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In developing homotopy theory in algebraic geometry, Michael Artin and Barry Mazur studied the \'etale homotopy types of schemes. Later, Eric Friedlander generalized them to the \'etale topological types of simplicial schemes. The aim of this paper is to extend further these theories to algebraic stacks. To achieve this goal, we exploit the derived functor approach of \'etale homotopy types by Ilan Barnea and Tomer Schlank, and use Daniel Isaksen's model category structure on pro-simplicial sets.

We show that any toroidal DM stack $X$ with finite diagonalizable inertia possesses a maximal toroidal coarsening $X_{tcs}$ such that the morphism $X\to X_{tcs}$ is logarithmically smooth. Further, we use torification results of [AT17] to construct a destackification functor, a variant of the main result of Bergh [Ber17], on the category of such toroidal stacks $X$. Namely, we associate to $X$ a sequence of blowings up of toroidal stacks $\widetilde{\mathcal{F}}_X\:Y\longrightarrow X$ such that $Y_{tc}$ coincides with the usual coarse moduli space $Y_{cs}$. In particular, this provides a toroidal resolution of the algebraic space $X_{cs}$. Both $X_{tcs}$ and $\widetilde{\mathcal{F}}_X$ are functorial with respect to strict inertia preserving morphisms $X'\to X$. Finally, we use coarsening morphisms to introduce a class of non-representable birational modifications of toroidal stacks called Kummer blowings up. These modifications, as well as our version of destackification, are used in our work on functorial toroidal resolution of singularities., 29 pages

Given a quasiprojective algebraic variety with a reductive group action, we describe a relationship between its equivariant derived category and the derived category of its geometric invariant theory quotient. This generalizes classical descriptions of the category of coherent sheaves on projective space and categorifies several results in the theory of Hamiltonian group actions on projective manifolds.This perspective generalizes and provides new insight into examples of derived equivalences between birational varieties. We provide a criterion under which two different GIT quotients are derived equivalent, and apply it to prove that any two generic GIT quotients of an equivariantly Calabi-Yau projective-over-affine variety by a torus are derived equivalent.We also use these techniques to study autoequivalences of the derived category of coherent sheaves of a variety arising from a variation of GIT quotient. We show that these autoequivalences are generalized spherical twists, and describe how they result from mutations of semiorthogonal decompositions. Beyond the GIT setting, we show that all generalized spherical twist autoequivalences of a dg-category can be obtained from mutation in this manner.Motivated by a prediction from mirror symmetry, we refine the main theorem describing the derived category of a GIT quotient. We produce additional derived autoequivalences of a GIT quotient and propose an interpretation in terms of monodromy of the quantum connection. We generalize this observation by proving a criterion under which a spherical twist autoequivalence factors into a composition of other spherical twists.Finally, our technique for studying the derived category of a GIT quotient relies on a special stratification of the unstable locus in GIT. In the final chapter we establish a new modular description of this stratification using the mapping stack from the quotient of the affine line by the multiplicative group to the quotient stack X/G. This is the first foundational step in extending the methods of GIT beyond global quotient stacks X/G to other stacks arising in algebraic geometry. We describe a method of constructing such stratifications for arbitrary algebraic stacks and show that it reproduces the GIT stratification as well as the classical stratification of the moduli stack of vector bundles on a smooth curve.

The first purpose of this dissertation is to introduce and develop a theory of toric stacks which encompasses and extends the notions of toric stacks defined by Lafforgue, Borisov-Chen-Smith, Fantechi-Mann-Nironi, Iwanari, and Satriano, as well as classical toric varieties. In addition to introducing a broader class of smooth toric stacks, the definition we introduce allows singularities. The second purpose is to characterize toric stacks in a "bottom up" fashion, similar to the treatment of smooth toric Deligne-Mumford stacks by Fantechi-Mann-Nironi and the characterization of toric varieties as "abstract toric schemes" which are reduced, separated, and normal.

