Book contents
- Frontmatter
- Contents
- Foreword by Jan Mycielski
- Preface
- Preface to the Paperback Edition
- Part I Paradoxical Decompositions, or the Nonexistence of Finitely Additive Measures
- Chapter 1 Introduction
- Chapter 2 The Hausdorff Paradox
- Chapter 3 The Banach-Tarski Paradox: Duplicating Spheres and Balls
- Chapter 4 Locally Commutative Actions: Minimizing the Number of Pieces in a Paradoxical Decomposition
- Chapter 5 Higher Dimensions and Non-Euclidean Spaces
- Chapter 6 Free Groups of Large Rank: Getting a Continuum of Spheres from One
- Chapter 7 Paradoxes in Low Dimensions
- Chapter 8 The Semigroup of Equidecomposability Types
- Part II Finitely Additive Measures, or the Nonexistence of Paradoxical Decompositions
- Appendix A Euclidean Transformation Groups
- Appendix B Jordan Measure
- Appendix C Unsolved Problems
- Addendum to Second Printing
- References
- List of Symbols
- Index
Chapter 8 - The Semigroup of Equidecomposability Types
from Part I - Paradoxical Decompositions, or the Nonexistence of Finitely Additive Measures
Published online by Cambridge University Press: 05 August 2012
- Frontmatter
- Contents
- Foreword by Jan Mycielski
- Preface
- Preface to the Paperback Edition
- Part I Paradoxical Decompositions, or the Nonexistence of Finitely Additive Measures
- Chapter 1 Introduction
- Chapter 2 The Hausdorff Paradox
- Chapter 3 The Banach-Tarski Paradox: Duplicating Spheres and Balls
- Chapter 4 Locally Commutative Actions: Minimizing the Number of Pieces in a Paradoxical Decomposition
- Chapter 5 Higher Dimensions and Non-Euclidean Spaces
- Chapter 6 Free Groups of Large Rank: Getting a Continuum of Spheres from One
- Chapter 7 Paradoxes in Low Dimensions
- Chapter 8 The Semigroup of Equidecomposability Types
- Part II Finitely Additive Measures, or the Nonexistence of Paradoxical Decompositions
- Appendix A Euclidean Transformation Groups
- Appendix B Jordan Measure
- Appendix C Unsolved Problems
- Addendum to Second Printing
- References
- List of Symbols
- Index
Summary
Certain proofs and theorems involving equidecomposability would be much simplified if we could add sets. For instance, if X could literally be added to X to form 2X, then the fact that X is paradoxical could be stated simply as X = 2X. In fact, this can be done if we expand the group action appropriately so that multiple copies of X can be formed. This new context for discussing equidecomposability will allow us to state and prove theorems that otherwise would be very cumbersome. One of these is a cancellation law for equidecomposability that has several uses, the most important of which is its use in Tarski's theorem (9.2) relating paradoxical decompositions and invariant measures. Another application will be a proof that any two subsets of S2 with nonempty interior are equidecomposable using rotations. This expanded context for equidecomposability will also yield a simpler proof of Theorem 4.5 that a locally commutative action of a free non-Abelian group is paradoxical.
Definition 8.1. Suppose the group G acts on X. Define an enlarged action as follows. Let X* = X × N and let G* = {(g, π): g ∈ G and π is a permutation of N}, and let the group G* act on X* by (g, π)(x, n) = (g(x), π(n)). If A ⊆ X*, then those n ∈ N such that A has at least one element with second coordinate n are called the levels of A.
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- The Banach-Tarski Paradox , pp. 109 - 122Publisher: Cambridge University PressPrint publication year: 1985