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専攻談話会(セミナー)

原則的に水曜日の13:30から15:00におこなっています。 客員教授、新任教員以外にも講演者を募集しています。 集中講義などでいらっしゃる先生など、お心あたりのある方は専攻幹事 青谷 (aotani at is.titech.ac.jp) までお知らせください。

なお、談話会を開くときには、日時、会場、講演者、題目の情報を専攻幹事までメールにてお送りください。

2015年度第6回

日時:12月24日, 15:20-16:20

場所:東工大大岡山キャンパス西八号館W809

講演者:Marvin K. Nakayama (New Jersey Institute of Technology)

Title: Efficient Monte Carlo Methods for Risk Estimation and its Error, With Applications to Safety Analyses of Nuclear Power Plants

Abstract: Analysts often measure risk through a quantile or a failure probability. The p-quantile of a continuous random variable is the constant for which p of the mass of its distribution lies below the quantile; e.g., the median is the 0.5-quantile. In project management, one may want to determine a time T such that the project has a 95% chance of completing by T, which is the 0.95-quantile. In finance, where a quantile is known as a value-at-risk, analysts frequently measure risk with the 0.99-quantile of a portfolio’s loss. For complex stochastic models, analytically computing a quantile is usually not possible, so Monte Carlo simulation is often employed. In addition to providing a point estimate for a quantile, we also want to measure the simulation estimate's statistical error, typically through a confidence interval (CI) for the quantile. Indeed, the U.S. Nuclear Regulatory Commission requires that licensees of nuclear power plants demonstrate compliance using a “95/95 criterion,” which entails ensuring (with 95% confidence) that a 0.95-quantile is less than a mandated limit.

In this talk we present some methods for constructing CIs for a quantile estimated via Monte Carlo simulation. Unfortunately, simple random sampling can produce unusably wide CIs, so analysts may apply variance-reduction techniques (VRTs), such as importance sampling or control variates, in simulations to decrease the statistical error. We first discuss forming a quantile CI using a finite difference, and the second approach employs a procedure known as sectioning, which is closely related to batching. The asymptotic validity of both CIs follows from a so-called Bahadur representation, which shows that a quantile estimator can be approximated by a linear transformation of a probability estimator. We have established Bahadur representations for a broad class of VRTs, including antithetic variates, control variates, Latin hypercube sampling, and importance sampling. We present some numerical results comparing the different CIs.

This work is supported, in part, by the National Science Foundation through grants CMMI-0926949, CMMI-1200065, DMS-1331010, CMMI-1537322.

Speaker Bio: Marvin K. Nakayama is a professor of computer science at the New Jersey Institute of Technology. He received a B.A. in mathematics-computer science from U.C. San Diego, and an M.S. and Ph.D. in operations research from Stanford University. He won second prize in the George E. Nicholson Student Paper Competition sponsored by INFORMS, received a CAREER Award from the National Science Foundation, and won the Best Theoretical Paper Award at the 2014 Winter Simulation Conference. He is the Simulation Area Editor for the INFORMS Journal on Computing and an associate editor for the ACM Transactions on Modeling and Computer Simulation. His research interests include simulation, applied probability, statistics, risk analysis, reliability, and modeling of cascading failures.

2015年度第5回

日時:11月16日15:00--16:00

場所:東工大大岡山キャンパス西八号館W904

講演者:Kostis Sagonas (Uppsala University)

Title: Compiling Erlang to Native Code

Abstract: For about fifteen years now, we been developing, maintaining, and extending the native code compiler for the concurrent functional programming language Erlang, to keep up-to-date with additions at the language level, the runtime system of Erlang/OTP, compiler technology advances, and new interesting architectures.

In this talk, we will review the internals of HiPE (the High-Performance Erlang) native code compiler and its ErLLVM backend, which targets the LLVM compiler infrastructure. Besides presenting the overall architecture of HiPE and ErLLVM and their integration in Erlang/OTP, we will try to critically discuss technical challenges and decisions we took, and present a detailed performance evaluation.

Time permitting, we may briefly introduce software tools for Erlang (Dialyzer, TypEr, tidier, PropEr, Concuerror and CutEr) built by the speaker's group.

