7.1. Positive scalar curvature
Proof of Proposition 4.4. — In the setting of the proposition, the oper- atorD has a well-defined localised index
indexZG(D)∈K∗(C∗(M;Z)G).
Then indexG(D)∈K∗(C∗(M)G) is the image of indexZG(D) under the map K∗(C∗(Z)G) =K∗(C∗(M;Z)G)→K∗(C∗(M)G),
and hence equal to zero.
To prove Theorem 4.6, we use the following equivariant version of a theorem of Vilms [39] that was proved in [19].
Theorem 7.1. — Letπ: M → B be a fibre bundle with fibreN and structure groupK. Suppose that M and B both have bounded geometry and proper, isometricG-actions makingπ G-equivariant. Let gN be aK- invariant Riemannian metric onN. Then there is aG-invariant Riemannian metricgM on M such that π is a G-equivariant Riemannian submersion with totally geodesic fibres.
Proof of Theorem 4.6. — LetκG/K denote the scalar curvature of the G-invariant Riemannian metricgG/K on the base of the fibre bundleM → G/K. Note that sinceG/K is a homogeneous space,κG/K is a finite con- stant. Let H ⊆T M be an Ehresmann connection. Then as in the proof of Theorem 7.1 above, we may lift gG/K to a G-invariant metric gH on H, as well as lift theK-invariant Riemannian metricgN onN to a metric on the vertical subbundleV ⊆T M. Define a G-invariant metric on M by gM :=gH⊕gV.
SinceNhas uniformly positive scalar curvatureκN, it satisfies inf{κN}=:
κ0 >0. Now let T and A denote the O’Neill tensors of the submersion π (their definitions can be found in [32]). By Theorem 7.1 above, the fibres ofM are totally geodesic, soT = 0. Pick an orthonormal basis of horizon- tal vector fields{Xi}. A result of Kramer ([30], p. 596), relates the scalar curvatures by
κM(p) =κG/K+κN(p)−X
i,j
kAXi(Xj)kp.
Since bothM and N have bounded geometry, it follows that their scalar curvaturesκM and κN are uniformly bounded. Therefore
sup
p∈M
X
i,j
kAXi(Xj)kp6A0<∞
for some positive constant A0. Upon scaling the fibre metric on N by a positive factort, we obtain
κM(p)>κG/K+t−2κ0−A0>0 whenever 0< t <
r κ0
−κG/K+A0, where we choose A0 > 0 large enough such that−κG/K +A0 > 0. Thus gM is a G-invariant metric of uniform positive scalar curvature onM.
7.2. The localised Baum–Connes conjecture
LetZ ⊂X be a closed,G-invariant, cocompact subset. Let HX be an admissible equivariant C0(X)-module. The map ϕXZ in (6.3) restricts to a map
ϕXZ:D∗red(Z)G→D∗red(X)G that mapsC∗(Z)G intoC∗(X;Z)G. Hence we obtain
(7.1) ϕXZ:D∗red(Z)G/C∗(Z)G→D∗red(X)G/C∗(X;Z)G. By Paschke duality, the analyticK-homology ofZ equals
K∗G(Z) =K∗+1(D∗red(Z)G/C∗(Z)G).
So (7.1) induces
(7.2) (ϕXZ)∗:K∗G(Z)→K∗G(X)loc. Using the maps (7.2), we obtain
(7.3) ϕX∗ : RK∗G(X)→K∗G(X)loc.
Lemma 7.2. — The following diagram commutes, whereµXG denotes the analytic assembly map forX:
RK∗G(X) µ
X G //
ϕX∗
K∗(Cred∗ G).
K∗G(X)loc
indexlocG
88
Proof. —If Z ⊂X is G-cocompact, then naturality of boundary maps with respect to∗-homomorphisms implies that the diagram
K∗G(Z) indexG//
(ϕXZ)∗
K∗(Cred∗ G).
K∗G(X)loc
indexlocG
88
commutes. By Corollary 4.3, the top horizontal arrow equals µZG, so the
claim follows after we take direct limits.
This lemma directly implies Proposition 4.9.
Remark 7.3. — The arguments in this subsection have two more conse- quences.
(1) If the map (7.3) is injective, then injectivity of the Baum–Connes assembly map implies injectivity of the map in Conjecture 4.8.
(2) If the map (7.3) is surjective, then Conjecture 4.8 implies surjectiv- ity of the Baum–Connes assembly map.
BIBLIOGRAPHY
[1] H. Abels, “Parallelizability of proper actions, globalK-slices and maximal compact subgroups”,Math. Ann.212(1974), p. 1-19.
[2] N. Anghel, “Remark on Callias’ index theorem”, Rep. Math. Phys. 28(1989), no. 1, p. 1-6.
[3] ——— , “On the index of Callias-type operators”,Geom. Funct. Anal. 3(1993), no. 5, p. 431-438.
[4] P. Baum, A. Connes & N. Higson, “Classifying space for proper actions and K-theory of groupC∗-algebras”, inC∗-algebras: 1943–1993 (San Antonio, 1993), Contemporary Mathematics, vol. 167, American Mathematical Society, 1994, p. 240- 291.
