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Ivancevic_Applied-Diff-Geom

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Technical Preliminaries: Tensors, Actions and Functors 131morphism f.The above example involves 1D curves in 3D space. More generally,topological quantum field theory studies nD manifolds embedded in (n+k)Dspace–time, which in the k → ∞ limit appear as ‘abstract’ nD manifolds.It appears that these are best described using certain ‘n−categories withduals’, meaning n−categories in which every j−morphism f has a dual f ∗ .Therefore, a tangle is a box in 3D space with knotted and linked stringembedded within it and a certain number of strands of that string emanatingfrom the surface of the box. There are no open ends of string insidethe box. We usually think of some subset of the strands as inputs to thetangle and the remaining strands as the outputs from the tangle. Usuallythe inputs are arranged to be drawn vertically and so that they enter tanglefrom below, while the outputs leave the tangle from above. The tangle itself(within the box) is arranged as nicely as possible with respect to a verticaldirection. This means that a definite vertical direction is chosen, and thatthe tangle intersects planes perpendicular to this direction transversely exceptfor a finite collection of critical points. These basic critical points arelocal maxima and local minima for the space curves inside the tangle. Twotangles configured with respect to the same box are ambient isotopic if thereis an isotopy in three space carrying one to the other that fixes the inputand output strands of each tangle. We can compose two tangles A and Bwhere the number of output strand of A is equal to the number of inputstrands of B. Composition is accomplished by joining each output strandof A to a corresponding input strand of B [Kauffman and Radford (1995);Kauffman and Radford (1999); Kauffman (1994)].A tangle diagram is a box in the plane, arranged parallel to a chosen verticaldirection with a left–right ordered sequence of input strands enteringthe bottom of the box, and a left–right ordered sequence of output strandsemanating from the top of the box. Inside the box is a diagram of thetangle represented with crossings (broken arc indicating the undercrossingline) in the usual way for knot and links. We assume, as above, that thetangle is represented so that it is transverse to lines perpendicular to thevertical except for a finite number of points in the vertical direction alongthe tangle. It is said that the tangle is well arranged, or Morse with respectto the vertical direction when these transversality conditions are met.At the critical points we will see a local maximum, a local minimum or acrossing in the diagram. Tangle composition is well–defined (for matchinginput/output counts) since the input and output strands have an ordering(from left to right for the reader facing the plane on which the tangle di-

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