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  • K(π, 1) Spaces in Algebraic Geometry

    By

    Piotr Achinger

    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 Arthur E. Ogus, Chair

    Professor Martin C. Olsson

    Professor Yasunori Nomura

    Spring 2015

  • K(π, 1) Spaces in Algebraic Geometry

    Copyright 2015

    by Piotr Achinger

  • Abstract

    K(π, 1) Spaces in Algebraic Geometry

    by

    Piotr Achinger

    Doctor of Philosophy in Mathematics

    University of California, Berkeley

    Professor Arthur E. Ogus, Chair

    The theme of this dissertation is the study of fundamental groups and classifying spaces in the context of the étale topology of schemes. The main result is the existence of K(π, 1) neighborhoods in the case of semistable (or more generally log smooth) reduction, generalizing a result of Gerd Faltings. As an application to p-adic Hodge theory, we use the existence of these neighborhoods to compare the cohomology of the geometric generic fiber of a semistable scheme over a discrete valuation ring with the cohomology of the associated Faltings topos. The other results include comparison theorems for the cohomology and homotopy types of several types of Milnor fibers. We also prove an `-adic version of a formula of Ogus, describing the monodromy action on the complex of nearby cycles of a log smooth family in terms of the log structure.

    1

  • “Proof is hard to come by.”

    –Proposition Joe

    i

  • Contents

    1 Introduction 1 1.1 A non-technical outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    1.1.1 The classical theory over the complex numbers . . . . . . . . . . . . . . 3 1.1.2 The algebraic theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.1.3 Semistable degenerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.1.4 K(π, 1) neighborhoods and p-adic Hodge theory . . . . . . . . . . . . . 15 1.1.5 Summary of the results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    1.2 Discussion of the main result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.2.1 The Fontaine conjectures in p-adic Hodge theory . . . . . . . . . . . . 18 1.2.2 Faltings’ topos and coverings by K(π, 1)’s . . . . . . . . . . . . . . . . . . 21 1.2.3 K(π, 1) neighborhoods in the log smooth case . . . . . . . . . . . . . . . 22

    1.3 Other results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1.3.1 Comparison theorems for Milnor fibers . . . . . . . . . . . . . . . . . . . 23 1.3.2 The monodromy formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    1.4 Conventions and notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

    2 Preliminaries 26 2.1 K(π, 1) spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

    2.1.1 K(π, 1) spaces in algebraic topology . . . . . . . . . . . . . . . . . . . . . . 26 2.1.2 K(π, 1) spaces in algebraic geometry . . . . . . . . . . . . . . . . . . . . . 29 2.1.3 Complements and examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    2.2 Logarithmic geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.1 Conventions about log geometry . . . . . . . . . . . . . . . . . . . . . . . 35

    ii

  • 2.2.2 Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

    2.2.3 Absolute cohomological purity . . . . . . . . . . . . . . . . . . . . . . . . 37

    2.2.4 Saturated morphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    2.3 Functoriality of cohomology pullback maps . . . . . . . . . . . . . . . . . . . . . 39

    2.3.1 Base change morphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

    2.3.2 Cohomology pullback morphisms . . . . . . . . . . . . . . . . . . . . . . 40

    2.3.3 Compatibility of base change and pullback . . . . . . . . . . . . . . . . . 41

    3 K(π, 1) neighborhoods and p-adic Hodge theory 43

    3.1 η-étale maps and Noether normalization . . . . . . . . . . . . . . . . . . . . . . . . 44

    3.1.1 Relative Noether normalization . . . . . . . . . . . . . . . . . . . . . . . . 44

    3.1.2 η-étale maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

    3.2 Existence of K(π, 1) neighborhoods . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

    3.2.1 Charts over a trait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

    3.2.2 Proof of the main theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    3.2.3 Relatively smooth log structures . . . . . . . . . . . . . . . . . . . . . . . . 50

    3.2.4 Obstacles in characteristic zero . . . . . . . . . . . . . . . . . . . . . . . . . 50

    3.3 A counterexample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

    3.3.1 Proof via rigid geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

    3.3.2 Proof via complex geometry . . . . . . . . . . . . . . . . . . . . . . . . . . 55

    3.4 The equicharacteristic zero case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

    3.5 Abhyankar’s lemma and extension of lcc sheaves . . . . . . . . . . . . . . . . . . 58

    3.6 The comparison theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

    4 Milnor fibers 64

    4.1 Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

    4.1.1 Classical Milnor fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

    4.1.2 Milnor fibers in the étale topology . . . . . . . . . . . . . . . . . . . . . . 67

    4.1.3 The completed Milnor fibers . . . . . . . . . . . . . . . . . . . . . . . . . . 68

    4.2 Algebraic vs completed Milnor fibers . . . . . . . . . . . . . . . . . . . . . . . . . . 69

    4.3 Classical vs algebraic Milnor fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

    iii

  • 5 Nearby cycles and monodromy 79 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.2 The complex analytic case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.3 Homological algebra and group cohomology . . . . . . . . . . . . . . . . . . . . . 89

    5.3.1 Homological algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.3.2 Cohomology of procyclic groups . . . . . . . . . . . . . . . . . . . . . . . 92

    5.4 The algebraic case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.5 Variants and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

    5.5.1 The case over a trait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 5.5.2 The case over a disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

    Bibliography 108

    iv

  • Acknowledgements

    First of all, I would like to thank my advisor, Professor Arthur Ogus, for his guidance during my years in graduate school. Despite his many duties as the chair of the department, he always managed to find the time to discuss mathematics with me.

    I am indebted to Professor Ahmed Abbes for suggesting to me the problem of construct- ing K(π, 1) neighborhoods, which became the crux of this Dissertation.

    I would like to thank Professor Martin Olsson for numerous fruitful discussions.

    I am lucky to have learned from Professors Abbes, Ogus, and Olsson not only the tech- niques of algebraic geometry, but also the importance of rigor, cautiousness, and good writ- ing style.

    Many thanks to Dorka Budacz, Wolfgang Schmalz, and Masha Vlasenko for providing illustrations for the Introduction, and to Ravi Fernando, Anna Seigal, Minseon Shin, and Alex Youcis for their helpful suggestions regarding the exposition.

    Special thanks to The Cheeseboard Collective.

    v

  • Chapter 1

    Introduction

    Let X be a sufficiently nice topological space — for example, a CW complex or a manifold (see §2.1.1 for the minimal assumptions we need). Assume that X is connected, and pick a base point x ∈X . We call X a K(π, 1) space if its higher homotopy groups

    πi (X , x), i = 2, 3, . . .

    are zero. The homotopy type of such a space is completely determined by its fundamental group π1(X , x), and in particular the cohomology of X with coefficients in every local sys- tem (a locally constant sheaf of abelian groups) can be identified with the group cohomology of the corresponding representation of π1(X , x).

    Similarly, in the context of algebraic geometry, a connected scheme X with a geometric point x is a called a K(π, 1) scheme if the cohomology of every étale local system agrees with the cohomology of the corresponding representation of the fundamental group πét1 (X , x). The importance of this notion was first revealed in the context of Artin’s proof of the com- parison theorem [SGA73b, Exp. XI, 4.4] between the étale cohomology of a smooth scheme X over C and the singular cohomology of the associated analytic space X an. The main step in the proof is the construction of a covering of X by K(π, 1) open subsets1