Preface: A few years ago our curiosity was aroused by reports on new methods in reinforcement learning, a field that was developed primarily within the artificial intelligence community, starting a few decades ago. These methods were aiming to provide effective suboptimal solutions to complex problems of planning and sequential decision making under uncertainty, that for a long time were thought to be intractable. Our first impression was that the new methods were ambitious, overly optimistic, and lacked firm foundation. Yet there were claims of impressive successes and indications of a solid core to the modern developments in reinforcement learning, suggesting that the correct approach to their understanding was through dynamic programming.
Three years later, after a lot of study, analysis, and experimentation, we believe that our initial impressions were largely correct. This is indeed an ambitious, often ad hoc, methodology, but for reasons that we now understand much better, it does have the potential of success with important and challenging problems. With a good deal of justification, it claims to deal effectively with the dual curses of dynamic programming and stochastic optimal control: Bellman's curse of dimensionality (the exponential computational explosion with the problem dimension is averted through the use of parametric approximate representations of the cost-to-go function), and the curse of modeling (an explicit system model is not needed, and a simulator can be used instead). Furthermore, the methodology has a logical structure and a mathematical foundation, which we systematically develop in this book. It draws on the theory of function approximation, the theory of iterative optimization and neural network training, and the theory of dynamic programming. In view of the close connection with both neural networks and dynamic programming, we settled on the "name neuro-dynamic programming" (NDP), which describes better in our opinion the nature of the subject than the older and more broadly applicable name "reinforcement learning."
Our objective in this book is to explain with mathematical analysis, examples, speculative insight, and case studies, a number of computational ideas and phenomena that collectively can provide the foundation for understanding and applying the NDP methodology. We have organized the book in three major parts.
Inevitably, some choices had to be made regarding the material to be covered. Given that the reinforcement learning literature often involves a mixture of heuristic arguments and incomplete analysis, we decided to pay special attention to the distinction between factually correct and incorrect statements, and to rely on rigorous mathematical proofs. Because some of these proofs are long and tedious, we have made an effort to organize the material so that most proofs can be omitted without loss of continuity on the part of the reader. For example, during a first reading, a reader could omit all of the proofs in Chapters 2-5, and proceed to subsequent chapters.
However, we wish to emphasize our strong belief in the beneficial interplay between mathematical analysis and practical algorithmic insight. Indeed, it is primarily through an effort to develop a mathematical structure for the NDP methodology that we will ever be able to identify promising or solid algorithms from the bewildering array of speculative proposals and claims that can be found in the literature.
The fields of neural networks, reinforcement learning, and approximate dynamic programming have been very active in the last few years and the corresponding literature has greatly expanded. A comprehensive survey of this literature is thus beyond our scope, and we wish to apologize in advance to researchers in the field for not citing their works. We have confined ourselves to citing the sources that we have used and that contain results related to those presented in this book. We have also cited a few sources for their historical significance, but our references are far from complete in this regard.
Finally, we would like to express our thanks to a number of individuals. Andy Barto and Michael Jordan first gave us pointers to the research and the state of the art in reinforcement learning. Our understanding of the reinforcement learning literature and viewpoint gained significantly from interactions with Andy Barto, Satinder Singh, and Rich Sutton. The first author collaborated with Vivek Borkar on the average cost Q-learning research discussed in Chapter 7, and with Satinder Singh on the dynamic channel allocation research discussed in Chapter 8. The first author also benefited a lot through participation in an extensive NDP project at Alphatech, Inc., where he interacted with David Logan and Nils Sandell, Jr. Our students contributed substantially to our understanding through discussion, computational experimentation, and individual research. In particular, they assisted with some of the case studies in Chapter 8, on parking (Keith Rogers), football (Steve Patek), tetris (Sergey Ioffe and Dimitris Papaioannou), and maintenance and combinatorial optimization (Cynara Wu). The joint researches of the first author with Jinane Abounadi and with Steve Patek are summarized in Sections 7.1 and 7.2, respectively. Steve Patek also offered tireless and invaluable assistance with the experimental implementation, validation, and interpretation of a large variety of untested algorithmic ideas. The second author has enjoyed a fruitful collaboration with Ben Van Roy that led to many results, including those in Sections 6.3, 6.7, 6.8, and 6.9. We were fortunate to work at the Laboratory for Information and Decision Systems at M.I.T., which provided us with a stimulating research environment. Funding for our research that is reported in this book was provided by the National Science Foundation, the Army Research Office through the Center for Intelligent Control Systems, the Electric Power Research Institute, and Siemens. We are thankful to Prof. Charles Segal of Harvard's Department of Classics for suggesting the original quotation that appears at the beginning of this preface. Finally, we are grateful to our families for their love, encouragement, and support while this book was being written.
Dimitri P. Bertsekas
John N. Tsitsiklis