2015年度第4回

日時:11月24日15:00--16:30

場所:東工大大岡山キャンパス西八号館E1001

講演者:Grady Booch (IBM Research)

Title: The Future of Software Engineering

Abstract: No matter what future we may envision, it relies on software that has not yet been written. Even now, software-intensive systems have woven themselves into the interstitial spaces of civilization, and we as individuals and as a species have slowly surrendered ourselves to computing. Looking back, we can identify several major and distinct styles whereby we have built such systems. We have come a long way, and even today, we certainly can name a number of best practices for software development that yield systems of quality. However, by no means can we stand still: the nature of the systems we build continues to change, and as they collectively weave themselves into our live, we must attend not only to the technical elements of software development, we must also attend to human needs. In this presentation we will look at the history of software engineering and offer some grand challenges for the future.

Biography: Grady Booch is Chief Scientist for Software Engineering at IBM Research where he is currently engaged in the architecting of cognitive systems encompassing both IBM's Watson as well as non-von Neumann platforms. Grady is also developing a major transmedia documentary for public broadcast on the intersection of computing and the human experience. Booch is an IBM Fellow, an ACM Fellow, an IEEE Fellow, and on behalf of the BCS has been awarded the Lovelace Medal and given the Turing Lecture. Author of six books, Grady wrote the long-running column "On Architecture" and now writes "On Computing" for IEEE Software; over his career he has published hundreds of technical articles and has lectured and consulted around the world. Co-author of the Unified Modeling Language, Grady has helped with the architecture of complex software-intensive systems in most every domain imaginable.

2015年度第3回

日時:8月28日11時から

場所:東工大大岡山キャンパス西八号館W811

講演者:Professor Daniel Keren (University of Haifa)

Title: Classification Using a Background Prior

Abstract: A canonical problem in machine learning is category classification (e.g. find all instances of human faces, cars etc., in an image). Typically, the input for training a classifier is a relatively small sample of positive examples, and a much larger sample of negative examples, which in current applications can consist of images from thousands of categories. The difficulty of the problem sharply increases with the dimension and size of the negative example set. In this talk I will describe an efficient and easy to apply classification algorithm, which replaces the negative samples by a prior and then constructs a "hybrid" classifier which separates the positive samples from this prior. The resulting classifier achieves an identical or better classification rate than SVM, while requiring far smaller memory and lower computational complexity to train and apply.

2015年度第2回

日時: 2015年7月29日(水) 14:00--16:00 会場: 西8号館W棟9階 W911号室

講演者: Walter Binder (Università della Svizzera italiana (USI), Switzerland)

題目: Accurate Profiling in the Presence of Dynamic Compilation

要旨: Many profilers based on bytecode instrumentation yield wrong results in the presence of an optimizing dynamic compiler, either due to not being aware of optimizations such as stack allocation and method inlining, or due to the inserted code disrupting such optimizations. To avoid such perturbations, we present a novel technique to make any profiler implemented at the bytecode level aware of optimizations performed by the dynamic compiler. We implement our approach in a state-of-the-art Java virtual machine and demonstrate its significance with several concrete profilers. We quantify the impact of escape analysis on allocation-hotspot profiling and on object-lifetime analysis, as well as the impact of method inlining on callsite profiling. We illustrate how our approach enables new kinds of profilers, such as a profiler for non-inlined callsites and a testing framework for locating performance bugs in dynamic compiler implementations. Joint work with Yudi Zheng and Lubomir Bulej.

略歴: Walter Binder is an associate professor in the Faculty of Informatics, Università della Svizzera italiana (USI), Switzerland. He holds an MSc, a PhD, and a Venia Docendi from Vienna University of Technology, Austria. His main research interests are in the areas of program analysis, virtual machines, parallel programming, and Cloud computing.

2015年度第1回

日時: 2015年6月2日(火) 17:00-18:30

会場: 西8号館W棟9階 W911号室

講演者: Raffi Khatchadourian (City University of New York)

題目: Open Problems in Automatically Refactoring Legacy Java Software to use New Features in Java 8

要旨: Java 8 is one of the largest upgrades to the popular language and framework in over a decade. In this talk, I will first overview several new, key features of Java 8 that can help make programs easier to read, write, and maintain, especially in regards to collections. These features include Lambda Expressions, the Stream API, and enhanced interfaces, many of which help bridge the gap between functional and imperative programming paradigms and allow for succinct concurrency implementations. Next, I will discuss several open issues related to automatically migrating (refactoring) legacy Java software to use such features correctly, efficiently, and as completely as possible. Solving these problems will help developers to maximally understand and adopt these new features thus improving their software.

