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Ivancevic_Applied-Diff-Geom

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130 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introductionare tangles (see [Baez (1997); Baez and Dolan (1998)]):We can think of this morphism f : x → y as representing the trajectories ofa collection of particles and antiparticles, where particles and antiparticlescan be created or annihilated in pairs. Reversing the direction of time, weget the ‘dual’ morphism f ∗ : y → x:..This morphism is not the inverse of f, since the composite f ◦ f ∗nontrivial tangle:is aIndeed, any groupoid becomes a ∗−category if we set f ∗ = f −1 for every.

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