# Converse of Poincaré-Hopf theorem

Let $M$ be a connected, compact, oriented manifold of dimension $n<7$. If any two maps $M \to M$ having equal degrees are homotopic, must $M$ be diffeomorphic to the $n$-sphere?

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Of course not. Consider a homotopy-sphere for example. –  Ryan Budney Jul 21 '13 at 7:01
There's infinitely many examples that aren't even homotopy spheres. Just take any manifold so that the group of homotopy-classes of homotopy self-equivalences is trivial. For example, a hyperbolic manifold with trivial isometry group. –  Ryan Budney Jul 21 '13 at 7:25
@Ryan: As I am sure you know, in dimensions less than seven and different from four, any homotopy sphere is diffeomorphic to the standard sphere; in dimension four, the smooth Poincaré conjecture is unresolved. –  Ricardo Andrade Jul 21 '13 at 7:47
It follows from Mostow rigidity. There's plenty of symmetry-less hyperbolic 3-manifolds. Every finite group is the isometry group of a hyperbolic 3-manifold, and the trivial group is the isometry group of infinitely many. –  Ryan Budney Jul 21 '13 at 10:22
@Ryan: in the hyperbolic manifold example, how about non-homotopic zero-degree maps? –  Sergei Ivanov Jul 21 '13 at 18:21


I have failed to find any reference with a proof that two self-maps of $\RR P^n$ which have the same degree are homotopic, although this result is stated without proof as theorem 2.1 in this article (McGibbon, Self-maps of projective spaces, Transactions of the A.M.S. 271 (1), 1982, pages 325-346). For completeness, I will give a proof below. The proof is more homotopy theoretical than for the case of $\CC P^3$ above, yet also uses fairly standard techniques.

[The rest of this answer is rather long, and you may ignore it if you are not interested in the proof for $\RR P^n$.]

Proof that two self-maps of $\RR P^n$ (with $n>1$ odd) are homotopic if they have the same degree

Assume that $n$ is an odd integer with $n>1$. I will actually prove the stronger result that two basepoint-preserving self-maps of $\RR P^n$ which have the same degree are homotopic via a basepoint-preserving homotopy. More precisely, I will prove that the map which takes (the pointed homotopy class of) a self-map to its degree $$\deg : [\RR P^n,\RR P^n]_\ast \To \ZZ$$ is a bijection. Let us start with a lemma which will be useful later on.

Lemma: A map $f:\RR P^n \to \RR P^n$ (with $n>1$) has even degree if and only if $f$ induces the trivial map on fundamental groups.

Proof: This proof involves some cohomological computations similar to the ones done earlier for $\CC P^3$. Note that $H^\ast(\RR P^n;\ZZ/2) = \ZZ/2[x]/(x^{n+1})$ where $x$ is in degree $1$. Since $\pi_1(\RR P^n)=\ZZ/2$, a self-map $f$ of $\RR P^n$ induces the trivial homomorphism on $\pi_1$ if and only if it induces the trivial homomorphism on $H^1(-;\ZZ/2)$, i.e. if and only if $f^\ast x = 0$. Letting $f^\ast x = k x$ with $k\in\ZZ/2$, then $f^\ast(x^n) = k^n x^n = k x^n$, so $k = \deg f \mod 2$. The result follows. ■

A cofibre sequence involving $\RR P^n$. Our starting point is the following description of $\RR P^n$. The space $\RR P^n$ is obtained by attaching a $n$-cell to $\RR P^{n-1}$ whose attaching map is the standard covering map $\pi : S^{n-1} \to \RR P^{n-1}$. In short, $\RR P^n$ is the homotopy cofibre of the map $\pi$. The associated homotopy cofibre sequence — extended to the right by one term — is: $$S^{n-1} \overset{\pi}{\To} \RR P^{n-1} \overset{i}{\To} \RR P^n \overset{q}{\To} S^n$$ Here, the first map $\pi$ is the standard quotient map, the second map $i$ is the inclusion, and the third map $q$ collapses $\RR P^{n-1}$ to a point.

The associated exact sequence in homotopy. Applying the functor $[-,\RR P^n]_\ast$ to the above cofibre sequence produces an exact sequence: $$[S^n,\RR P^n]_\ast \overset{q^\ast}{\To} [\RR P^n,\RR P^n]_\ast \overset{i^\ast}{\To} [\RR P^{n-1},\RR P^n]_\ast \overset{\pi^\ast}{\To} [S^{n-1},\RR P^n]_\ast$$ This is an exact sequence of pointed sets. Moreover, the leftmost term is a group, and the sequence verifies a stronger equivariant exactness property at the term $[\RR P^n,\RR P^n]_\ast$ which will be explained and used later.

Calculation of the terms of the exact sequence. First, let us calculate all terms of the sequence other than $[\RR P^n,\RR P^n]_\ast$:

• we have isomorphisms $[S^n,\RR P^n]_\ast = \pi_n(\RR P^n) = \pi_n(S^n) = \ZZ$ as groups (by using the covering map $S^n \to \RR P^n$ which induces an isomorphism on higher homotopy groups);

• similarly, $[S^{n-1},\RR P^n]_\ast = \pi_{n-1}(\RR P^n) = \pi_{n-1}(S^n) = 0$;

• $[\RR P^{n-1},\RR P^n]_\ast = [\RR P^{n-1},\RR P^\infty]_\ast = \widetilde{H}^1(\RR P^{n-1};\ZZ/2) = \ZZ/2$ by using cellular approximation again.

Summarizing, the previous exact sequence of pointed sets is $$\ZZ \overset{q^\ast}{\To} [\RR P^n,\RR P^n]_\ast \overset{i^\ast}{\To} \ZZ/2 \overset{\pi^\ast}{\To} 0$$ In particular, $i^\ast$ is a surjective function.

