Abstract
We present a history of the Baum–Connes conjecture, the methods involved, the current status, and the mathematics it generated.
To Alain Connes, for providing lifelong inspiration
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- 1.
This is important, e.g., for applications to foliations, see Chapter 7.
- 2.
In the case of SL 3(Z), surjectivity of μ r is the open problem; the LHS of the Baum–Connes conjecture was computed in [SG08].
- 3.
Recall that B(d, n) is defined as the quotient of the non-abelian free group F d by the normal subgroup generated by all n’s powers in F d.
- 4.
- 5.
- 6.
We believe that Connes and Moscovici actually had \(C^{\ast }_r(G)\), not C ∗(G), in mind when writing this.
- 7.
As Connes once pointed out: “E Γ is a point on which Γ acts freely!.”
- 8.
Although published only in 1995, the celebrated “ Conspectus” was first circulated in 1981.
- 9.
K-homology is the homology theory dual to topological K-theory. It was shown by Atiyah [Ati70] that an elliptic (pseudo-)differential operator on M defines an element in K 0(M).
- 10.
When B Γ is a general CW-complex we must replace K 0(B Γ) by , where X runs along compact subsets of B Γ.
- 11.
In terms of Kasparov theory, to be defined in Chapter 3 below, this can be expressed using Kasparov product: \(\beta [D]=[\mathcal {L}_\Gamma ]\otimes _{C(B\Gamma )}[D]\).
- 12.
- 13.
This is actually the same map as the map β from Section 2.5.
- 14.
The fact that it is indeed an equivalence relation does not appear in [BC00].
- 15.
Note that the proof is given there only for discrete groups, but the proof goes over to locally compact group.
- 16.
- 17.
For a nice proof of that result NOT appealing to Atiyah’s L 2-index theorem, see lemma 7.1 in [MV03].
- 18.
Or is a-(T)-menable, according to Gromov.
- 19.
A concrete example of a non-cocompact lattice in Sp(n, 1), is Sp(n, 1)(H(Z)), the group of points of the real algebraic group Sp(n, 1) over the ring H(Z) of integral quaternions. For such a group Conjecture 5 is still open.
- 20.
See the discussion of strong property (T) in 6.3.
- 21.
Until the end of Proposition 7.6, we denote a countable group by Γ∞ rather than Γ, as we view Γ∞ as the limit of its finite quotients Γk.
- 22.
If Γ∞ has property (T), e π is the image in \(C^{\ast }_\pi (\Gamma _\infty )\) of the Kazhdan projection \(e_{\mathcal {G}}\in C^{\ast }_{\mathrm {max}}(\Gamma _\infty )\) from Proposition 5.4.
- 23.
The re-interpretation goes as follows: fix an auxiliary orientation on the edges of E, allowing one to define the coboundary operator d : ℓ 2(V ) → ℓ 2(E) : ϕ↦dϕ, where dϕ(e) = ϕ(e +) − ϕ(e −). Observe that Δ = d ∗d, so that \(\langle \Delta \phi ,\phi \rangle =\|d\phi \|{ }^2=\frac {1}{2}\sum _{x\sim y}|\phi (x)-\phi (y)|{ }^2\). By the Rayleigh quotient, \(\frac {1}{\lambda _1}\) is the smallest constant K > 0 such that ∥ϕ∥2 ≤ K∥dϕ∥2 for every ϕ ⊥ 1. We leave the rest as an exercise.
- 24.
Recall that a group acting properly isometrically on a CAT(0) cube complex, has the Haagerup property, see, e.g., Corollary 1 in [Val18].
- 25.
Amenability (resp. property (T)) can be defined by a fixed point property: existence of a fixed point for affine actions on compact convex sets (resp. affine isometric actions on Hilbert spaces). This makes clear that it is preserved under extensions.
- 26.
Under the assumption that | Γ| coarsely embeds into Hilbert space, the assumption that B Γ is a finite complex was removed by Skandalis et al. [STY02], using their groupoid approach to CBC.
- 27.
Recall from Section 8.2.3 that an entourage is a subset of X × X on which d(., .) is bounded.
- 28.
- 29.
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Acknowledgement
Thanks are due to J.-B. Bost, R. Coulon, N. Higson, V. Lafforgue, P.-Y. Le Gall, H. Oyono-Oyono, N. Ozawa, and M. de la Salle for useful conversations and exchanges.
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Aparicio, M.P.G., Julg, P., Valette, A. (2019). The Baum–Connes conjecture: an extended survey. In: Chamseddine, A., Consani, C., Higson, N., Khalkhali, M., Moscovici, H., Yu, G. (eds) Advances in Noncommutative Geometry. Springer, Cham. https://doi.org/10.1007/978-3-030-29597-4_3
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