2014年度第2回

日時: 2014年8月13日(水) 13:30-14:30

会場: 西8号館W棟8階W809号室

講演者: Victor Magron (Laboratory for Analysis and Architecture of Systems (LAAS)-CNRS)

題目: New applications of moment-SOS hierarchies

要旨: Semidefinite programming is relevant to a wide range of mathematic fields, including combinatorial optimization, control theory, matrix completion. In 2001, Lasserre introduced a hierarchy of semidefinite relaxations for particular polynomial instances of the Generalized Moment Problem (GMP). My talk emphasizes new applications of this moment-SOS hierarchy, investigated during my PhD and Postdoc research.

In the context of formal proofs for nonlinear optimization, one can combine the moment-SOS hierarchy with maxplus approximation of semiconvex functions. Such a framework is mandatory for formal certification of nonlinear inequalities, occurring by thousands in the proof of Kepler Conjecture by Hales.

I also present how to approximate, as closely as desired, the Pareto curve associated with bicriteria polynomial optimization problems or the image of semialgebraic sets under polynomial maps. For each problem, one builds a hierarchy of semidefinite programs, so that the sequence of bounds converges in L1 norm.

Finally, this hierarchy allows to analyze programs containing loop invariants with polynomial assignments.

2014年度第1回

日時: 2014年7月1日(火) 10:45-12:15

会場: 西8号館W棟11階W1101号室

講演者: Katerina Mitrokotsa (Department of Computer Science and Engineering, Chalmers University of Technology)

題目: Authentication in constrained settings

要旨: Access to restricted services and/or places requires authentication. However, authentication is sometimes performed in: i) noisy conditions, ii) hostile environments and iii) constrained settings. By noisy conditions, we refer to noise in the communication channel that may lead to modification of the transmitted information. By hostile environments we mainly refer to environments where attackers may attempt to impersonate legitimate users, while by constrained settings we refer to environments that may include communication among wireless devices with limited resources.

Authentication is a decision making problem where we need to decide whether or not to accept the credentials of an identity-carrying entity. In the context of cryptographic authentication, we have extensively investigated the family of distance bounding protocols. These can be used as the main countermeasure against relay attacks. We analyse the security of such protocols. These authentication problems will also be briefly connected to the problem of privacy-preservation.

(講演者紹介: Katerina Mitrokotsa is an assistant professor at the department of Computer Science and Engineering at Chalmers University of Technology. Her main research interests lie in information and network security, privacy- preservation, machine learning for security and applied cryptography. Formerly, she held positions as a Marie Curie fellow at EPFL, as professor at the University of Applied Sciences of Western Switzerland (HES-SO), as a postdoctoral researcher in TU Delft and as a visitor assistant professor at the department of Computer Science at Vrije Universiteit in Amsterdam. She has been awarded the Rubicon Research Grant by the Netherlands Organization for Scientific Research (NWO) and a Marie Curie Intra European Fellowship. Currently, among others she serves as associate editor for the IEEE Communications Letters and the Computers & Security Journal (Elsevier). She has served on the PCs of INFOCOM, ACNS, Africacrypt, Indocrypt and multiple other well-known conferences in the area on communications and information security.)

2013年度第9回

日時: 2014年3月10日 10:00 - 12:00

会場: 西8号館W棟10階W1008号室

講演者: 方菱 氏 (産業技術総合研究所セキュアシステム研究部門システムライフサイクル研究グループ

題目: 確率モデル検査による止まらないマイコンFUJIMIの評価

要旨: マイクロプロセッサユニット(MPU)は、静電気放電等外部ノイズ信号を 受けた場合、システム機能がフリーズや誤動作する場合が多々ある。FUJIMIはレ ジリエンス(resilience)技術である。この技術は定期的にCPUコアをリセット し、フリーズや誤動作から回復させる。しかし、一般にレジリエンス技術は複雑 な相互作用因子に依存するため、評価が困難である。

本研究は、上記のFUJIMIシステムを確率モデルによって定性と定量的な評価を 行った。定性評価は6 PCTL式により、システムを正しく回復できる性質を証明し た。定量評価は、システム障害の低減率とADT(業界標準である年平均ダウンタ イム)によってシステムを改善したことを具体的な数値によって示した。実験結 果によって、我々の評価方法は費用対効果が高く、信頼性があることが示された。

