## Posts tagged ‘sheaves’

### Schemes in algebraic geometry 3 : glued schemes and sheaves

André Weil was among the first ones to point out the importance of having a local description of varieties, especially projective spaces, which can always locally be described as an affine space with completion by a hyperplane at infinity, and projective varieties, which similarly look like varieties in affine space. The use of sheaves in local description of spaces was magnified by Cartan and Serre, in the context of complex analytic spaces, and generalised to the algebraic setting by Serre in Faisceaux algébriques cohérents.

The projective space is the simplest example of an algebro-geometric object which cannot be described by the prime spectrum or the functor of points of a ring. For example, there is no obvious ring whose ideals describe varieties in projective space, which come from homogeneous equations. We would like to give a correct definition of gluing affine lines (with coordinates $z$ and $1/z$) to define the projective line $\mathbb P^1$ as the gluing of $\mathbb A^1$ with $\mathbb A^1 \to \mathbb P^1$ given by $z \mapsto 1/z$. For functors of points, the latest article by Alain Connes and Caterina Consani, gives a definition. For prime spectra, one has to be aware that gluing only topological spaces do not give meaningful information on algebraic properties. This is illustrated by the case of differentiable manifolds, which are not the same as topological manifolds: gluing differentiable manifolds has to induce a correspondance between differentiable functions (this is equivalent to the requirement that gluing maps between charts be differentiable).
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### Monads in mathematics 4 : the bar and cobar constructions

The use of monads and comonads in homological algebra is as old as the theory: Godement’s standard construction refers to the use of monads in cohomology theory, and is said to be the first study of a general method for constructing acyclic resolutions. Later the theory was explored in greater depth by Eilenberg, Moore, Barr, Beck. The term bar resolution is now most commonly used to describe the process. Monads derived from operads were also studied by Lawvere, Mitchell, Bénabou, under the name of algebraic theories, with aim towards topoi and logic.

Bar resolutions (see J. Baez website and the LNM Seminar on Triples and Categorical Homology Theory) are a way to canonically (i.e. functorially) describe an arbitrary algebra over a monad using only free algebras. It is a particular case of definition with generators and relations which is often the only way to describe infinite mathematical objects with finite expressions (computer algebra systems usually deal with finitely generated objects, and use generators and relations to describe their elements), but bar resolutions somehow are the universal way of doing this.

Most cohomology theories fit in the framework of bar constructions, though in various apparently unrelated ways. However, a visible common denominator of most constructions is the use of simplicial methods. This makes them of some use in homotopical algebra: bar resolutions are used to define canonical cofibrant resolutions of objects, which explains their uses in definition of derived functors.
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### Extensions of sheaves and the local-global spectral sequence

GIven a topological space $X$, algebraic topologists would sometimes be interested in sheaves over this space. In most cases, these sheaves are sheaves of functions with some special property, or sheaves of modules over these rings. We could casually say thet the notion of sheaf is some mix of topology and algebra (more generally, categories), which allows to remember where are localized objects. For example, the set of continuous functions over a compact space can sometimes allow to recover an isomorphic space, but its mere normed algebra structure does not immediately say things like: “i am greater here than there”, “i am positive here and negative there”…

So a sheaf is not only determined by a set of fonctions (the commonly used term is section) defined over the whole space, but also by sections over open sets, and by the restriction maps between these various sets of sections. A sheaf has also a gluing theory : if we choose sections over open sets with equal restrictions on the intersections, they must glue to a section over the union of the open sets (if not, our object does not deserve to be a sheaf, so it is only a presheaf).

Virtually anything which can be restricted to open sets and defined locally defines a sheaf: continuous functions form a sheaf, but functions with $||f||_{\infty} = 1$ do not usually form a sheaf (this property is not local). A sheaf of abelian groups is a sheaf of things which can be (locally) added and substracted, a sheaf of rings has sections which can be multiplied, and we can also define sheaves of modules over a sheaf of rings.

A morphism of sheaves $f: F \to G$ is a map which is determined by the image of localized sections. In the case of abelian groups, the kernel sheaf is the sheaf of sections of F which are locally in the kernel of f, while the quotient sheaf (cokernel) is something looking like G where sections are identified if locally it should be so. A morphism is thus injective or surjective if locally it is so. Sheaves of abelian groups (or sheaves of modules over some sheaf of rings) form an abelian category, and it makes sense to speak of the extension group $\mathrm{Ext}^1(F,G)$ for two such sheaves F and G. Suppose we are working with sheaves of modules (especially coherent sheaves) over a sheaf of rings $\mathcal O_X$. How should we compute this group ?