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Title: Computational aspects of knots and knot transformation
Author: Saleh, Rafiq Asad Muthanna
ISNI:       0000 0004 2732 3950
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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In this thesis we study the computational aspects of knots and knot trans- formations. Most of the problems of recognising knot properties (such as planarity, unknottedness, equivalence) are known to be decidable, however for many problems their precise time or space complexity is still unknown. On the other hand, their complexity in terms of computational power of devices needed to recognise the knot properties was not studied yet. In this thesis we address this problem and provide first known bounds for some knot problems within this context. In order to estimate and characterise complexity of knot problems represented by Gauss words, we consider vari- ous tools and mathematical models including automata models over infinite alphabets, standard computational models and definability in logic. In particular we show that the planarity problem of signed and unsigned Gauss words can be recognised by a two-way deterministic register au- tomata. Then we translate this result in terms of classical computational models to show that these problems belong to the log-space complexity class L, Further we consider definability questions in terms of first order logic and its extensions and show that planarity of both signed and unsigned Gauss words cannot be expressed by a formula of first-order predicate logic, while extensions of first-order logic with deterministic transitive closure operator allow to define planarity of both signed unsigned Gauss words. Follow- ing the same line of research we provide lower and upper bounds for the planarity problem of Gauss paragraphs and unknottedness. In addition we consider knot transformations in terms of string rewriting systems and provide a refined classification of Reidemeister moves formu- lated as string rewriting rules for Gauss words. Then we analyse the reach- ability properties for each type and present some bounds on the complexity of the paths between two knot diagrams reachable by a sequence of moves of the same type. Further we consider a class of non-isomorphic knot diagrams generated by type I moves from the unknot and discover that the sequence corresponding to the number of diagrams with respect to the number of crossings is equal to a sequence related to a class of Eulerian maps with respect to the number of edges. We then investigate the bijective mapping between the two classes of objects and as a result we present two algo- rithms to demonstrate the transformations from one object to the other. It is known that unknotting a knot may lead to a significant increase in number of crossings during the transformations. We consider the question of designing a set of rules that would not lead to the increase in the number of crossings during knot transformations. In particular we introduce a new set moves in this regard which can be used to substitute one of the rules of type II that increases the number of crossings. We show that such new moves coupled with Reidemeister moves can unknot all known examples of complex trivial knot diagrams without increasing number of crossings.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available