30 research outputs found
A trust region-type normal map-based semismooth Newton method for nonsmooth nonconvex composite optimization
We propose a novel trust region method for solving a class of nonsmooth and
nonconvex composite-type optimization problems. The approach embeds inexact
semismooth Newton steps for finding zeros of a normal map-based stationarity
measure for the problem in a trust region framework. Based on a new merit
function and acceptance mechanism, global convergence and transition to fast
local q-superlinear convergence are established under standard conditions. In
addition, we verify that the proposed trust region globalization is compatible
with the Kurdyka-{\L}ojasiewicz (KL) inequality yielding finer convergence
results. We further derive new normal map-based representations of the
associated second-order optimality conditions that have direct connections to
the local assumptions required for fast convergence. Finally, we study the
behavior of our algorithm when the Hessian matrix of the smooth part of the
objective function is approximated by BFGS updates. We successfully link the KL
theory, properties of the BFGS approximations, and a Dennis-Mor{\'e}-type
condition to show superlinear convergence of the quasi-Newton version of our
method. Numerical experiments on sparse logistic regression and image
compression illustrate the efficiency of the proposed algorithm.Comment: 56 page
Variational Properties of Decomposable Functions Part II: Strong Second-Order Theory
Local superlinear convergence of the semismooth Newton method usually
requires the uniform invertibility of the generalized Jacobian matrix, e.g.
BD-regularity or CD-regularity. For several types of nonlinear programming and
composite-type optimization problems -- for which the generalized Jacobian of
the stationary equation can be calculated explicitly -- this is characterized
by the strong second-order sufficient condition. However, general
characterizations are still not well understood. In this paper, we propose a
strong second-order sufficient condition (SSOSC) for composite problems whose
nonsmooth part has a generalized conic-quadratic second subderivative. We then
discuss the relationship between the SSOSC and another second order-type
condition that involves the generalized Jacobians of the normal map. In
particular, these two conditions are equivalent under certain structural
assumptions on the generalized Jacobian matrix of the proximity operator. Next,
we verify these structural assumptions for -strictly decomposable
functions via analyzing their second-order variational properties under
additional geometric assumptions on the support set of the decomposition pair.
Finally, we show that the SSOSC is further equivalent to the strong metric
regularity condition of the subdifferential, the normal map, and the natural
residual. Counterexamples illustrate the necessity of our assumptions.Comment: 28 pages; preliminary draf
Nonmonotone globalization for Anderson acceleration via adaptive regularization
Anderson acceleration (AA) is a popular method for accelerating fixed-point iterations, but may suffer from instability and stagnation. We propose a globalization method for AA to improve stability and achieve unified global and local convergence. Unlike existing AA globalization approaches that rely on safeguarding operations and might hinder fast local convergence, we adopt a nonmonotone trust-region framework and introduce an adaptive quadratic regularization together with a tailored acceptance mechanism. We prove global convergence and show that our algorithm attains the same local convergence as AA under appropriate assumptions. The effectiveness of our method is demonstrated in several numerical experiments
Accelerating ADMM for efficient simulation and optimization
The alternating direction method of multipliers (ADMM) is a popular approach for solving optimization problems that are potentially non-smooth and with hard constraints. It has been applied to various computer graphics applications, including physical simulation, geometry processing, and image processing. However, ADMM can take a long time to converge to a solution of high accuracy. Moreover, many computer graphics tasks involve non-convex optimization, and there is often no convergence guarantee for ADMM on such problems since it was originally designed for convex optimization. In this paper, we propose a method to speed up ADMM using Anderson acceleration, an established technique for accelerating fixed-point iterations. We show that in the general case, ADMM is a fixed-point iteration of the second primal variable and the dual variable, and Anderson acceleration can be directly applied. Additionally, when the problem has a separable target function and satisfies certain conditions, ADMM becomes a fixed-point iteration of only one variable, which further reduces the computational overhead of Anderson acceleration. Moreover, we analyze a particular non-convex problem structure that is common in computer graphics, and prove the convergence of ADMM on such problems under mild assumptions. We apply our acceleration technique on a variety of optimization problems in computer graphics, with notable improvement on their convergence speed
Anderson acceleration for nonconvex ADMM based on Douglas-Rachford splitting
The alternating direction multiplier method (ADMM) is widely used in computer graphics for solving optimization problems that can be nonsmooth and nonconvex. It converges quickly to an approximate solution, but can take a long time to converge to a solution of high-accuracy. Previously, Anderson acceleration has been applied to ADMM, by treating it as a fixed-point iteration for the concatenation of the dual variables and a subset of the primal variables. In this paper, we note that the equivalence between ADMM and Douglas-Rachford splitting reveals that ADMM is in fact a fixed-point iteration in a lower-dimensional space. By applying Anderson acceleration to such lower-dimensional fixed-point iteration, we obtain a more effective approach for accelerating ADMM. We analyze the convergence of the proposed acceleration method on nonconvex problems, and verify its effectiveness on a variety of computer graphics problems including geometry processing and physical simulation
Variational Properties of Decomposable Functions. Part I: Strict Epi-Calculus and Applications
We provide systematic studies of the variational properties of decomposable
functions which are compositions of an outer support function and an inner
