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Changes of MindAn Essay on Rational Belief Revision$
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Neil Tennant

Print publication date: 2012

Print ISBN-13: 9780199655755

Published to Oxford Scholarship Online: September 2012

DOI: 10.1093/acprof:oso/9780199655755.001.0001

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A Formal Theory of Contraction

A Formal Theory of Contraction

(p.95) Chapter 4 A Formal Theory of Contraction
Changes of Mind

Neil Tennant

Oxford University Press

This is the heart of the formal theory. Mathematically rigorous definitions are provided of all the formal notions that have been gently introduced in the earlier discussion. The main data type of a step is defined, and the central concept of a dependency network is defined in terms of steps. The concept of a minimally mutilating contraction can then be explicated. Interest in the computational complexity of the contraction problem is motivated by thoroughly surveying known results about the (sometimes horrendous) complexities of various other decision problems of a logical nature. This is in order to provide a context within which the complexity results for contraction should strike the reader as both interesting and welcome. The contraction problem is rigorously characterized, including the hard version that involves the (now precisely explicated) desideratum of minimal mutilation. The simplest version of the contraction problem is shown to be NP-complete; the harder version, involving minimal mutilation, is shown to be at just the next level up in the so-called polynomial hierarchy

Keywords:   step, dependency network, minimal mutilation, contraction, computational complexity, decision problem, NP-complete

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