This thesis pertains to the algebraic K-theory of tame Artin stacks. Building on earlier work of Vezzosi and Vistoli in equivariant K-theory, which we translate in stacky language, we give a description of the algebraic K-groups of tame quotient stacks. Using a strategy of Vistoli, we recover Grothendieck-Riemann-Roch-like formulae for tame quotient stacks that refine Toën’s Grothendieck-Riemann-Roch formula for Deligne-Mumford stacks (as it was realized that the latter pertains to quotient stacks since it relies on the resolution property). Our formulae differ from Toën’s in that, instead of using the standard inertia stack, we use the cyclotomic inertia stack introduced by Abramovich, Graber and Vistoli in the early 2000s. Our results involve the rational part of the K'-theory of the object considered. We establish a few conjectures, the main one (Conjecture 6.3) pertaining to the covariance of our Lefschetz-Riemann-Roch map for proper morphisms of tame stacks (not necessarily representable). Other future works might be dedicated to the study of torsion in K'-groups as well as more general Artin stacks.

We develop a new approach to the study of supersymmetric gauge theories on ALE spaces using the theory of framed sheaves on root toric stacks, which illuminates relations with gauge theories on $\mathbb{R}^4$ and with two-dimensional conformal field theory. We construct a stacky compactification of the minimal resolution $X_k$ of the $A_{k-1}$ toric singularity $\mathbb{C}^2/\mathbb{Z}_k$, which is a projective toric orbifold $\mathscr{X}_k$ such that $\mathscr{X}_k\setminus X_k$ is a $\mathbb{Z}_k$-gerbe. We construct moduli spaces of torsion free sheaves on $\mathscr{X}_k$ which are framed along the compactification gerbe. We prove that this moduli space is a smooth quasi-projective variety, compute its dimension, and classify its fixed points under the natural induced toric action. We use this construction to compute the partition functions and correlators of chiral BPS operators for $\mathcal{N}=2$ quiver gauge theories on $X_k$ with nontrivial holonomies at infinity. The partition functions are computed with and without couplings to bifundamental matter hypermultiplets and expressed in terms of toric blowup formulas, which relate them to the corresponding Nekrasov partition functions on the affine toric subsets of $X_k$. We compare our new partition functions with previous computations, explore their connections to the representation theory of affine Lie algebras, and find new constraints on fractional instanton charges in the coupling to fundamental matter. We show that the partition functions in the low energy limit are characterised by the Seiberg-Witten curves, and in some cases also by suitable blowup equations involving Riemann theta-functions on the Seiberg-Witten curve with characteristics related to the nontrivial holonomies., 106 pages; v2: minor corrections; v3: minor corrections, comments added, references added; v4: further minor corrections and comments added, references added and updated; Final version published in Advances in Mathematics

We formulate a notion of “geometric reductivity” in an abstract categorical setting which we refer to as adequacy. The main theorem states that the adequacy condition implies that the ring of invariants is finitely generated. This result applies to the category of modules over a bialgebra, the category of comodules over a bialgebra, and the category of quasi-coherent sheaves on an algebraic stack of finite type over an affine base.

Given an algebraic stack with quasiaffine diagonal, we show that each G_m-gerbe comes from a central separable algebra. In other words, Taylor's bigger Brauer group equals the etale cohomology in degree two with coefficients in G_m. This gives new results also for schemes. We use the method of twisted sheaves explored by de Jong and Lieblich., Comment: 12 pages

One of the main obstacles for proving Riemann–Roch for algebraic stacks is the lack of cohomology and homology theories that are closer to the K-theory and G-theory of algebraic stacks than the traditional cohomology and homology theories for algebraic stacks. In this paper we study in detail a family of cohomology and homology theories which we call Bredon-style theories that are of this type and in the spirit of the classical Bredon cohomology and homology theories defined for the actions of compact topological groups on topological spaces. We establish Riemann–Roch theorems in this setting: it is shown elsewhere that such Riemann–Roch theorems provide a powerful tool for deriving formulae involving virtual fundamental classes associated to dg-stacks, for example, moduli stacks of stable curves provided with a virtual structure sheaf associated to a perfect obstruction theory. We conclude the present paper with a brief application of this nature.

We prove that every separated Artin stack of finite type over a noetherian base scheme admits a proper covering by a quasi-projective scheme. An application of this result is a version of the Grothendieck existence theorem for Artin stacks.

