in the Krylov
subspace, even when the estimated statistics become erroneous (e.g., due to
sudden changes of environments). Therefore, compared with those existing
methods, the proposed algorithm is more suited to adaptive filtering
applications. The algorithm is analyzed based on a modified version of the
adaptive projected subgradient method (APSM). Numerical examples demonstrate
that the proposed algorithm enjoys better tracking performance than the
existing methods for the interference suppression problem in code-division
multiple-access (CDMA) systems as well as for simple system identification
problems.Comment: 10 figures. In IEEE Transactions on Signal Processing, 201
Adaptive Quadratic-Metric Parallel Subgradient Projection Algorithm and its Application to Acoustic Echo Cancellation
Publication in the conference proceedings of EUSIPCO, Florence, Italy, 200
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