A Localization Theorem for Derived Loop Spaces and Periodic Cyclic Homology by Harrison I-Yuan Chen A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Mathematics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor David Nadler, Chair Professor Denis Auroux Professor Richard Borcherds Professor Ling Hon Lam Summer 2018 A Localization Theorem for Derived Loop Spaces and Periodic Cyclic Homology Copyright 2018 by Harrison I-Yuan Chen 1 Abstract A Localization Theorem for Derived Loop Spaces and Periodic Cyclic Homology by Harrison I-Yuan Chen Doctor of Philosophy in Mathematics University of California, Berkeley Professor David Nadler, Chair Motivated by a theorem in the K-theoretic setting relating the localization of K0(X=T ) over a closed point z 2 Spec(K0(BT )) to the Borel-Moore homology of the fixed points BM z H• (X ; C), we prove an equivariant localization theorem for smooth quotient stacks by re- ductive groups G in the setting of derived loop spaces and periodic cyclic homology, realizing a Jordan decomposition of loops described by Ben-Zvi and Nadler. We show that the derived loop space L(X=G) is a family of twisted unipotent loop spaces over Aff(L(BG)) = G==G; more precisely, the fiber over a formal neighborhood of a semisimple orbit [z] 2 G==G is the unipotent loop space of the classical fixed points with a twisted S1-action. We further study the relationship between unipotent loop spaces and formal loop spaces, and prove that their Tate S1-invariant functions are isomorphic. Applying a theorem of Bhatt identifying derived de Rham cohomology with Betti cohomology, we obtain an equivariant localiza- tion theorem for periodic cyclic homology in the smooth case, identifying the completion of HP (Perf(X=G)) at z 2 G==G with the 2-periodic equivariant singular cohomology of the z-fixed points H•(Xz =Gz ; k)((u)). i I dedicate this work to the mountains. To the dominating shadow of Mount Shasta at dawn, to the morning fog breaking under the rime-covered summit of Lassen Peak, to the clouds of Taiwan pillowing into the valleys of Mount Jade, to Moose Lake and its granite shores glowing red under the light of the setting sun, to canyon dryfalls obstructing the path to Wahguyhe Peak, to sun cups on Mount Lyell melting into the soggy banks of an unnamed lake, to the lichened rock fields littering the gentle slopes of Mount Galen, to the swaying tufts of tall grass on the bare dome of Sutton Mountain. ii Contents Contents ii 1 Introduction 1 2 Background 5 2.1 Infinity categories . 6 2.2 t-structures . 11 2.3 Derived stacks and prestacks . 16 2.4 Formal and derived completions . 18 2.5 Quasicoherent and ind-coherent sheaves . 21 2.6 Group actions on categories and equivariant objects . 28 2.7 Circle actions and mixed complexes . 31 2.8 Hochschild homology and cyclic homology . 34 2.9 Hochcschild homology is a localizing invariant . 38 2.10 Calculations of Hochschild homology . 42 3 Equivariant Localization for Derived Loop Spaces and Periodic Cyclic Homology 51 3.1 Derived loop spaces and its variants . 51 3.2 Twisted circle actions on loop spaces . 61 3.3 Localization for formal loop spaces . 64 3.4 Localization for unipotent loop spaces . 69 3.5 Tate S1-equivariant functions on formal loop spaces compute Betti cohomology 70 3.6 Equivariant localization for periodic cyclic homology . 72 3.7 Comparing global functions on unipotent and formal loop spaces . 77 4 Convolution Methods in Geometric Representation Theory 84 1 4.1 Toy example: convolution pattern in Hochschild homology: A =Gm . 84 4.2 The Hochschild homology of Coh(B\G=B)................... 86 4.3 The Hochschild homology of Coh(Lu(B\G=B)) . 92 Bibliography 98 iii Acknowledgments I would like to express my immense gratitude to my advisor, David Nadler. The content of this work arose from questions he posed to me, and through our many conversations his ideas have undoubtedly seeped into every crevice of this work. Beyond mathematics, David was a thoughtful listener and offered much-needed encouragement in spite of my reticence. I feel fortunate to have been his student, and for the many facets of his support I am deeply thankful. I would also like to thank David Ben-Zvi, Bhargav Bhatt, Daniel Halpern-Leistner, Anatoly Preygel, and Pavel Safranov for the mathematical insights which they have shared with me, as well as Shishir Agrawal, Gurbir Dhillon, Benjamin Gammage, Daniel Lowengrub, and Gus Schrader for the numerous mathematical discussions we have had throughout the years. I've learned much from these mathematicians and hope to continue to do so in the future. I thank Jamal for feeding me during my time at Berkeley; I will miss his lunch plates wherever I go. I thank all my friends here who have