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What is (Artificial) Intelligence? The science of Artificial Intelligence (AI) might be defined as the construction of intelligent systems and their analysis. A natural definition of systems is anything which has an input and an output stream. Intelligence is more complicated. It can have many faces like creativity, solving problems, pattern recognition, classification, learning, induction, deduction, building analogies, optimization, surviving in an environment, language processing, knowledge and many more. A formal definition incorporating every aspect of intelligence, however, seems difficult. An important observation is that most, if not all known facets of intelligence can be formulated as goal driven or, more precisely, as maximizing some utility function. It is, therefore, sufficient to study goal driven AI. E.g. the (biological) goal of animals and humans is to survive and spread. The goal of AI systems should be to be useful to humans. The problem is that, except for special cases, we know neither the utility function, nor the environment in which the system will operate, in advance. The mathematical theory, coined AIXI, (see next item) represents a formal solution of these problems.

Universal Artificial Intelligence. Sequential decision theory formally solves the problem of rational agents in uncertain worlds if the true environmental probability distribution is known. Solomonoff's theory of universal induction formally solves the problem of sequence prediction for unknown distribution. With a formal solution I mean a rigorous mathematically definition, uniquely specifying the solution. I unified both theories and gave strong arguments that the resulting universal AIXI model behaves optimally in any computable environment. I also made some progress towards a computable AI theory. I constructed an algorithm AIXItl, which is superior to any other time t and space l bounded agent. The computation time of AIXItl is of the order 2l. The constant 2l is still too large to allow a direct implementation. My main focus is on theoretical studies related to the AIXI model and on further reducing the time-complexity of AIXItl down to a point where it runs on todays computers with reasonable computation time. This apporach may be characterized as a mathematical top-down approach to AI.

Compression Prize. I am sponsoring a prize of up to 50'000€ for compressing human knowledge, widely known as the Hutter Prize. The compression contest is motivated by the fact that being able to compress well is closely related to acting intelligently, thus reducing the slippery concept of intelligence to hard file size numbers. In order to compress data, one has to find regularities in them, which is intrinsically difficult (many researchers live from analyzing data and finding compact models). So compressors beating the current "dumb" compressors need to be smart(er). Since the prize wants to stimulate developing "universally" smart compressors, we need a "universal" corpus of data. Arguably the online encyclopedia Wikipedia is a good snapshot of the Human World Knowledge. So the ultimate compressor of it should "understand" all human knowledge, i.e. be really smart.

My AI research projects (till 2006). My AI research is centered around Universal Artificial Intelligence in general and the optimal AIXI in particular. I outlined for a number of problem classes, including sequence prediction, strategic games, function minimization, reinforcement and supervised learning, how they fit into the general AIXI model and how AIXI formally solves them. The fastest algorithm for all well-defined problems arose from the construction of the time-bounded AIXItl model. I proved tight error and loss bounds for Solomonoff's universal sequence prediction scheme. I also work on robust predictions, optimization algorithms, including a novel selection scheme for evolutionary algorithms, a linear time approximation algorithm for the knapsack problem, reinforcement planning, and direct policy search. Early student work is on classifier systems, reinforcement learning Hebb nets, and computer graphics. I made my PhD in particle physics and performed applied research in a medical software company on various topics.

Kolmogorov Complexity. Kolmogorov defined the complexity of a string as the length of its shortest description on a universal Turing machine. A string is simple if it can be described by a short program, like ``the string of one million ones'', and is complex if there is no such short description, like for a random string whose shortest description is specifying it bit-by-bit. Kolmogorov complexity is a key concept in (algorithmic) information theory and for universal sequence prediction. The Algorithmic Information Theory page contains references to tutorials, courses, books, papers, people homepages, events, and others. There is a (moderated) mailing list intended for people in Computer Sciences, Statistics, Mathematics, and other areas or disciplines with interests in Kolmogorov Complexity, Solomonoff Induction, Levin Search, Martin-Loef Randomness, and related areas. Researchers in these fields are encouraged to join the list and participate.

Recommended TextBooks and Courses which are most relevant for understanding and modelling intelligent behavior in general, and for developing the AIXI model in particular. For novices I suggest to start with the Reinforcement Learning book by Sutton and Barto. It requires no background knowledge, describes the key ideas, open problems, and great applications of this field. The Artificial Intelligence book by Russell and Norvig gives a comprehensive overview over AI in general. The Kolmogorov Complexity book by Li and Vitanyi is an excellent introduction to algorithmic information theory. If you have some background knowledge in reinforcement learning or decision theory and algorithmic information theory you may be interested in my Theory of Universal Artificial Intelligence.

Job openings. I'm always looking for good PhD students and occasionally have money for PostDocs. In 2009 I hired Peter Sunehag as a PostDoc for working on Universal AI&RL. In 2008 I hired Rakib Ahmed as a PostDoc for working on a computer vision project. In 2006 I co-hired Paola Ranco as a PhD student for a BioStatistics project. In 2005 I hired Daniil Ryabko as a PostDoc, and in 2003 I hired Shane Legg for a PhD and Jan Poland as a PostDoc for investigating the AIXI model.

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