| 1. |
- Anagnostopoulos, George, et al.
(författare)
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Strain Engineering in Highly Wrinkled CVD Graphene/Epoxy Systems
- 2018
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 10:49, s. 43192-43202
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Tidskriftsartikel (refereegranskat)abstract
- Chemical vapor deposition (CVD) is regarded as a promising fabrication method for the automated, large-scale, production of graphene and other two-dimensional materials. However, its full commercial exploitation is limited by the presence of structural imperfections such as folds, wrinkles, and even cracks that downgrade its physical and mechanical properties. For example, as shown here by means of Raman spectroscopy, the stress transfer from an epoxy matrix to CVD graphene is on average 30% of that of exfoliated monolayer graphene of over 10 μm in dimensions. However, in terms of electrical response, the situation is reversed; the resistance has been found here to decrease by the imposition of mechanical deformation possibly due to the opening up of the structure and the associated increase of electron mobility. This finding paves the way for employing CVD graphene/epoxy composites or coatings as conductive "networks" or bridges in cases for which the conductivity needs to be increased or at least retained when the system is under deformation. The tuning/control of such systems and their operative limitations are discussed here.
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| 2. |
- Alur, Rajeev, et al.
(författare)
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Compositional Modeling and Analysis of Multi-Hop Control Networks
- 2011
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Ingår i: IEEE Transactions on Automatic Control. - 0018-9286 .- 1558-2523. ; 56:10, s. 2345-2357
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Tidskriftsartikel (refereegranskat)abstract
- We propose a mathematical framework for modeling and analyzing multi-hop control networks designed for systems consisting of multiple control loops closed over a multi-hop (wireless) communication network. We separate control, topology, routing, and scheduling and propose formal syntax and semantics for the dynamics of the composed system, providing an explicit translation of multi-hop control networks to switched systems. We propose formal models for analyzing robustness of multi-hop control networks, where data is exchanged through a multi-hop communication network subject to disruptions. When communication disruptions are long, compared to the speed of the control system, we propose to model them as permanent link failures. We show that the complexity of analyzing such failures is NP-hard, and discuss a way to overcome this limitation for practical cases using compositional analysis. For typical packet transmission errors, we propose a transient error model where links fail for one time slot independently of the past and of other links. We provide sufficient conditions for almost sure stability in presence of transient link failures, and give efficient decision procedures. We deal with errors that have random time span and show that, under some conditions, the permanent failure model can be used as a reliable abstraction. Our approach is compositional, namely it addresses the problem of designing scalable scheduling and routing policies for multiple control loops closed on the same multi-hop control network. We describe how the translation of multi-hop control networks to switched systems can be automated, and use it to solve control and networking co-design challenges in some representative examples, and to propose a scheduling solution in a mineral floatation control problem that can be implemented on a time triggered communication protocols for wireless networks.
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| 3. |
- Arun, K. G., et al.
(författare)
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New horizons for fundamental physics with LISA
- 2022
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Ingår i: Living Reviews in Relativity. - : Springer Science and Business Media LLC. - 1433-8351 .- 2367-3613. ; 25:1
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Forskningsöversikt (refereegranskat)abstract
- The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.
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| 4. |
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| 5. |
- Barack, Leor, et al.
(författare)
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Black holes, gravitational waves and fundamental physics : a roadmap
- 2019
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 36:14
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Forskningsöversikt (refereegranskat)abstract
- The grand challenges of contemporary fundamental physics dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'.
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| 6. |
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| 7. |
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| 8. |
- Speranzon, Alberto, 1975-
(författare)
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Coordination, Consensus and Communication in Multi-robot Control Systems
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Analysis, design and implementation of cooperative control strategies for multi-robot systems under communication constraints is the topic of this thesis. Motivated by a rapidly growing number of applications with networked robots and other vehicles, fundamental limits on the achievable collaborative behavior are studied for large teams of autonomous agents. In particular, a problem is researched in detail in which the group of agents is supposed to agree on a common state without any centralized coordination. Due to the dynamics of the individual agents and their varying connectivity, this problemis an extension of the classical consensus problemin computer science. It captures a crucial component of many desirable features of multi-robot systems, such as formation, flocking, rendezvous, synchronizing and covering. Analytical bounds on the convergence rate to consensus are derived for several systemconfigurations. It is shown that static communication networks that exhibit particular symmetries yield slow convergence, if the connectivity of each agent does not scale with the total number of agents. On the other hand, some randomly varying networks allow fast convergence even if the connectivity is low. It is furthermore argued that if the data being exchanged between the agents are quantized, it may heavily degrade the performance. The extent to which certain quantization schemes are more suitable than others is quantified through relations between the number of agents and the required total network bit rate. The design of distributed coordination and estimation schemes based on the consensus algorithm is presented. A receding horizon coordination strategy utilizing subgradient optimization is developed. Robustness and implementation aspects are discussed. A new collaborative estimation method is also proposed. The implementation of multi-robot control systems is difficult due to the high systemcomplexity. In the final part of this thesis, a hierarchical control architecture appropriate for a class of coordination tasks is therefore suggested. It allows a formal verification of the correctness of the implemented control algorithms.
