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# [2018-11-30] Prof. Leonid Libkin, University of Edinburgh, "Certain Answers Meet Zero-One Laws" (English Speech)

專題討論演講公告

張貼人：白師瑜 ╱ 公告日期：2018-11-22**Title:**Certain Answers Meet Zero-One Laws

**Date:**2018-11-30 2:20pm-3:30pm

**Location:**R103, CSIE

**Speaker:**Prof. Leonid Libkin, University of Edinburgh

**Hosted by:**Prof. Tony Tan

**Abstract:**

The talk will start with presenting a brief overview of querying incomplete information in databases and the main computational challenges it presents. Querying incomplete data invariably relies on the very coarse classification of query answers into those that are certain and those that are not. Such a classification is often very costly, and we propose to refine it by measuring how close an answer is to certainty.

This measure is defined as the probability that the query is true under a random interpretation of missing information in a database. Since there are infinitely many such interpretations, to pick one at random we adopt the approach used in the study of asymptotic properties and 0-1 laws for logical sentences and define the measure as the limit of a sequence. We show that in the standard model of missing data, the 0-1 law is observed: this limit always exists and can be only 0 or 1 for a very large class of queries. Thus, query answers are either almost certainly true, or almost certainly false, and this classification behaves very well computationally. When databases satisfy constraints, the measure is defined as the conditional probability of the query being true if the constraints are true. This can now be an arbitrary rational number, which is always computable. Another refinement of the notion of certainty views answers with a larger set of interpretations that make them true as better ones. We pinpoint the exact complexity of finding best answers for first-order queries.

**Biography:**

Leonid Libkin is Professor of Foundations of Data Management in the School of Informatics at the University of Edinburgh. He was previously a Professor at the University of Toronto and a member of research staff at Bell Laboratories in Murray Hill. He received his PhD from the University of Pennsylvania in 1994. His main research interests are in the areas of data management and applications of logic in computer science. He has written five books and over 200 technical papers. His awards include a Royal Society Wolfson Research Merit Award, a Marie Curie Chair Award, and six Best Paper Awards. He has chaired programme committees of major database conferences (ACM PODS, ICDT) and was the conference chair of the 2010 Federated Logic Conference. He has given many invited conference talks and has served on multiple program committees and editorial boards. He is an ACM fellow, a fellow of the Royal Society of Edinburgh, and a member of Academia Europaea.

最後修改時間：2018-11-22 AM 9:22