Schur's lemma (Riemannian geometry)

In Riemannian geometry, Schur's lemma is a result that says, heuristically, whenever certain curvatures are pointwise constant then they are forced to be globally constant. The proof is essentially a one-step calculation, which has only one input: the second Bianchi identity.

The Schur lemma for the Ricci tensor

Suppose (M, g) is a smooth Riemannian manifold with dimension n. Recall that this defines for each element p of M:

The Schur lemma states the following:

Suppose that n is not equal to two. If there is a function κ on M such that Ricp = κ(p)gp for all p in M then dκ = 0. Equivalently, κ is constant on each connected component of M; this could also be phrased as asserting that each connected component of M is an Einstein manifold.

The Schur lemma is a simple consequence of the "twice-contracted second Bianchi identity," which states that

understood as an equality of smooth 1-forms on M. Substituting in the given condition Ricp = κ(p)gp, one finds that

Alternative formulations of the assumptions

Let B be a symmetric bilinear form on an n-dimensional inner product space (V, g). Then

Additionally, note that if B = κg for some number κ, then one automatically has κ = 1/ntrgB. With these observations in mind, one can restate the Schur lemma in the following form:

Let (M, g) be a connected smooth Riemannian manifold whose dimension is not equal to two. Then the following are equivalent:

  • There is a function κ on M such that Ricp = κ(p)gp for all p in M
  • There is a number κ such that Ricp = κgp for all p in M, i.e. (M, g) is Einstein
  • One has Ricp = 1/nRpgp for all p in M, i.e. the traceless Ricci tensor is zero

If (M, g) is a connected smooth pseudo-Riemannian manifold, then the first three conditions are equivalent, and they imply the fourth condition.

Note that the dimensional restriction is important, since every two-dimensional Riemannian manifold which does not have constant curvature would be a counterexample.

The Schur lemma for the Riemann tensor

The following is an immediate corollary of the Schur lemma for the Ricci tensor.

Let be a connected smooth Riemannian manifold whose dimension n is not equal to two. Then the following are equivalent:

  • There is a function κ on M such that secp(V) = κ(p) for all p in M and all two-dimensional linear subspaces V of TpM
  • There is a number κ such that secp(V) = κ for all p in M and all two-dimensional linear subspaces V of TpM, i.e. (M, g) has constant curvature
  • secp(V) = 1/n(n-1)Rp for all p in M and all two-dimensional linear subspaces V of TpM
  • for all p in M
  • the sum of the Weyl curvature and semi-traceless part of the Riemann tensor is zero
  • both the Weyl curvature and the semi-traceless part of the Riemann tensor are zero

The Schur lemma for Codazzi tensors

Let (M, g) be a smooth Riemannian or pseudo-Riemannian manifold of dimension n. Let h he a smooth symmetric (0,2)-tensor field whose covariant derivative, with respect to the Levi-Civita connection, is completely symmetric. The symmetry condition is an analogue of the Bianchi identity; continuing the analogy, one takes a trace to find that

If there is a function κ on M such that hp = κ(p)gp for all p in M, then upon substitution one finds

Hence n > 1 implies that κ is constant on each connected component of M. As above, one can then state the Schur lemma in this context:

Let (M, g) be a connected smooth Riemannian manifold whose dimension is not equal to one. Let h be a smooth symmetric (0,2)-tensor field whose covariant derivative is totally symmetric as a (0,3)-tensor field. Then the following are equivalent:

  • there is a function κ on M such that hp = κ(p)gp for all p in M
  • there is a number κ such that hp = κgp for all p in M
  • hp = 1/n(trghp)gp for all p in M, i.e. the traceless form of h is zero
  • for all p in M
  • for all p in M

If (M, g) is a connected and smooth pseudo-Riemannian manifold, then the first three are equivalent, and imply the fourth and fifth.

Applications

The Schur lemmas are frequently employed to prove roundness of geometric objects. A noteworthy example is to characterize the limits of convergent geometric flows.

