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Ivancevic_Applied-Diff-Geom

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132 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introductionagram is drawn). Note that the cardinality of the set of input strands oroutput strands can be equal to zero. If they are both zero, then the tangleis simply a knot or link diagram arranged well with respect to the verticaldirection [Kauffman and Radford (1995); Kauffman and Radford (1999);Kauffman (1994)].The Reidemeister moves are a set of moves on diagrams that combinatoriallygenerate isotopy for knots, links and and tangles [Reidemeister(1948)]. If two tangles are equivalent in 3D space, then corresponding diagramsof these tangles can be obtained one from another, by a sequenceof Reidemeister moves. Each move is confined to the tangle box and keepsthe input and output strands of the tangle diagram fixed.Two (tangle) diagrams are said to be regularly isotopic if one can beobtained from the other by a sequence of Reidemeister moves of type 0,2,3(move number 1 is not used in regular isotopy).If A and B are given tangles, we denote the composition of A and B byAB where the diagram of A is placed below the diagram of B and the outputstrands of A are connected to the input strands of B. If the cardinalitiesof the sets of input and output strands are zero, then we simple place onetangle below the other to form the product [Kauffman and Radford (1995);Kauffman and Radford (1999); Kauffman (1994)].Along with tangle composition, as defined in the previous paragraph, wealso have an operation of product or juxtaposition of tangles. To juxtaposetwo tangles A and B simply place their diagrams side by side with A tothe left of B and regard this new diagram as a new tangle whose inputsare the inputs of A followed by the inputs of B, and whose outputs are theoutputs of A followed by the outputs of B. We denote the tangle productof A and B by A ⊗ B.It remains to describe the equivalence relation on tangles that makesthem represent regular isotopy classes of embedded string. Every tangle isa composition of elementary tangles where an elementary tangle is one ofthe following list: a cup (a single minimum – zero inputs, two outputs),a cap (a single maximum – two inputs, zero outputs), a crossing (a singlelocal crossing diagram – two inputs and two outputs).2.3.11 Ultimate Conceptual Machines:Weak n−CategoriesAs traditionally conceived, an n−category is an algebraic structurehaving objects or 0−morphisms, 1−morphisms between 0−morphisms,

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