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| 9. |
- Stamatakis, George, et al.
(författare)
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Autonomous Maintenance in IoT Networks via AoI-driven Deep Reinforcement Learning
- 2021
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Ingår i: IEEE CONFERENCE ON COMPUTER COMMUNICATIONS WORKSHOPS (IEEE INFOCOM WKSHPS 2021). - : IEEE. - 9781665404433 - 9781665447140
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Konferensbidrag (refereegranskat)abstract
- Internet of Things (IoT) with its growing number of deployed devices and applications raises significant challenges for network maintenance procedures. In this work, we formulate a problem of autonomous maintenance in IoT networks as a Partially Observable Markov Decision Process. Subsequently, we utilize Deep Reinforcement Learning algorithms (DRL) to train agents that decide if a maintenance procedure is in order or not and, in the former case, the proper type of maintenance needed. To avoid wasting the scarce resources of IoT networks we utilize the Age of Information (AoI) metric as a reward signal for the training of the smart agents. AoI captures the freshness of the sensory data which are transmitted by the IoT sensors as part of their normal service provision. Numerical results indicate that AoI integrates enough information about the past and present states of the system to be successfully used in the training of smart agents for the autonomous maintenance of the network.
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| 10. |
- Stamatakis, George, et al.
(författare)
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Control of Status Updates for Energy Harvesting Devices that Monitor Processes with Alarms
- 2019
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Ingår i: 2019 IEEE GLOBECOM WORKSHOPS (GC WKSHPS). - : IEEE. - 9781728109602
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Konferensbidrag (refereegranskat)abstract
- In this work, we derive optimal transmission policies for an energy harvesting status update system. The system monitors a stochastic process which can be in one of two states of operation, a normal state or an alarm state. We capture the freshness of status updates for each state of the stochastic process by using a separate Age of Information (AoI) variable for each state and by extending the definition of AoI to account for the stochastic process state changes. We assume that the demand for status updates is higher when the stochastic process is in the alarm state and utilize a transition cost function that increases non-linearly with AoI when the stochastic process is in the alarm state and linearly otherwise. We formulate the problem at hand as a Markov Decision Process, evaluate numerically the derived policies and illustrate their effectiveness for reserving energy in anticipation of future alarm states.
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| 11. |
- Stamatakis, George, et al.
(författare)
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Controlling Status Updates in a Wireless System with Heterogeneous Traffic and Age of Information Constraints
- 2019
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Ingår i: 2019 IEEE GLOBAL COMMUNICATIONS CONFERENCE (GLOBECOM). - : IEEE. - 9781728109626
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Konferensbidrag (refereegranskat)abstract
- In this work, we derive optimal status update generation policies for a wireless source node that has a single queue which serves two applications, one that is Age of Information (AoI) sensitive and one that is not. Additionally, the AoI-sensitive application sets a strict upper bound on the AoI. The latter constraint is satisfied by utilizing an ultra-reliable low-latency communication link. We formulate a dynamic programming problem and utilize the framework of Markov Decision Processes to derive optimal policies for the generation of status update packets. We compare the derived optimal policies against baseline policies. Results indicate that existing status update policies fail to capture the effect of AoI insensitive traffic on queueing delay and AoI and thus perform suboptimally.
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| 12. |
- Stamatakis, George J., et al.
(författare)
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Semantics-Aware Active Fault Detection in IoT
- 2022
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Ingår i: 2022 20TH INTERNATIONAL SYMPOSIUM ON MODELING AND OPTIMIZATION IN MOBILE, AD HOC, AND WIRELESS NETWORKS (WIOPT 2022). - : IEEE. - 9783903176492 - 9781665460767 ; , s. 161-168
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Konferensbidrag (refereegranskat)abstract
- In this work we address a problem of active fault detection in an IoT scenario, whereby a monitor probes a remote device in order to detect faults and acquire fresh information. However, probing can have a significant impact on the IoT networks energy and communication resources. To address this problem we utilize Age of Information as a measure of the freshness of information at the monitor and adopt a semanticsaware communication approach between the monitor and the remote device. In semantics-aware communications, the processes of generating and transmitting information are treated jointly in order to consider the importance of information and the purpose of communication. We formulate the problem as a Partially Observable Markov Decision Process and propose a computationally efficient stochastic approximation algorithm to approximate the optimal policy. Finally, we present numerical results that exhibit the advantage of our approach compared to a conventional delay-based probing policy.
