6 research outputs found
Oracle Complexity Reduction for Model-free LQR: A Stochastic Variance-Reduced Policy Gradient Approach
We investigate the problem of learning an -approximate solution for
the discrete-time Linear Quadratic Regulator (LQR) problem via a Stochastic
Variance-Reduced Policy Gradient (SVRPG) approach. Whilst policy gradient
methods have proven to converge linearly to the optimal solution of the
model-free LQR problem, the substantial requirement for two-point cost queries
in gradient estimations may be intractable, particularly in applications where
obtaining cost function evaluations at two distinct control input
configurations is exceptionally costly. To this end, we propose an
oracle-efficient approach. Our method combines both one-point and two-point
estimations in a dual-loop variance-reduced algorithm. It achieves an
approximate optimal solution with only
two-point cost information
for
Learning Personalized Models with Clustered System Identification
We address the problem of learning linear system models from observing
multiple trajectories from different system dynamics. This framework
encompasses a collaborative scenario where several systems seeking to estimate
their dynamics are partitioned into clusters according to their system
similarity. Thus, the systems within the same cluster can benefit from the
observations made by the others. Considering this framework, we present an
algorithm where each system alternately estimates its cluster identity and
performs an estimation of its dynamics. This is then aggregated to update the
model of each cluster. We show that under mild assumptions, our algorithm
correctly estimates the cluster identities and achieves an approximate sample
complexity that scales inversely with the number of systems in the cluster,
thus facilitating a more efficient and personalized system identification
process
Regional stability analysis of transitional fluid flows
A method to bound the maximum energy perturbation for which regional stability of transitional fluid flow models can be guaranteed is introduced. The proposed method exploits the fact that the fluid model’s nonlinearities are both lossless and locally bounded and uses the axes lengths of the ellipsoids for the trajectory set containment as variables in the stability conditions. Compared to existing approaches based on quadratic constraints, the proposed method leads to an average increase in the maximum allowable energy perturbation of ≈ 29% for the 4-state Waleffe-Kim-Hamilton (WKH) shear flow model and of ≈ 38% for the 9-state reduced model of Couette flow
Expression Analysis of Dopamine Receptor Subtypes in Normal Human Pituitaries, Nonfunctioning Pituitary Adenomas and Somatotropinomas, and the Association between Dopamine and Somatostatin Receptors with Clinical Response to Octreotide-LAR in Acromegaly
Context: Dopamine receptor (DR) and somatostatin receptor subtype expression in pituitary adenomas may predict the response to postsurgical therapies
Stoma-free survival after anastomotic leak following rectal cancer resection: worldwide cohort of 2470 patients
Background: The optimal treatment of anastomotic leak after rectal cancer resection is unclear. This worldwide cohort study aimed to provide an overview of four treatment strategies applied. Methods: Patients from 216 centres and 45 countries with anastomotic leak after rectal cancer resection between 2014 and 2018 were included. Treatment was categorized as salvage surgery, faecal diversion with passive or active (vacuum) drainage, and no primary/secondary faecal diversion. The primary outcome was 1-year stoma-free survival. In addition, passive and active drainage were compared using propensity score matching (2: 1). Results: Of 2470 evaluable patients, 388 (16.0 per cent) underwent salvage surgery, 1524 (62.0 per cent) passive drainage, 278 (11.0 per cent) active drainage, and 280 (11.0 per cent) had no faecal diversion. One-year stoma-free survival rates were 13.7, 48.3, 48.2, and 65.4 per cent respectively. Propensity score matching resulted in 556 patients with passive and 278 with active drainage. There was no statistically significant difference between these groups in 1-year stoma-free survival (OR 0.95, 95 per cent c.i. 0.66 to 1.33), with a risk difference of -1.1 (95 per cent c.i. -9.0 to 7.0) per cent. After active drainage, more patients required secondary salvage surgery (OR 2.32, 1.49 to 3.59), prolonged hospital admission (an additional 6 (95 per cent c.i. 2 to 10) days), and ICU admission (OR 1.41, 1.02 to 1.94). Mean duration of leak healing did not differ significantly (an additional 12 (-28 to 52) days). Conclusion: Primary salvage surgery or omission of faecal diversion likely correspond to the most severe and least severe leaks respectively. In patients with diverted leaks, stoma-free survival did not differ statistically between passive and active drainage, although the increased risk of secondary salvage surgery and ICU admission suggests residual confounding
Stoma-free Survival After Rectal Cancer Resection With Anastomotic Leakage: Development and Validation of a Prediction Model in a Large International Cohort.
Objective:To develop and validate a prediction model (STOMA score) for 1-year stoma-free survival in patients with rectal cancer (RC) with anastomotic leakage (AL).Background:AL after RC resection often results in a permanent stoma.Methods:This international retrospective cohort study (TENTACLE-Rectum) encompassed 216 participating centres and included patients who developed AL after RC surgery between 2014 and 2018. Clinically relevant predictors for 1-year stoma-free survival were included in uni and multivariable logistic regression models. The STOMA score was developed and internally validated in a cohort of patients operated between 2014 and 2017, with subsequent temporal validation in a 2018 cohort. The discriminative power and calibration of the models' performance were evaluated.Results:This study included 2499 patients with AL, 1954 in the development cohort and 545 in the validation cohort. Baseline characteristics were comparable. One-year stoma-free survival was 45.0% in the development cohort and 43.7% in the validation cohort. The following predictors were included in the STOMA score: sex, age, American Society of Anestesiologist classification, body mass index, clinical M-disease, neoadjuvant therapy, abdominal and transanal approach, primary defunctioning stoma, multivisceral resection, clinical setting in which AL was diagnosed, postoperative day of AL diagnosis, abdominal contamination, anastomotic defect circumference, bowel wall ischemia, anastomotic fistula, retraction, and reactivation leakage. The STOMA score showed good discrimination and calibration (c-index: 0.71, 95% CI: 0.66-0.76).Conclusions:The STOMA score consists of 18 clinically relevant factors and estimates the individual risk for 1-year stoma-free survival in patients with AL after RC surgery, which may improve patient counseling and give guidance when analyzing the efficacy of different treatment strategies in future studies