Consider a smooth variety $X$ and a smooth divisor $D\subset X$. Kim and Sato (arXiv:0806.3819) define a natural compactification of $(X\setminus D)^n$, denoted $X_D^{[n]}$, which is a moduli space of stable configurations of $n$ points lying on expansions of $(X,D)$ in the sense of Jun Li (arXiv:math/0009097, arXiv:math/0110113). The purpose of this note is to generalize Kim and Sato's construction to the case where $X$ is an algebraic stack; and to construct an analogous projective moduli space $W_\pi^{[n]}$ for a degeneration $\pi:W \to B$. We construct $X^n_D$ and $W_\pi^{[n]}$ and prove their properness using a universal construction introduced in our paper arXiv:1110.2976 with Cadman and Wise. We then use these spaces for a concrete application, as explained in the next paragraph. In arXiv:1103.5132, a degeneration formula for Gromov--Witten invariants of schemes and stacks is developed, generalizing the approach of Jun Li. This in particular requires proving properness of Li's stack of pre-deformable stable maps in the case where the target $(X,D)$ or $W\to B$ is a Deligne--Mumford stack. One could simply adapt Li's proof, or follow the age-old tradition of imposing such endeavor as an exercise on "the interested reader". Instead, we prefer to provide a different proof here, which uses the properness of $X_D^{[n]}$ and $W_\pi^{[n]}$. Similar ideas are used by Kim, Kresch and Oh (arXiv:1105.6143) to prove the properness of their space of ramified maps. This note is identical to the text available on our web pages since March 2013. It is posted now as it has become an essential ingredient in others' work., Comment: 16 pages

We establish Tannaka duality for noetherian algebraic stacks with affine stabilizer groups. Our main application is the existence of Hom-stacks in great generality., Comment: 41 pages; various improvements; quasi-separatedness of Hom-stack (Appendix B); more counter-examples (Section 10); former Appendix B is now in arXiv:1606.08517; some corrections and improvements

Using notions of homogeneity we give new proofs of M. Artin's algebraicity criteria for functors and groupoids. Our methods give a more general result, unifying Artin's two theorems and clarifying their differences., 34 pages

We prove some finiteness theorems for the Picard functor of an algebraic stack, in the spirit of SGA 6, exp. XII and XIII. In particular, we give a stacky version of Raynaud's relative representability theorem, we give sufficient conditions for the existence of the torsion component of the Picard functor, and for the finite generation of the Neron-Severi groups or of the Picard group itself. We give some examples and applications. In an appendix, we prove the semicontinuity theorem for a (non necessarily tame) algebraic stack., 29 pages. Final version, including the remarks of the referee. To appear in Adv. Math

We give an introduction to Joyce's construction of the motivic Hall algebra of coherent sheaves on a variety M. When M is a Calabi-Yau threefold we define a semi-classical integration map from a Poisson subalgebra of this Hall algebra to the ring of functions on a symplectic torus. This material will be used in arxiv:1002.4374 to prove some basic properties of Donaldson-Thomas curve-counting invariants on Calabi-Yau threefolds., Comment: 35 pages

62 pages, in French; International audience; In this article we study the Picard functor and the Picard stack of an algebraic stack. We give a new and direct proof of the representability of the Picard stack. We prove that it is quasi-separated, and that the connected component of the identity is proper when the fibers of X are geometrically normal. We study some examples of Picard functors of classical stacks. In an appendix, we review the lisse-étale cohomology of abelian sheaves on an algebraic stack.

We introduce and study a class of algebraic stacks with finite inertia in positive and mixed characteristic, which we call tame algebraic stacks. They include tame Deligne-Mumford stacks, and are arguably better behaved than general Deligne-Mumford stacks. We also give a complete characterization of finite flat linearly reductive schemes over an arbitrary base. Our main result is that tame algebraic stacks are \'etale locally quotient by actions of linearly reductive finite group schemes. In a subsequent paper we will show that tame algebraic stacks admit a good theory of stable maps., Comment: 31 pages, 3 sections and 1 appendix

We use twisted stable maps to compute the number of rational degree d plane curves having prescribed contacts to a smooth plane cubic., Comment: 27 pages, v2: typos corrected and references added

We suggest a construction of virtual fundamental classes of certain types of moduli spaces., Comment: LaTeX, Postscript file available at http://www.math.ubc.ca/people/faculty/behrend/inc.ps

We construct the algebraic stack of formal groups and use it to provide a new perspective onto a recent result of M. Hovey and N. Strickland on comodule categories for Landweber exact algebras. This leads to a geometric understanding of their results as well as to a generalisation.