2013年度第8回

日時: 2014年1月15日 15:05 - 16:35

会場: 西8号館W棟8階W809号室

講演者: 三浦英之 氏 (東京工業大学 数理・計算科学専攻)

題目: 非圧縮Navier-Stokes流に対する特異点除去可能性について

要旨: 非圧縮Navier-Stokes方程式は非圧縮粘性流体の流速場と圧力場の関係を 記述すると信じられている偏微分方程式である.その解の時間大域的な滑らかさ の問題は, 1934年にJ.Lerayによって提案されて以来,多くの研究が行われてい るが,現在までのところ未解決である.この問題に対する一つのアプローチとし て,ある点で解が特異にならないための十分条件を考察する特異点除去可能性の 研究がある.この研究の最近の進展を,講演者の結果も含めて紹介したい.

2013年度第7回

日時: 2013年12月16日(月)13:20 - 14:30 その後,簡単なお茶会をします

会場: 西8号館W棟10F1008コラボレーションルーム

講演者: Ravi Montenegro (U. Mass, Lowell)

題目: The Kruskal Count: how a card trick can help us break codes

要旨: The Kruskal Count is a card trick popularized by Martin Gardner. John Pollard independently used the same idea to construct a tool that can be used to break some types of encryption. The Birthday Paradox can explain why your ipod seems to play too many songs from the same albums. Pollard again used similar intuition on cryptographic problems. We discuss these curious results and explain how they are examples of random walks, also known as Markov Chains.

2013年度第6回

日時: 2013年12月4日(水) 16:30〜

会場: 大岡山西8号館 W棟 W1008室

講演者: 石崎 一明 (IBM東京基礎研究所)

題目: 静的型付き言語用Just-In-Timeコンパイラの再利用による、動的型付き言語用コンパイラの実装と最適化

要旨: Web関連のソフトウェア開発において、JavaScript, PHP, Python,Rubyなどの動的型付き言語の人気が高い。これらの言語では、最初インタプリタによって実装され、その後言語の人気の高さとともに大きくなる高速化の要求に応えるためにコンパイラが開発される、という過程をたどることが多い。新たな言語のコンパイラ開発のために、言語毎にコンパイラを一から作成するのは実装にかかる労力が非常に大きい。我々は、この労力を削減し効率的な開発のために、既存の静的型付き言語Just-In-Timeコンパイラの再利用Repurpose-JIT compiler)というアプローチを用いて、Python言語のコンパイラを実装した。Repurpose-JIT compilerの実装においては、以下の2点が問題となり、高性能を得ることが難しかった。1)コンパイル対象とする言語の命令と再利用しようとするコンパイラの中間言語の命令の粒度の違い、2)高品質なコード生成にとって重要な型情報をソースコードから得ることの困難さ。本発表では、これらの問題点を解決する最適化を提案し、特に実行時情報を用いたコードの特殊化が有効であることを実験結果に基づいて紹介する。さらに、今回の経験から得られた、コンパイラ以外の性能に影響を与える要因についても紹介する。 This is joint work with Jose Castanos, David Edelsohn, Priya Nagpurkar, Toshio Nakatani, Takeshi Ogasawara, and Peng Wu.

参考文献:

[1] Kazuaki Ishizaki, Takeshi Ogasawara, Jose Castanos, Priya Nagpurkar, David Edelsohn, and Toshio Nakatani. Adding dynamically-typed language support to a statically-typed language compiler: performance evaluation, analysis, and tradeoffs, VEE 2012, 2012

[2] Jose Castanos, David Edelsohn, Kazuaki Ishizaki, Priya Nagpurkar, Toshio Nakatani, Takeshi Ogasawara, and Peng Wu. On the Benefits and Pitfalls of Extending a Statically Typed Language JIT Compiler for Dynamic Scripting Languages, OOPSLA 2012, 2012

発表スライドは次のリンクからご覧になれます. http://www.slideshare.net/ishizaki/titech20131204-public

2013年度第5回

日時: 2013年11月7日(水) 13:20-14:50

会場: 大岡山西8号館W棟 W809号室

講演者: 鈴木大慈 氏(東京工業大学 数理・計算科学専攻)