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| 13. |
- Stamatakis, George, et al.
(författare)
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Optimal Policies for Status Update Generation in an IoT Device With Heterogeneous Traffic
- 2020
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Ingår i: IEEE Internet of Things Journal. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 2327-4662. ; 7:6, s. 5315-5328
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Tidskriftsartikel (refereegranskat)abstract
- A large body of applications that involve monitoring, decision making, and forecasting require timely status updates for their efficient operation. Age of Information (AoI) is a newly proposed metric that effectively captures this requirement. Recent research on the subject has derived AoI optimal policies for the generation of status updates and AoI optimal packet queueing disciplines. Unlike previous research, we focus on low-end devices that typically support monitoring applications in the context of the Internet of Things. We acknowledge that these devices host a diverse set of applications some of which are AoI sensitive while others are not. Furthermore, due to their limited computational resources, they typically utilize a simple first-in-first-out (FIFO) queueing discipline. We consider the problem of optimally controlling the status update generation process for a system with a source-destination pair that communicates via a wireless link, whereby the source node is composed of a FIFO queue and serves two applications, one that is AoI sensitive and one that is not. We formulate this problem as a dynamic programming problem and utilize the framework of Markov decision processes to derive the optimal policy for the generation of status update packets. Due to the lack of comparable methods in the literature, we compare the derived optimal policies against baseline policies such as the zero-wait policy. Results indicate that the baseline policy fails to capture the complex system dynamics that determine the relationship between the frequency of status update generation and the resulting queueing delay and thus perform poorly. To the best of our knowledge, the derived optimal policy does not exhibit a simple structure; thus, we utilized the baseline policies, whose operation is intuitive, to gain insight into the inner workings of the optimal policy.
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| 14. |
- Stamatakis, George, et al.
(författare)
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Semantics-Aware Active Fault Detection in Status Updating Systems
- 2024
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Ingår i: IEEE Open Journal of the Communications Society. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 2644-125X. ; 5, s. 1182-1196
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Tidskriftsartikel (refereegranskat)abstract
- With its growing number of deployed devices and applications, the Internet of Things (IoT) raises significant challenges for network maintenance procedures. In this work, we address a problem of active fault detection in an IoT scenario, whereby a monitor can probe a remote device to acquire fresh information and facilitate fault detection. However, probing could significantly impact the system's energy and communication resources. To this end, we utilize the Age of Information to measure the freshness of information at the monitor and adopt a semantics-aware communication approach between the monitor and the remote device. In semantics-aware communications, the processes of generating and transmitting information are treated jointly to consider the importance of information and the purpose of communication. We formulate the problem as a Partially Observable Markov Decision Process and show analytically that the optimal policy is of a threshold type. Finally, we use a computationally efficient stochastic approximation algorithm to approximate the optimal policy and present numerical results that exhibit the advantage of our approach compared to a conventional delay-based probing policy.
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| 15. |
- Tepelenis, Kostas, et al.
(författare)
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Laparoscopic versus open approach to neurolytic celiac plexus block in inoperable pancreatic cancer
- 2018
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Ingår i: ANZ journal of surgery. - : WILEY. - 1445-1433 .- 1445-2197. ; 88:11, s. E767-E771
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Tidskriftsartikel (refereegranskat)abstract
- Background Methods Neurolytic celiac plexus block (NCPB) is an effective method of palliative pain control. The purpose of this study was to evaluate the feasibility and effectiveness of the laparoscopic NCPB versus open approach. Eight patients (Group A) underwent diagnostic laparoscopy which revealed an inoperable pancreatic cancer. Forty millilitres of solution (20 mL of 95% ethanol mixed with 20 mL of xylocaine) was injected into either side of para-aortic soft tissue. The same solution was injected in 10 patients (Group B), with inoperable pancreatic body cancer diagnosed during laparotomy. Results Conclusions There were no intraoperative or post-operative, NCPB related, complications. Patients in both groups, reported significant pain relief in the early post-operative period. Using the visual analogue scale preoperatively, in second post-operative day, first and third post-operative month, no significant different was observed between the two groups. The mean hospital stay in both groups was 2.1 versus 5.2 (P = 0.0005) and the mean survival 8.1 versus 7.9 months (ns). The NCPB is feasible method for palliation in inoperable pancreatic cancer. Laparoscopic NCPB gives excellent results and could still be considered in selected cases, as an effective alternative during staging laparoscopy.
