# Braids and Coverings: Selected Topics by Vagn Lundsgaard Hansen

By Vagn Lundsgaard Hansen

This ebook is predicated on a graduate path taught via the writer on the college of Maryland. The lecture notes were revised and augmented through examples. the 1st chapters advance the basic conception of Artin Braid teams, either geometrically and through homotopy idea, and talk about the hyperlink among knot idea and the combinatorics of braid teams via Markou's Theorem. the ultimate chapters supply a close research of polynomial overlaying maps, that could be seen as a homomorphism of the elemental staff of the bottom area into the Artin Braid staff on n strings.

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Extra resources for Braids and Coverings: Selected Topics

Example text

Second, by specifying a topology on the full subcategory of affine schemes 30 Aff . 2]. Let C be a category and C be a full subcategory of C, both of them admitting fibre products which are chosen to be compatible with respect to the inclusion functor. Suppose that we are given a pretopology P on C and a pretopology P on C . ). Suppose furthermore that the following two conditions are satisfied. (T1) For all objects X ∈ Ob (C) there exists a covering family {fα : Xα → X}α∈A ∈ Cov (X), such that Xα ∈ Ob (C ) for all α ∈ A .

G ~~ γ Proof. 34. 40. By property (T2) from above there exists a family {hδ : Zδ → X}δ∈D ∈ Cov (X) such that Zδ QQQ QQQ QQQhδ QQQ QQQ  fα,β fαQQ( /X / Xα,β Xα > DD ~ ~ DD ~ ~ DD ~~ g DD ~~ γ " Yγ commutes, and this is doing the job. 3 Standard topologies Let C = Sch, and let C = Aff be the full subcategory of affine schemes. Let the pretopology on C be given by the Zariski topology. In this section we shall describe the most frequently used topologies on the category of schemes. 42 Definition.

Let X ∈ Ob (C ) be an affine scheme. We define the following sets of families of morphisms. (i) Cov etf (X) := {fα : Xα → X}α∈A : fα ´etale, finite, Xα → X surjective α∈A (ii) Cov et (X) := {fα : Xα → X}α∈A : fα ´etale, Xα → X surjective α∈A    {fα : Xα → X}α∈A : fα flat, quasi-finite, of finite presentation, (iii) Cov f ppf (X) :=  Xα → X surjective  α∈A (iv) Cov f pqc (X) := {fα : Xα → X}α∈A : fα flat, Xα → X surjective      . e. only finite sets A of indices, are permitted. 43 Remark.