[5] R. Bott&R. Seeley, “Some remarks on the paper of Callias”,Commun. Math.
Phys.62(1978), no. 3, p. 235-245.
[6] M. Braverman, “The index theory on non-compact manifolds with proper group action”,J. Geom. Phys.98(2015), p. 275-284.
[7] M. Braverman&S. Cecchini, “Callias-type operators in von Neumann algebras”, J. Geom. Anal.28(2018), no. 1, p. 546-586.
[8] J. Brüning&H. Moscovici, “L2-index for certain Dirac-Schrödinger operators”, Duke Math. J.66(1992), no. 2, p. 311-336.
[9] U. Bunke, “AK-theoretic relative index theorem and Callias-type Dirac operators”, Math. Ann.303(1995), no. 2, p. 241-279.
[10] U. Bunke &A. Engel, “The coarse index class with support”,https://arxiv.
org/abs/1706.06959, 2018.
[11] C. Callias, “Axial anomalies and index theorems on open spaces”,Commun. Math.
Phys.62(1978), no. 3, p. 213-234.
[12] C. Carvalho&V. Nistor, “An index formula for perturbed Dirac operators on Lie manifolds”,J. Geom. Anal.24(2014), no. 4, p. 1808-1843.
[13] S. Cecchini, “Callias-type operators inC∗-algebras and positive scalar curvature on noncompact manifolds”,J. Topol. Anal.12(2020), no. 4, p. 897-939.
[14] A. Connes&H. Moscovici, “TheL2-index theorem for homogeneous spaces of Lie groups”,Ann. Math.115(1982), no. 2, p. 291-330.
[15] G. Gong, Q. Wang&G. Yu, “Geometrization of the strong Novikov conjecture for residually finite groups”,J. Reine Angew. Math.621(2008), p. 159-189.
[16] M. Gromov &H. B. Lawson, Jr., “Positive scalar curvature and the Dirac op- erator on complete Riemannian manifolds”,Publ. Math., Inst. Hautes Étud. Sci.
(1984), no. 58, p. 83-196.
[17] H. Guo, “Index of Equivariant Callias-Type Operators and Invariant Metrics of Positive Scalar Curvature”,J. Geom. Anal.31(2021), no. 1, p. 1-34.
[18] H. Guo, P. Hochs & V. Mathai, “Coarse geometry and Callias quantisation”, Trans. Am. Math. Soc.374(2021), no. 4, p. 2479-2520.
[19] H. Guo, V. Mathai&H. Wang, “Positive scalar curvature and Poincaré duality for proper actions”,J. Noncommut. Geom.13(2019), no. 4, p. 1381-1433.
[20] H. Guo, Z. Xie& G. Yu, “A Lichnerowicz Vanishing Theorem for the Maximal Roe Algebra”,https://arxiv.org/abs/1905.12299, 2021.
[21] U. Haagerup&A. Przybyszewska, “Proper metrics on locally compact groups, and proper affine isometric actions on Banach spaces”,https://arxiv.org/abs/
math/0606794, 2006.
[22] N. Higson&J. Roe,AnalyticK-homology, Oxford Mathematical Monographs, Oxford University Press, 2000, xviii+405 pages.
[23] N. Higson, J. Roe& G. Yu, “A coarse Mayer–Vietoris principle”,Math. Proc.
Camb. Philos. Soc.114(1993), no. 1, p. 85-97.
[24] P. Hochs&V. Mathai, “Geometric quantization and families of inner products”, Adv. Math.282(2015), p. 362-426.
[25] P. Hochs&Y. Song, “An equivariant index for proper actions III: The invariant and discrete series indices”,Differ. Geom. Appl.49(2016), p. 1-22.
[26] P. Hochs, B.-L. Wang&H. Wang, “An equivariant Atiyah–Patodi–Singer index theorem for proper actions I: the index formula”, https://arxiv.org/abs/1904.
11146, 2020.
[27] ——— , “An equivariant Atiyah–Patodi–Singer index theorem for proper actions II: theK-theoretic index”,https://arxiv.org/abs/2006.08086, 2020.
[28] C. Kottke, “An index theorem of Callias type for pseudodifferential operators”,J.
K-Theory 8(2011), no. 3, p. 387-417.
[29] ——— , “A Callias-type index theorem with degenerate potentials”,Commun. Par- tial Differ. Equations40(2015), no. 2, p. 219-264.
[30] W. Kramer, “The scalar curvature on totally geodesic fiberings”, Ann. Global Anal. Geom.18(2000), no. 6, p. 589-600.
[31] D. Kucerovsky, “A short proof of an index theorem”,Proc. Am. Math. Soc.129 (2001), no. 12, p. 3729-3736.
[32] B. O’Neill, “The fundamental equations of a submersion”,Mich. Math. J. 13 (1966), p. 459-469.
[33] J. Roe, “Coarse cohomology and index theory on complete Riemannian manifolds”, Mem. Am. Math. Soc.104(1993), no. 497, p. x+90.
[34] ——— ,Index theory, coarse geometry, and topology of manifolds, CBMS Regional Conference Series in Mathematics, vol. 90, American Mathematical Society, 1996, x+100 pages.