17 research outputs found
Local Linear Convergence of ISTA and FISTA on the LASSO Problem
We establish local linear convergence bounds for the ISTA and FISTA
iterations on the model LASSO problem. We show that FISTA can be viewed as an
accelerated ISTA process. Using a spectral analysis, we show that, when close
enough to the solution, both iterations converge linearly, but FISTA slows down
compared to ISTA, making it advantageous to switch to ISTA toward the end of
the iteration processs. We illustrate the results with some synthetic numerical
examples
Sensitivity Analysis for Mirror-Stratifiable Convex Functions
This paper provides a set of sensitivity analysis and activity identification
results for a class of convex functions with a strong geometric structure, that
we coined "mirror-stratifiable". These functions are such that there is a
bijection between a primal and a dual stratification of the space into
partitioning sets, called strata. This pairing is crucial to track the strata
that are identifiable by solutions of parametrized optimization problems or by
iterates of optimization algorithms. This class of functions encompasses all
regularizers routinely used in signal and image processing, machine learning,
and statistics. We show that this "mirror-stratifiable" structure enjoys a nice
sensitivity theory, allowing us to study stability of solutions of optimization
problems to small perturbations, as well as activity identification of
first-order proximal splitting-type algorithms. Existing results in the
literature typically assume that, under a non-degeneracy condition, the active
set associated to a minimizer is stable to small perturbations and is
identified in finite time by optimization schemes. In contrast, our results do
not require any non-degeneracy assumption: in consequence, the optimal active
set is not necessarily stable anymore, but we are able to track precisely the
set of identifiable strata.We show that these results have crucial implications
when solving challenging ill-posed inverse problems via regularization, a
typical scenario where the non-degeneracy condition is not fulfilled. Our
theoretical results, illustrated by numerical simulations, allow to
characterize the instability behaviour of the regularized solutions, by
locating the set of all low-dimensional strata that can be potentially
identified by these solutions
Local Linear Convergence Analysis of Primal-Dual Splitting Methods
In this paper, we study the local linear convergence properties of a
versatile class of Primal-Dual splitting methods for minimizing composite
non-smooth convex optimization problems. Under the assumption that the
non-smooth components of the problem are partly smooth relative to smooth
manifolds, we present a unified local convergence analysis framework for these
methods. More precisely, in our framework we first show that (i) the sequences
generated by Primal-Dual splitting methods identify a pair of primal and dual
smooth manifolds in a finite number of iterations, and then (ii) enter a local
linear convergence regime, which is characterized based on the structure of the
underlying active smooth manifolds. We also show how our results for
Primal-Dual splitting can be specialized to cover existing ones on
Forward-Backward splitting and Douglas-Rachford splitting/ADMM (alternating
direction methods of multipliers). Moreover, based on these obtained local
convergence analysis result, several practical acceleration techniques are
discussed. To exemplify the usefulness of the obtained result, we consider
several concrete numerical experiments arising from fields including
signal/image processing, inverse problems and machine learning, etc. The
demonstration not only verifies the local linear convergence behaviour of
Primal-Dual splitting methods, but also the insights on how to accelerate them
in practice