Gromov-Lawson conjecture
If is a metric of positive scalar curvature (cf. also Metric) on a compact spin manifold (cf. also Spinor structure), results of A. Lichnerowicz [a4] show that there are no harmonic spinors; consequently, the \hat{A}-genus of M vanishes. M. Gromov and H.B. Lawson [a2], [a3] showed that if a manifold M _ { 1 } can be obtained from a manifold M _ { 2 } which admits a metric of positive scalar curvature, by surgeries in codimension at least 3, then M _ { 1 } admits a metric of positive scalar curvature. They wondered if this might be the only obstruction to the existence of a metric of positive scalar curvature in the spinor context if the dimension m was at least 5. (This restriction is necessary to ensure that certain surgery arguments work.) S. Stolz [a9] showed this was the case in the simply-connected setting: if M is a simply-connected spin manifold of dimension at least 5 (cf. also Simply-connected domain), then M admits a metric of positive scalar curvature if and only if the \hat{A}-genus of M vanishes. This invariant takes values in \bf Z if m \equiv 0 modulo 8, in \mathbf{Z}_{2} if m \equiv 1,2 modulo 8, in 2 \mathbf{Z} if m \equiv 4 modulo 8, and vanishes if m \equiv 3,5,6,7 modulo 8.
The situation is more complicated in the presence of a fundamental group \pi. Let \mathcal{Z} _ { m} ^{\pi } be the Grothendieck group of finitely generated \mathbf{Z}_{2}-graded modules over the Clifford algebra \operatorname { Clif } ({\bf R} ^ { m } ) which have a \pi action commuting with the \operatorname { Clif } ({\bf R} ^ { m } ) action. The inclusion i of \operatorname { Clif } ({\bf R} ^ { m } ) in \operatorname{Clif}( \mathbf R ^ { m + 1 } ) induces a dual pull-back i ^ { * } from \mathcal{Z} _ { m + 1 } ^ { \pi } to \mathcal{Z} _ { m} ^{\pi }. The real K-theory groups of \mathbf{R} \pi are given by:
\begin{equation*} K O _ { m } ( {\bf R} \pi ) = {\cal Z} _ { m } ^ { \pi } / i ^ { * } {\cal Z} _ { m + 1 } ^ { \pi }. \end{equation*}
J. Rosenberg [a6] defined a K-theory-valued invariant \alpha taking values in this group which generalizes the \hat{A}-genus. It was conjectured that this might provide a complete description of the obstruction to the existence of a metric of positive scalar curvature; this refined conjecture became known as the Gromov–Lawson–Rosenberg conjecture.
The conjecture was established for spherical space form groups [a1] and for finite Abelian groups of rank at most 2 and odd order [a8]. It has also been established for a (short) list of infinite groups, including free groups, free Abelian groups, and fundamental groups of orientable surfaces [a5]. S. Schick [a7] has shown that the conjecture, in the form due to J. Rosenberg, is false by exhibiting a compact spin manifold with fundamental group \mathbf{Z}\oplus\mathbf{Z}\oplus\mathbf{Z}\oplus\mathbf{Z}\oplus\mathbf{Z}_3 which does not admit a metric of positive scalar curvature but for which the \alpha invariant vanishes. It is not known (1998) if the conjecture holds for finite fundamental groups.
References
[a1] | B. Botvinnik, P. Gilkey, S. Stolz, "The Gromov–Lawson–Rosenberg conjecture for groups with periodic cohomology" J. Diff. Geom. , 46 (1997) pp. 374–405 |
[a2] | M. Gromov, H.B. Lawson, "Spin and scalar curvature in the presence of a fundamental group I" Ann. of Math. , 111 (1980) pp. 209–230 |
[a3] | M. Gromov, H.B. Lawson, "The classification of simply connected manifolds of positive scalar curvature" Ann. of Math. , 111 (1980) pp. 423–434 |
[a4] | A. Lichnerowicz, "Spineurs harmoniques" C.R. Acad. Sci. Paris , 257 (1963) pp. 7–9 |
[a5] | J. Rosenberg, S. Stolz, "A "stable" version of the Gromov–Lawson conjecture" Contemp. Math. , 181 (1995) pp. 405–418 |
[a6] | J. Rosenberg, "C ^ { * }-algebras, positive scalar curvature, and the Novikov conjecture" Publ. Math. IHES , 58 (1983) pp. 197–212 |
[a7] | T. Schick, "A counterexample to the (unstable) Gromov–Lawson–Rosenberg conjecture" Topology , 37 (1998) pp. 1165–1168 |
[a8] | R. Schultz, "Positive scalar curvature and odd order Abelian fundamental groups" Proc. Amer. Math. Soc. , 125 : 3 (1997) pp. 907–915 |
[a9] | S. Stolz, "Simply connected manifolds of positive scalar curvature" Ann. of Math. , 136 (1992) pp. 511–540 |
Gromov-Lawson conjecture. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Gromov-Lawson_conjecture&oldid=54690