題目: マルチプルカーネル学習およびガウス過程事前分布を用いたスパース加法モデル推定

要旨: Multiple Kernel Learning (MKL) およびガウス過程事前分布を用いたMKLのベイズ的変種についてその統計的性質を議論する.MKLは画像認識の分野などで成功を収めているが,オリジナルのL1正則化を用いるよりもL1とL2の中間的な正則化を用いたものが多くの問題で高い精度を出すことが実験的に報告されている.そこで,L1とL2の中間的な正則化としてelasticnet型の正則化やLpノルムを考えた場合に,それら中間的な正則化がどのように汎化性能に影響を与えるかを理論的に考察する.一方,MKLのベイズ的変種を用いれば,デザインへの条件を課さないでも速い収束レートを導出することができることも紹介する.

参考文献:

[1] Taiji Suzuki, and Masashi Sugiyama: Fast learning rate of multiple kernel learning: trade-off between sparsity and smoothness. The Annals of Statistics, vol. 41, number 3, pp. 1381-1405, 2013.

[2] Taiji Suzuki: PAC-Bayesian Bound for Gaussian Process Regression and Multiple Kernel Additive Model. Conference on Learning Theory (COLT2012), 2012, JMLR Workshop and Conference Proceedings 23: 8.1--8.20, 2012.

[3] Taiji Suzuki: Unifying Framework for Fast Learning Rate of Non-Sparse Multiple Kernel Learning. Advances in Neural Information Processing Systems 24 (NIPS2011). pp.1575--1583.

2013年度第4回

日時: 2013年7月24日(水) 13:20-14:50

会場: 大岡山西8号館W棟 W1008号室

講演者: 寺嶋 郁二 氏(東京工業大学 数理・計算科学専攻)

題目: クラスター変換とブレイド群

要旨: クラスター変換の理論は,2002年に二人の数学者 S.Fomin と A.Zelevinsky によって導入されました.この講演ではクラスター変換を用いて,ブレイド群の表現を具体的かつ幾何的に構成できることを説明します.

2013年度第3回

日時: 2013年7月18日(木) 13:20-14:50

会場: 大岡山西8号館W棟8F W809号室

講演者: 野村 俊一 氏 (東京工業大学 数理・計算科学専攻)

題目: アクチュアリーの業務、試験、課題について

要旨: 「アクチュアリー」とは,保険・年金の業界で活躍する数理業務のスペシャリストである. 本発表では、自身の経験を踏まえてアクチュアリーに関する以下の内容を紹介する. ・アクチュアリーの役割と前職(損害保険会社)での経験 ・アクチュアリー資格試験を通るまでの軌跡と勉強方法 ・アクチュアリーが現在取り組んでいる主要な課題

2013年度第2回

日時: 2013年5月29日(水) 15:30-

会場: 大岡山西8号館W棟10F コラボレーションルーム(W1008)

講演者: Ian Piumarta氏 (Viewpoints Research Institute)

題目: To trap a better mouse

要旨: Edsger Dijkstra once said: "The tools we use have a profound and devious influence on our thinking habits, and therefore on our thinking abilities." When we write programs, the single most important tool we're using is the programming language. When we become fluent in a programming language, we tend to think of our solutions directly in terms of its data structures and algorithms. Unfortunately there is no such thing as a programming language that is good for everything. In our lives as programmers we will inevitably encounter problems for which a natural and elegant language has not yet been invented in which to express a solution. We then have a choice: either think poor thoughts in a 'general purpose' programming language, or invent new languages in which to think much better thoughts. Which option we choose can mean the difference between a successful project and a failed project. In this talk I will review some of the reasons why language is important, show which parts of both natural and artificial languages tend to be malleable, and suggest some ways of thinking about languages that might lead to better ways of modelling and implementing the languages in which we program -- and, therefore, think.

2013年度第1回

日時: 2013年4月18日(木) 13:20-14:50

会場: 西8号館W棟8階W809号室

講演者: Anne Bouillard 氏 (ENS Paris, 東京工業大学 数理・計算科学専攻)

題目: Exact Worst-case Delay for FIFO-multiplexing Tandems

要旨: This talk is about computing the actual worst-case end-to end delay for a flow in a tandem of network under some general assumptions using the network calculus theory, that is originally based on the (min,plus) algebra. Unfortunately, using those algebraic methods lead to very pessimistic bounds. We will see that we can indeed model this network as a mixed interger linear program and drastically improve those bounds.

