Self-maps of simply connected manifolds

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1 Question

Let us call an oriented closed connected manifold flexible if it admits a self-map that has non-trivial degree (i.e., degree not equal to 1, 0, or -1).

Question 1.1. Do there exist closed simply connected manifolds (of non-zero dimension) that are not flexible?

Remark 1.2. In the following, for simplicity, we implicitly assume that all manifolds are of non-zero dimension.

2 Examples and partial answers

2.1 Examples of flexible manifolds

  • Of course, all spheres are flexible.
  • All odd-dimensional real projective spaces are flexible; all complex projective spaces are flexible.
  • Products of flexible manifolds with oriented closed connected manifolds are flexible; in particular, tori are flexible.
  • All closed simply connected 3-manifolds are flexible.
  • All closed simply connected 4-manifolds are flexible [Duan&Wang2004, Corollary 2].
  • ...

2.2 Examples of manifolds that are not flexible

Notice that there are many oriented closed connected manifolds that are not flexible.

Example 2.1.

  • All oriented closed connected manifolds with non-zero simplicial volume are not flexible (because the simplicial volume is functorial.
  • This includes, for instance, oriented closed connected manifolds of non-positive sectional curvature. (More examples and explanations of these facts can be found on the page on simplicial volume.)

However, by a theorem of Gromov, the simplicial volume of closed simply connected manifolds is always zero. So the simplicial volume cannot be used to discover closed simply connected manifolds that are not flexible.

3 Solution

This problem is was in fact Solved in [Arkowitz&Lupton2000, Examples 5.1 & 5.2]. There they give examples of 1-connected rational Poincare complexes, \mathcal{M}_1 and \mathcal{M}_2 of dimensions 208 and 228. Both of these algebras have a finite set of homotopy classes of self maps and so are inflexible They also state that the alebras \mathcal{M}_1 and \mathcal{M}_1 can be realised by simply connected manifolds M_1 and M_2. It follows that M_1 and M_2 are inflexible.

The realisation of \mathcal{M}_1 and $\mathcal{M}_2 relies upon a theorem proven independently by Barge and Sullivan. A special case of this theorem is as follows

Theorem 3.1 c.f. [Barge1976, Theorem 1] and [Sullivan1977, Theorem 13.2]. Let \mathcal{M} be a simply connected differential rational Poincare algebra of dimension n. If either

  • n \neq 4k or
  • or n = 4k and the intersection form of \mathcal{M} represents the trivial element of W_0(\Qq, the Witt group of \Qq,
then there is a close simply connected PL manifold
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with rational homotopy type given by \mathcal{M}. If n \neq 4k then
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may be given a smooth structure and if n = 4k there
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can be smoothed except possible at one point.

Remark 3.2. Note that [Arkowitz&Lupton2000] simply state that the theorem of Barge-Sullivan applies and do not give arguments that \mathcal{M}_1 or \mathcal{M}_2 satisfy the hypotheses of Barge. See the discussion page for more information.

4 Further discussion

...

5 References

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