For example, a key part of Richard Hamilton's 1982 breakthrough on the Ricci flow[1] was his "pinching estimate" which, informally stated, says that for a Riemannian metric which appears in a 3-manifold Ricci flow with positive Ricci curvature, the eigenvalues of the Ricci tensor are close to one another relative to the size of their sum. If one normalizes the sum, then, the eigenvalues are close to one another in an absolute sense. In this sense, each of the metrics appearing in a 3-manifold Ricci flow of positive Ricci curvature "approximately" satisfies the conditions of the Schur lemma. The Schur lemma itself is not explicitly applied, but its proof is effectively carried out through Hamilton's calculations.

In the same way, the Schur lemma for the Riemann tensor is employed to study convergence of Ricci flow in higher dimensions. This goes back to Gerhard Huisken's extension of Hamilton's work to higher dimensions,[2] where the main part of the work is that the Weyl tensor and the semi-traceless Riemann tensor become zero in the long-time limit. This extends to the more general Ricci flow convergence theorems, some expositions of which directly use the Schur lemma.[3] This includes the proof of the differentiable sphere theorem.

The Schur lemma for Codazzi tensors is employed directly in Huisken's foundational paper on convergence of mean curvature flow, which was modeled on Hamilton's work.[4] In the final two sentences of Huisken's paper, it is concluded that one has a smooth embedding with

where is the second fundamental form and is the mean curvature. The Schur lemma implies that the mean curvature is constant, and the image of this embedding then must be a standard round sphere.

Another application relates full isotropy and curvature. Suppose that is a connected thrice-differentiable Riemannian manifold, and that for each the group of isometries acts transitively on This means that for all and all there is an isometry such that and This implies that also acts transitively on i.e. for every there is an isometry such that and Since isometries preserve sectional curvature, this implies that is constant for each The Schur lemma implies that has constant curvature. A particularly notable application of this is that any spacetime which models the cosmological principle must be the warped product of an interval and a constant-curvature Riemannian manifold. See O'Neill (1983, page 341).

Stability

Recent research has investigated the case that the conditions of the Schur lemma are only approximately satisfied.

Consider the Schur lemma in the form "If the traceless Ricci tensor is zero then the scalar curvature is constant." Camillo De Lellis and Peter Topping[5] have shown that if the traceless Ricci tensor is approximately zero then the scalar curvature is approximately constant. Precisely:

  • Suppose is a closed Riemannian manifold with nonnegative Ricci curvature and dimension Then, where denotes the average value of the scalar curvature, one has

Next, consider the Schur lemma in the special form "If is a connected embedded surface in whose traceless second fundamental form is zero, then its mean curvature is constant." Camillo De Lellis and Stefan Müller[6] have shown that if the traceless second fundamental form of a compact surface is approximately zero then the mean curvature is approximately constant. Precisely

  • there is a number such that, for any smooth compact connected embedded surface one has
where is the second fundamental form, is the induced metric, and is the mean curvature

As an application, one can conclude that itself is 'close' to a round sphere.

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References

  1. Hamilton, Richard S. (1982). "Three-manifolds with positive Ricci curvature". J. Differential Geometry. 17 (2): 255–306.
  2. Huisken, Gerhard (1985). "Ricci deformation of the metric on a Riemannian manifold". J. Differential Geom. 21 (1): 47–62.
  3. Böhm, Christoph; Wilking, Burkhard (2008). "Manifolds with positive curvature operators are space forms". Ann. of Math. (2). 167 (3): 1079–1097.
  4. Huisken, Gerhard (1984). "Flow by mean curvature of convex surfaces into spheres". J. Differential Geom. 20 (1): 237–266.
  5. De Lellis, Camillo; Topping, Peter M. (2012). "Almost-Schur lemma". Calc. Var. Partial Differential Equations. 443 (3–44): 347–354.
  6. De Lellis, Camillo; Müller, Stefan (2005). "Optimal rigidity estimates for nearly umbilical surfaces". J. Differential Geom. 69 (1): 75–110.
  • Shoshichi Kobayashi and Katsumi Nomizu. Foundations of differential geometry. Vol. I. Interscience Publishers, a division of John Wiley & Sons, New York-London 1963 xi+329 pp.
  • Barrett O'Neill. Semi-Riemannian geometry. With applications to relativity. Pure and Applied Mathematics, 103. Academic Press, Inc. [Harcourt Brace Jovanovich, Publishers], New York, 1983. xiii+468 pp. ISBN 0-12-526740-1
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