2012年度第4回

日時: 2013年3月21日(木) 15:00-16:30

会場: 西8号館E棟10階E1011号室

講演者: Martin Mu"ller 氏

題目: ''Move Quality in Monte Carlo Simulation: A Case Study using the Fuego Go Program''

要旨: Despite half a dozen years of intense research, designing effective simulation policies for Monte Carlo Tree search in Go is still considered something of a black art, and driven largely by trial and error. Important ideas that have evolved include pattern- and tactics-based playouts, simulation balancing, and several schemes to dynamically modify simulation policies online. In this study, we take an in-depth look at what happens when the Go program Fuego runs its playouts. We develop several methods to evaluate the quality of moves played in these simulations, and we evaluate the contribution of the different components of Fuego's playout policy. We study the distribution of both the number of blunders, or result-changing moves, and the absolute loss - in terms of number of points - for many variations of the Fuego playout policy. We use this study to identify an improvement to the Fuego default policy.

(Joint work with Sumudu Fernando)

2012年度第3回

日時: 2013年3月8日(金) 16:00-17:00

会場: 西8号館W棟10階1008号室

講演者: Benjamin Burton 氏(The University of Queensland)

題目: Untangling knots using combinatorial optimisation

要旨: Unknot recognition is the algorithmic problem of determining whether a knot (i.e., a closed loop) in 3-dimensional space can be untangled. It is a major unsolved problem as to whether this problem has a fast polynomial time solution. Here we present the first conclusive algorithm for unknot recognition which, although exponential time in theory, exhibits a clear polynomial time behaviour in exhaustive practical experiments. The algorithm makes significant use of techniques from combinatorial optimisation, and uses a branch-and-bound framework with linear programming steps. We also introduce the topological software package Regina (in which the algorithm was implemented), and discuss the relevance of our results to other difficult problems in computational topology.

This is joint work with Melih Ozlen.

講演後には Burton 氏との懇親会を予定しています。奮ってご参加ください。

2012年度第2回

日時: 2012年11月12日(月) 15:00-16:30

会場: 西8号館E棟10階1011号室

講演者: S.R.K. Branavan 氏

題目: Grounding Linguistic Analysis in Control Applications

要旨: Natural languages are the medium in which the majority of humanity's collective knowledge is recorded and communicated. If machines were able to automatically access and leverage this information, they could effectively perform many tasks that are currently considered to be computationally intractable, thus requiring human involvement. Today, the only way to infuse human knowledge into computational algorithms is to have humans in the loop - i.e., to manually encode the knowledge into heuristics, through annotations, or directly into the model structure itself.

Our ultimate goal is to automate this process, so that machines can access required knowledge directly from text. One path to this goal is to perform a semantic interpretation of text by grounding the textual information in the objects, actions and dynamics of the physical world. From a linguistic viewpoint, this grounding of language in control applications presents a very natural notion of language semantics. I.e., by allowing us to define semantics with respect to the control application, and avoid imposing subjective human notions of correctness.

In this talk, I will explore two aspects of the connection between language and control applications: first, how the semantic analysis of language can be driven by control performance; and second, how information from text can be leveraged to improve performance in complex control systems. These two aspects are in fact complementary, and language analysis is central to them both. Addressing these aspects jointly is particularly challenging. However, as our empirical results show, connecting the semantic analysis of language to control applications allow us to leverage the synergy between the two to simultaneously learn both language analysis and application control with little or no prior knowledge.

2012年度第1回

日時: 2012年4月19日(木) 13:20-14:50

会場: 西8号館W棟8階W809号室

講演者: Tuan Phung-Duc 氏 (東京工業大学 数理・計算科学専攻)

題目: Markov Chain and Queue with State-dependent Transition Rates

要旨: In this talk, we consider continuous time Markov chains with state-dependent transition rates arising from either retrial queues or infinite server queues. First, for multiserver retrial queues, we show that in the case of three and four servers, the joint stationary distribution of the number of busy servers and the number of retrying customers can be expressed in terms of continued fractions. Furthermore, we formulate the general case with any number of servers by a level-dependent QBD process for which a memory saving algorithm based on matrix continued fractions is presented. Next, we briefly present a topic on infinite server queues with synchronized abandonment, where we develop a new methodology to obtain either closed form solution or recursion for the stationary distribution of the number of busy servers.

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