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| 16. |
- Tsiamis, Anastasios, et al.
(författare)
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Learning to Control Linear Systems can be Hard
- 2022
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Ingår i: Proceedings of 35th Conference on Learning Theory, COLT 2022. - : ML Research Press. ; , s. 3820-3857
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Konferensbidrag (refereegranskat)abstract
- In this paper, we study the statistical difficulty of learning to control linear systems. We focus on two standard benchmarks, the sample complexity of stabilization, and the regret of the online learning of the Linear Quadratic Regulator (LQR). Prior results state that the statistical difficulty for both benchmarks scales polynomially with the system state dimension up to system-theoretic quantities. However, this does not reveal the whole picture. By utilizing minimax lower bounds for both benchmarks, we prove that there exist nontrivial classes of systems for which learning complexity scales dramatically, i.e. exponentially, with the system dimension. This situation arises in the case of underactuated systems, i.e. systems with fewer inputs than states. Such systems are structurally difficult to control and their system theoretic quantities can scale exponentially with the system dimension dominating learning complexity. Under some additional structural assumptions (bounding systems away from uncontrollability), we provide qualitatively matching upper bounds. We prove that learning complexity can be at most exponential with the controllability index of the system, that is the degree of underactuation.
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| 17. |
- Tsiamis, Anastasios, et al.
(författare)
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Statistical Learning Theory for Control : A Finite-Sample Perspective
- 2023
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Ingår i: IEEE Control Systems. - : Institute of Electrical and Electronics Engineers (IEEE). - 1066-033X .- 1941-000X. ; 43:6, s. 67-97
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Tidskriftsartikel (refereegranskat)abstract
- Learning algorithms have become an integral component to modern engineering solutions. Examples range from self-driving cars and recommender systems to finance and even critical infrastructure, many of which are typically under the purview of control theory. While these algorithms have already shown tremendous promise in certain applications [1], there are considerable challenges, in particular, with respect to guaranteeing safety and gauging fundamental limits of operation. Thus, as we integrate tools from machine learning into our systems, we also require an integrated theoretical understanding of how they operate in the presence of dynamic and system-theoretic phenomena. Over the past few years, intense efforts toward this goal - an integrated theoretical understanding of learning, dynamics, and control - have been made. While much work remains to be done, a relatively clear and complete picture has begun to emerge for (fully observed) linear dynamical systems. These systems already allow for reasoning about concrete failure modes, thus helping to indicate a path forward. Moreover, while simple at a glance, these systems can be challenging to analyze. Recently, a host of methods from learning theory and high-dimensional statistics, not typically in the control-theoretic toolbox, have been introduced to our community. This tutorial survey serves as an introduction to these results for learning in the context of unknown linear dynamical systems (see 'Summary'). We review the current state of the art and emphasize which tools are needed to arrive at these results. Our focus is on characterizing the sample efficiency and fundamental limits of learning algorithms. Along the way, we also delineate a number of open problems. More concretely, this article is structured as follows. We begin by revisiting recent advances in the finite-sample analysis of system identification. Next, we discuss how these finite-sample bounds can be used downstream to give guaranteed performance for learning-based offline control. The final technical section discusses the more challenging online control setting. Finally, in light of the material discussed, we outline a number of future directions.
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| 18. |
- Ziemann, Ingvar, et al.
(författare)
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A Tutorial on the Non-Asymptotic Theory of System Identification
- 2023
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Ingår i: 2023 62nd IEEE Conference on Decision and Control, CDC 2023. - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 8921-8939
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Konferensbidrag (refereegranskat)abstract
- This tutorial serves as an introduction to recently developed non-asymptotic methods in the theory of-mainly linear-system identification. We emphasize tools we deem particularly useful for a range of problems in this domain, such as the covering technique, the Hanson-Wright Inequality and the method of self-normalized martingales. We then employ these tools to give streamlined proofs of the performance of various least-squares based estimators for identifying the parameters in autoregressive models. We conclude by sketching out how the ideas presented herein can be extended to certain nonlinear identification problems. Note: For reasons of space, proofs have been omitted in this version and are available in an online version: https://arxiv.org/abs/2309.03873.
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