smooth mapping under certain constraint qualifications. We put a particular
focus on the strict twice epi-differentiability and the associated strict
second subderivative of such functions. Calculus rules for the (strict) second
subderivative and twice epi-differentiability of decomposable functions are
derived which allow us to link the (strict) second subderivative of
decomposable mappings to the simpler outer support function. Applying the
variational properties of the support function, we establish the equivalence
between strict twice epi-differentiability of decomposable functions,
continuous differentiability of its proximity operator, and the strict
complementarity condition. This allows us to fully characterize the strict
saddle point property of decomposable functions. In addition, we give a formula
for the strict second subderivative for decomposable functions whose outer
support set is sufficiently regular. This provides an alternative
characterization of the strong metric regularity of the subdifferential of
decomposable functions. Finally, we verify that these introduced regularity
conditions are satisfied by many practical functions.Comment: 28 pages; preliminary draf
Descent Properties of an Anderson Accelerated Gradient Method With Restarting
Anderson Acceleration (AA) is a popular acceleration technique to enhance the
convergence of fixed-point iterations. The analysis of AA approaches typically
focuses on the convergence behavior of a corresponding fixed-point residual,
while the behavior of the underlying objective function values along the
accelerated iterates is currently not well understood. In this paper, we
investigate local properties of AA with restarting applied to a basic gradient
scheme in terms of function values. Specifically, we show that AA with
restarting is a local descent method and that it can decrease the objective
function faster than the gradient method. These new results theoretically
support the good numerical performance of AA when heuristic descent conditions
are used for globalization and they provide a novel perspective on the
convergence analysis of AA that is more amenable to nonconvex optimization
problems. Numerical experiments are conducted to illustrate our theoretical
findings.Comment: 23 pages; 4 figure
The ‘Perfect’ Conversion: Dramatic Increase in CO2 Efflux from Shellfish Ponds and Mangrove Conversion in China
Aquaculture, particularly shellfish ponds, has expanded dramatically and become a major cause of mangrove deforestation and “blue carbon” loss in China. We present the first study to examine CO2 efflux from marine aquaculture/shellfish ponds and in relation to land-use change from mangrove forests in China. Light and dark sediment CO2 efflux from shellfish ponds averaged at 0.61 ± 0.07 and 0.90 ± 0.12 kg CO2 m−2 yr−1 (= 37.67 ± 4.89 and 56.0 ± 6.13 mmol m−2 d−1), respectively. The corresponding rates (−4.21 ± 4.54 and 41.01 ± 4.15 mmol m−2 d−1) from the adjacent mangrove forests that were devoid of aquaculture wastewater were lower, while those from the adjacent mangrove forests (3.48 ± 7.83 and 73.02 ± 5.76 mmol m−2 d−1)) receiving aquaculture wastewater markedly increased. These effluxes are significantly higher than those reported for mangrove forests to date, which is attributable to the high nutrient levels and the physical disturbance of the substrate associated with the aquaculture operation. A rise of 1 °C in the sediment temperature resulted in a 6.56% rise in CO2 released from the shellfish ponds. Combined with pond area data, the total CO2 released from shellfish ponds in 2019 was estimated to be ~12 times that in 1983. The total annual CO2 emission from shellfish ponds associated with mangrove conversion reached 2–5 Tg, offsetting the C storage by mangrove forests in China. These are significant environmental consequences rather than just a simple shift of land use. Around 30% higher CO2 emissions are expected from aquaculture ponds (including shellfish ponds) compared to shellfish ponds alone. Total annual CO2 emission from shellfish ponds will likely decrease to the level reported in early 1980 under the pond area-shrinking scenario, but it will be more than doubled under the business-as-usual scenario projected for 2050. This study highlights the necessity of curbing the expansion of aquaculture ponds in valuable coastal wetlands and increasing mangrove restoration to abandoned ponds
The âPerfectâ Conversion: Dramatic Increase in CO<sub>2</sub> Efflux from Shellfish Ponds and Mangrove Conversion in China
Aquaculture, particularly shellfish ponds, has expanded dramatically and become a major cause of mangrove deforestation and âblue carbonâ loss in China. We present the first study to examine CO2 efflux from marine aquaculture/shellfish ponds and in relation to land-use change from mangrove forests in China. Light and dark sediment CO2 efflux from shellfish ponds averaged at 0.61 ± 0.07 and 0.90 ± 0.12 kg CO2 mâ2 yrâ1 (= 37.67 ± 4.89 and 56.0 ± 6.13 mmol mâ2 dâ1), respectively. The corresponding rates (â4.21 ± 4.54 and 41.01 ± 4.15 mmol mâ2 dâ1) from the adjacent mangrove forests that were devoid of aquaculture wastewater were lower, while those from the adjacent mangrove forests (3.48 ± 7.83 and 73.02 ± 5.76 mmol mâ2 dâ1)) receiving aquaculture wastewater markedly increased. These effluxes are significantly higher than those reported for mangrove forests to date, which is attributable to the high nutrient levels and the physical disturbance of the substrate associated with the aquaculture operation. A rise of 1 °C in the sediment temperature resulted in a 6.56% rise in CO2 released from the shellfish ponds. Combined with pond area data, the total CO2 released from shellfish ponds in 2019 was estimated to be ~12 times that in 1983. The total annual CO2 emission from shellfish ponds associated with mangrove conversion reached 2â5 Tg, offsetting the C storage by mangrove forests in China. These are significant environmental consequences rather than just a simple shift of land use. Around 30% higher CO2 emissions are expected from aquaculture ponds (including shellfish ponds) compared to shellfish ponds alone. Total annual CO2 emission from shellfish ponds will likely decrease to the level reported in early 1980 under the pond area-shrinking scenario, but it will be more than doubled under the business-as-usual scenario projected for 2050. This study highlights the necessity of curbing the expansion of aquaculture ponds in valuable coastal wetlands and increasing mangrove restoration to abandoned ponds