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In [[mathematics]], the '''Haagerup property''', named after [[Uffe Haagerup]] and also known as [[Mikhail Gromov (mathematician)|Gromov]]'s '''a-T-menability''', is a property of [[Group (mathematics)|group]]s that is a strong negation of [[Kazhdan]]'s [[property (T)]]. Property (T) is considered a representation-theoretic form of rigidity, so the Haagerup property may be considered a form of strong nonrigidity; see below for details. | |||
The Haagerup property is interesting to many fields of mathematics, including [[harmonic analysis]], [[representation theory]], [[operator K-theory]], and [[geometric group theory]]. | |||
Perhaps its most impressive consequence is that groups with the Haagerup Property satisfy the [[Baum-Connes conjecture]] and the related [[Novikov conjecture]]. Groups with the Haagerup property are also uniformly [[embedding|embeddable]] into a [[Hilbert space]]. | |||
==Definitions== | |||
Let <math>G</math> be a [[second countable]] [[locally compact]] group. The following properties are all equivalent, and any of them may be taken to be definitions of the Haagerup property: | |||
#There is a [[proper function|proper]] [[continuous function|continuous]] conditionally [[negative definite]] [[Function (mathematics)|function]] <math>\Psi\colon G \to \Bbb{R}^+</math>. | |||
#<math>G</math> has the '''Haagerup approximation property''', also known as '''Property <math>C_0</math>''': there is a sequence of normalized continuous [[Positive-definite function on a group|positive-definite function]]s <math>\phi_n</math> which vanish at infinity on <math>G</math> and converge to 1 [[uniform convergence|uniformly]] on [[compact subset]]s of <math>G</math>. | |||
#There is a [[strongly continuous]] [[unitary representation]] of <math>G</math> which [[weak containment|weakly contains]] the [[trivial representation]] and whose matrix coefficients vanish at infinity on <math>G</math>. | |||
#There is a proper continuous affine isometric action of <math>G</math> on a [[Hilbert space]]. | |||
==Examples== | |||
There are many examples of groups with the Haagerup property, most of which are geometric in origin. The list includes: | |||
*All [[compact group]]s (trivially). Note all compact groups also have [[property (T)]]. The converse holds as well: if a group has both property (T) and the Haagerup property, then it is compact. | |||
*[[SO(n,1)]] | |||
*[[SU(n)|SU(n,1)]] | |||
*Groups acting properly on trees or on <math>\Bbb{R}</math>-trees | |||
*[[Coxeter groups]] | |||
*[[Amenable]] groups | |||
*Groups acting properly on [[CAT(0)]] [[cubical complex]]es | |||
==References== | |||
*Cherix, Cowling, Jolissaint, Julg, and Valette (2001). ''Groups with the Haagerup Property (Gromov's a-T-menability)'' | |||
[[Category:Representation theory]] | |||
[[Category:Geometric group theory]] |
Revision as of 03:06, 23 January 2014
In mathematics, the Haagerup property, named after Uffe Haagerup and also known as Gromov's a-T-menability, is a property of groups that is a strong negation of Kazhdan's property (T). Property (T) is considered a representation-theoretic form of rigidity, so the Haagerup property may be considered a form of strong nonrigidity; see below for details.
The Haagerup property is interesting to many fields of mathematics, including harmonic analysis, representation theory, operator K-theory, and geometric group theory.
Perhaps its most impressive consequence is that groups with the Haagerup Property satisfy the Baum-Connes conjecture and the related Novikov conjecture. Groups with the Haagerup property are also uniformly embeddable into a Hilbert space.
Definitions
Let be a second countable locally compact group. The following properties are all equivalent, and any of them may be taken to be definitions of the Haagerup property:
- There is a proper continuous conditionally negative definite function .
- has the Haagerup approximation property, also known as Property : there is a sequence of normalized continuous positive-definite functions which vanish at infinity on and converge to 1 uniformly on compact subsets of .
- There is a strongly continuous unitary representation of which weakly contains the trivial representation and whose matrix coefficients vanish at infinity on .
- There is a proper continuous affine isometric action of on a Hilbert space.
Examples
There are many examples of groups with the Haagerup property, most of which are geometric in origin. The list includes:
- All compact groups (trivially). Note all compact groups also have property (T). The converse holds as well: if a group has both property (T) and the Haagerup property, then it is compact.
- SO(n,1)
- SU(n,1)
- Groups acting properly on trees or on -trees
- Coxeter groups
- Amenable groups
- Groups acting properly on CAT(0) cubical complexes
References
- Cherix, Cowling, Jolissaint, Julg, and Valette (2001). Groups with the Haagerup Property (Gromov's a-T-menability)