19 research outputs found
Reliable Error Estimates for Optimal Control of Linear Elliptic PDEs with Random Inputs
We discretize a risk-neutral optimal control problem governed by a linear
elliptic partial differential equation with random inputs using a Monte Carlo
sample-based approximation and a finite element discretization, yielding finite
dimensional control problems. We establish an exponential tail bound for the
distance between the finite dimensional problems' solutions and the
risk-neutral problem's solution. The tail bound implies that solutions to the
risk-neutral optimal control problem can be reliably estimated with the
solutions to the finite dimensional control problems. Numerical simulations
illustrate our theoretical findings.Comment: 26 pages, 11 figure
Consistency of sample-based stationary points for infinite-dimensional stochastic optimization
We consider stochastic optimization problems with possibly nonsmooth
integrands posed in Banach spaces and approximate these stochastic programs via
a sample-based approaches. We establish the consistency of approximate Clarke
stationary points of the sample-based approximations. Our framework is applied
to risk-averse semilinear PDE-constrained optimization using the average
value-at-risk and to risk-neutral bilinear PDE-constrained optimization.Comment: 20 page
Sample Average Approximations of Strongly Convex Stochastic Programs in Hilbert Spaces
We analyze the tail behavior of solutions to sample average approximations
(SAAs) of stochastic programs posed in Hilbert spaces. We require that the
integrand be strongly convex with the same convexity parameter for each
realization. Combined with a standard condition from the literature on
stochastic programming, we establish non-asymptotic exponential tail bounds for
the distance between the SAA solutions and the stochastic program's solution,
without assuming compactness of the feasible set. Our assumptions are verified
on a class of infinite-dimensional optimization problems governed by
affine-linear partial differential equations with random inputs. We present
numerical results illustrating our theoretical findings.Comment: 20 pages, 4 figure
Sample Size Estimates for Risk-Neutral Semilinear PDE-Constrained Optimization
The sample average approximation (SAA) approach is applied to risk-neutral
optimization problems governed by semilinear elliptic partial differential
equations with random inputs. After constructing a compact set that contains
the SAA critical points, we derive nonasymptotic sample size estimates for SAA
critical points using the covering number approach. Thereby, we derive upper
bounds on the number of samples needed to obtain accurate critical points of
the risk-neutral PDE-constrained optimization problem through SAA critical
points. We quantify accuracy using expectation and exponential tail bounds.
Numerical illustrations are presented.Comment: 26 pages, 10 figure
Asymptotic Consistency for Nonconvex Risk-Averse Stochastic Optimization with Infinite Dimensional Decision Spaces
Optimal values and solutions of empirical approximations of stochastic
optimization problems can be viewed as statistical estimators of their true
values. From this perspective, it is important to understand the asymptotic
behavior of these estimators as the sample size goes to infinity. This area of
study has a long tradition in stochastic programming. However, the literature
is lacking consistency analysis for problems in which the decision variables
are taken from an infinite dimensional space, which arise in optimal control,
scientific machine learning, and statistical estimation. By exploiting the
typical problem structures found in these applications that give rise to hidden
norm compactness properties for solution sets, we prove consistency results for
nonconvex risk-averse stochastic optimization problems formulated in infinite
dimensional space. The proof is based on several crucial results from the
theory of variational convergence. The theoretical results are demonstrated for
several important problem classes arising in the literature.Comment: 24 page
The BDNFVal66Met SNP modulates the association between beta-amyloid and hippocampal disconnection in Alzheimer’s disease