Description of the arrow $i^\ast$ of the exact sequence. The next step is to identify the function $i^\ast$ appearing in the exact sequence. Consider the following isomorphisms characterizing the target of $i^\ast$: $$[\RR P^{n-1},\RR P^n]_\ast = \widetilde{H}^1(\RR P^{n-1};\ZZ/2) = \Hom\bigl(\pi_1(\RR P^{n-1}),\ZZ/2\bigr) = \Hom\bigl(\pi_1(\RR P^{n-1}),\pi_1(\RR P^n)\bigr)$$ Recall that the inclusion $i:\RR P^{n-1}\to \RR P^n$ induces an isomorphism on $\pi_1$. Therefore, the function $$i^\ast : [\RR P^n,\RR P^n]_\ast \To \Hom\bigl(\pi_1(\RR P^{n-1}),\pi_1(\RR P^n)\bigr) = \ZZ/2$$ takes $f:\RR P^n \to \RR P^n$ to $0\in\ZZ/2$ if and only if $f$ induces the trivial homomorphism on fundamental groups. The lemma at the beginning of this proof now implies that $i^\ast$ takes a map $f$ to its degree mod $2$, $\deg_2 f$.

In conclusion, the exact sequence becomes: $$\ZZ = [S^n,\RR P^n]_\ast \overset{q^\ast}{\To} [\RR P^n,\RR P^n]_\ast \overset{\deg_2}{\To} \ZZ/2$$ where the second arrow $\deg_2$ returns the degree mod $2$ of a self-map of $\RR P^n$. We know from before that $\deg_2 = i^\ast$ is surjective.

Equivariant exactness property of the exact sequence. At this point, we require the special equivariant exactness property of the long exact sequence in homotopy associated to a cofibre sequence. In this case, it states:

• the group $\ZZ = \pi_n(\RR P^n) = [S^n,\RR P^n]_\ast$ (with its usual group structure) acts on $[\RR P^n,\RR P^n]_\ast$;

• $f$ and $g$ map to the same element via $\deg_2 : [\RR P^n,\RR P^n]_\ast \to \ZZ/2$ if and only if there exists $\alpha\in [S^n,\RR P^n]_\ast$ such that $\alpha\cdot f = g$ in $[\RR P^n,\RR P^n]_\ast$.

The action of $\alpha \in [S^n,\RR P^n]_\ast$ takes $f \in [\RR P^n,\RR P^n]_\ast$ to: $$\alpha\cdot f : \RR P^n \overset{\sigma}{\To} S^n \vee \RR P^n \overset{\alpha+f}{\To} \RR P^n$$ where the map $\sigma$ simply "pinches a small spherical bubble off of $\RR P^n$". Note that $\deg(\alpha\cdot f) = \deg\alpha + \deg f$.

Conclusion of the proof. We are now ready to finish the proof. First we will show the injectivity of the degree function. Assume that $f$ and $g$ are pointed self-maps of $\RR P^n$ which have the same degree. Then their degrees mod $2$ also coincide and, by the exactness property above, there exists $\alpha$ such that $\alpha\cdot f = g$. But $$\deg f = \deg g = \deg(\alpha\cdot f) = \deg\alpha + \deg f$$ so that $\deg\alpha = 0$. It follows that $\alpha = 0$ in $\pi_n(\RR P^n)$, since elements in $\pi_n(\RR P^n) = \pi_n(S^n)$ are determined by their degree (here we require that $n$ is odd). Thus $g = \alpha\cdot f = f$ in $[\RR P^n,\RR P^n]_\ast$.

Finally, we show that any integer is the degree of some map $\RR P^n \to \RR P^n$. Recall that the function $\deg_2 : [\RR P^n,\RR P^n]_\ast \to \ZZ/2$ taking a map to its degree mod $2$ is surjective. In other words, there are self-maps of $\RR P^n$ with even and odd degrees. Since $\deg(\alpha\cdot f) = \deg\alpha + \deg f$, the desired conclusion follows by observing that any even integer is the degree of some map $\alpha : S^n \to \RR P^n$ — which holds because the covering map $S^n \to \RR P^n$ has degree $2$ and induces an isomorphism on $\pi_n$.

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Beautiful. ${}$ –  Mariano Suárez-Alvarez Jul 21 '13 at 22:41

To complete @RyanBudney's suggestion (and @Misha's comment), there are 3-manifolds in the snappea census which have trivial isometry group, so in particular have no non-trivial degree 1 self-maps, and which have trivial first betti number. If the map is degree $>0$, then this follows from Gromov's proof of Mostow rigidity (using the Gromov norm, the degree must $=\pm 1$, and the map is homotopic to an isometry). If a degree zero map from a 3-manifold to itself is homotopically non-trivial, then it must surject the fundamental group of some covering space. If this covering space is finite-index, then since the group is co-finitely Hopfian, this map must induce an isomorphism on fundamental group, and thus must be degree $\pm 1$, a contradiction. Thus, the image group must be infinite index. These groups have positive betti number, a contradiction.

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Dear @Ian Agol: Very nice answer. Thank you for explaining this. If you do not mind, could you please elaborate briefly (or perhaps give a reference for) your remark between parenthesis regarding the Gromov norm? Thank you very much. Also, it may be helpful to make clear that the snappea census covers only hyperbolic manifolds (if I am not mistaken), as most people are unlikely to be familiar with snappea. –  Ricardo Andrade Nov 12 '13 at 21:15
Ricardo: Gromov norm is a homotopy invariant and is multiplicative under finite covers. –  Igor Belegradek Nov 12 '13 at 23:54
@RicardoAndrade: Here's an overview: map.mpim-bonn.mpg.de/Simplicial_volume –  Ian Agol Nov 13 '13 at 4:09