kept me in good company throughout the years; amongst them Shishir Agrawal, Anna Lieb, Daniel Lowengrub, Eugenia Rosu, Gus Schrader, Minseon Shin, Ethan van Andel and Markus Vazquez for their always friendly faces within the department, and my roommates Catherine Chen, Gina Noh, Annie Ouyang and Markus Vazquez for making a place feel like home. I thank my aunt Danchu Yu and uncle Ron Chu for their hospitality and for trusting me with their house key. It occurs to me that this key is the sole item which I have always carried with me throughout my seven years in Berkeley. I thank my parents, Harry Chen and Hanfei Yu, for always encouraging me to pursue my interests and for their patience towards an often difficult son. I remember how my mother had taught me arithmetic as a child via games of counting (and eating) grapes, a lesson surrounding the logic behind a restaurant's slogan as we drove to Burlington, and how my father sketched secants of curves limiting to tangents on the back of a narrow junk-mail coupon. I also thank my brothers Timothy and Winston, who have inspired me in ways they may not know. Finally, I would like to thank Shu Tan, for her love which has never wavered. 1 Chapter 1 Introduction Many phenomena in representation theory can be realized via categorical invariants applied to geometric objects: Borel, Weil and Bott realized representations of a complex reductive group via equivariant sheaves on the flag variety, Beilinson and Bernstein realized representations of a finite-type Lie algebra via D-modules on the flag variety, Springer realized representations of Weyl groups via Borel-Moore homology on the nilpotent cone, and Ginzburg, Kazhdan and Lusztig realized representations of the affine Hecke algebra, a q-deformation of the group algebra of the extended affine Weyl group which specializes at prime powers to Iwahori-Hecke algebras, via the 0-truncated algebraic K-theory on the Steinberg stack. The last result in this list is a \decategorified” statement, obtained by applying an \under- ived" invariant to a category of interest. That is, the Grothendieck group of the derived cat- egory of coherent sheaves on the Steinberg stack K0(Coh(St=GGm )) has a K0(Coh(BGGm ))- algebra structure with multiplicative structure given by convolution and linear structure by pullback; in fact, it is a central algebra over K0(Coh(BGGm )). This convolution algebra is identified with the affine Hecke algebra, which leads to a classification of irreducible repre- sentations of the affine Hecke algebra. A central part of its story is a localization theorem in K-theory, which relates the localizations and specializations of K0(Coh(St=GGm )) at fixed pa- rameters of the center Spec(K0(Coh(BGGm ))) = G==G × Gm to the equivariant and ordinary cohomology of fixed points on the Steinberg stack. This thesis grew out of an interest in developing a parallel theory of the affine Hecke alge- bra in the setting of cyclic homology, which leads directly to the consideration of derived loop spaces, whose global functions and circle action compute Hochschild and cyclic homology. In this thesis we prove a foundational result parallel to the localization theorem described above: a localization theorem in periodic cyclic homology. We further outline the broad program that we hope to achieve, and provide several toy examples. Central to our argument is the following idea, which we learned from David Ben-Zvi and David Nadler. Traditionally, Hochschild homology has been computed algebraically via a cyclic bar complex on generators of a category. It is an invariant of small stable 1- CHAPTER 1. INTRODUCTION 2 categories, or equivalently, compactly generated presentable 1-categories. However, the theory of dualizable 1-categories under the Lurie tensor product allows us to generalize this notion to dualizable categories. Due to geometric descriptions of the dualizing structure of categories of sheaves on derived stacks X, the Hochschild homology can be naturally identified with global functions or distributions on the derived loop space [8]: HH(Perf(X)) = O(LX); HH(Coh(X)) = !(LX): When X is a derived scheme, we can explicitly identify the loop space LX with the shifted odd tangent bundle via a derived variant of the Hochschild-Kostant-Rosenberg theorem since derived loops on X are Zariski local. Introducing the S1-action, results in [10] [6] [51] allow us to identify the periodic cyclic homology with 2-periodic (derived) de Rham (co)chain complexes: • an an HP (Perf(X)) ' CdR(X ; k) ⊗k k((u)) HP (Coh(X)) ' C•;dR(X ; k) ⊗k k((u)) where u 2 C•(BS1; k) is the degree 2 Chern class. Our ultimate goal is to describe the periodic cyclic homology in the case when X is taken to be a quotient stack.