In Alzheimer’s disease (AD), a single-nucleotide polymorphism in the gene encoding brain-derived neurotrophic factor (BDNFVal66Met) is associated with worse impact of primary AD pathology (beta-amyloid, Aβ) on neurodegeneration and cognitive decline, rendering BDNFVal66Met an important modulating factor of cognitive impairment in AD. However, the effect of BDNFVal66Met on functional networks that may underlie cognitive impairment in AD is poorly understood. Using a cross-validation approach, we first explored in subjects with autosomal dominant AD (ADAD) from the Dominantly Inherited Alzheimer Network (DIAN) the effect of BDNFVal66Met on resting-state fMRI assessed functional networks. In seed-based connectivity analysis of six major large-scale networks, we found a stronger decrease of hippocampus (seed) to medial-frontal connectivity in the BDNFVal66Met carriers compared to BDNFVal homozogytes. BDNFVal66Met was not associated with connectivity in any other networks. Next, we tested whether the finding of more pronounced decrease in hippocampal-medial-frontal connectivity in BDNFVal66Met could be also found in elderly subjects with sporadically occurring Aβ, including a group with subjective cognitive decline (N = 149, FACEHBI study) and a group ranging from preclinical to AD dementia (N = 114, DELCODE study). In both of these independently recruited groups, BDNFVal66Met was associated with a stronger effect of more abnormal Aβ-levels (assessed by biofluid-assay or amyloid-PET) on hippocampal-medial-frontal connectivity decreases, controlled for hippocampus volume and other confounds. Lower hippocampal-medial-frontal connectivity was associated with lower global cognitive performance in the DIAN and DELCODE studies. Together these results suggest that BDNFVal66Met is selectively associated with a higher vulnerability of hippocampus-frontal connectivity to primary AD pathology, resulting in greater AD-related cognitive impairment
Consistency of Monte Carlo Estimators for Risk-Neutral PDE-Constrained Optimization
We apply the sample average approximation (SAA) method to risk-neutral
optimization problems governed by nonlinear partial differential equations
(PDEs) with random inputs. We analyze the consistency of the SAA optimal values
and SAA solutions. Our analysis exploits problem structure in PDE-constrained
optimization problems, allowing us to construct deterministic, compact subsets
of the feasible set that contain the solutions to the risk-neutral problem and
eventually those to the SAA problems. The construction is used to study the
consistency using results established in the literature on stochastic
programming. The assumptions of our framework are verified on three nonlinear
optimization problems under uncertainty.Comment: Revised introduction and appendix; 25 page
Osseous microarchitecture in frequent fracture zones of the distal clavicle.
BACKGROUND
Fracture classifications of the distal clavicle are based on ligamentous integrity. The influence of osseous microarchitecture on fracture occurrence, morphology, and the lesion's stability has not yet been investigated. We aimed to characterize osseous microarchitecture according to common fracture classification systems based on ligamentous integrity and investigated the possible effects of age, gender, and osteoporosis in distal clavicle fractures.
METHODS
N = 20 human cadaveric distal clavicles were scanned using XtremeCT with an isometric voxel size of 82 μm. In the sagittal plane, each data set was evaluated in 11 sections of approximately 7 mm thickness. Three topographic regions were defined: the bone lateral to the trapezoid (LTR), intertubercular (ITR), and medial to the conoid (MCR) ligament. Cortical bone mineral density (BMD) [mgHA/cm3] and cortical porosity (1- (BV/TV) [%]) were determined and evaluated relative to age and gender.
RESULTS
Along the mediolateral axis, there was an >20-fold increase in median cortical porosity (P ≤ .001). There were significant differences in cortical porosity between LTR and ITR (P ≤ .001) but not between ITR and MCR (P = .09). In ITR, cortical porosity was significantly greater in >60-year-old compared to younger donors (P = .01). For BMD, there was an >2-fold decrease toward the distal apex (P ≤ .001). BMD was significantly greater in ITR compared to LTR (P ≤ .001) and in MCR compared to ITR (P = .02). In ITR and MCR, clavicles of >60-year-old donors had significantly lower BMD values compared to younger donors (P 60-year-olds (P > .6).
CONCLUSION
The distal clavicle features a characteristic bony microarchitecture. The present study revealed a significant difference in bone quality of lateral, intertubercular, and medial zones of the distal clavicle and could specify target areas and strategies for surgical treatment of unstable fractures. Age, gender, and osteoporosis have a limited effect on bone quality and fracture genesis. In contrast, ligamentous quality is supposed to exert a substantial influence on fracture characteristics, especially in ITR. Fracture morphology of the distal clavicle is determined by a bony-ligamentous conjunction, which remains to be characterized