# Periodic Orbit property

A topological space $X$ satisfies "Periodic orbit property", briefly POP, if for every continuous map

$f:X \to X$, there exist a natural number $n$ and a point $x_{0}\in X$ such that $f^{n}(x_{0})=x_{0}$

Obviously fixed point property(FPP) implies POP.

For a natural number $n$,a topological space $X$ is called $n-POP$ if for every continuous map $f$ on $X$, $f^{n}$ has a fixed point.(Ex: $\mathbb{S}^{2n}$ is a 2-POP manifold, using degree of maps)

The Question:

Is there an example of a manifold $M$ which satisfies POP but for every $n\in \mathbb{N}$, there is a continuous map $f$ on $M$ such that $f^{n}$ has no fixed point?

Namely: we search for a manifold for which every self map has a periodic orbit, but there is no any control on periods.

Equivalently:

Is there a manifold $M$ which is POP but not $n-POP$ for all $n\in \mathbb{N}$?

In particular, can we say:

"every compact POP manifold is necessarily a $n$-POP manifold, for some $n$"?

Motivating by Lefschetz fixed-point theorem, we ask that:

What algebraic topological criterion, can be introduced for consideration of this property(POP)?

Edit: According to the very interesting answer of Qiaochu Yuan, in the orientable case, the question is equivalent to the following:

Let M be a closed orientable manifold. Is it true that $M$ is not POP if and only if $\chi(M)=0$?

Note: I think the continuation of the argument of Qiaochu Yuan for his first statement is not easy, for arbitrary manifold. Because for the simplest case $S^{3}$ we had the famous conjecture of "existence of a vector field on $S^{3}$ without periodic orbit. In fact consideration of non vanishing vector fields is necessary but not sufficient. Periodic orbits of vector fields are important, too. Moreover, perhaps an approach which is not based on "vector fields" could be useful, for example consideration of oriention reversing diffeomorphisms.

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Let $R_{\alpha}:S^1\to S^1$ be an irrational rotation, then $R^n_{\alpha}$ has no fixed point for every $n\geqslant 1$ –  Juan Valdez Dec 24 '13 at 20:08
So $S^{1}$ is not a POP manifold. But I search for a manifold which is POP but is not n-POP for all $n\in \mathbb{N}$. –  Ali Taghavi Dec 25 '13 at 8:00
What are examples of manifolds that satisfy POP but not FPP? –  Benjamin Dickman Jan 1 at 12:38
$S^{2n}$ is an example. The antipodal map has no fixed point, So $S^{2n}$ does not satisfy FPP. On the other hand, if $f$ is a self map on $S^{2n}$ without fixed point,Then deg(f)=-1. See Algebraic topology(Allen Hatcher). So deg($f^{2}$)=1. Then $f^{2}$ has fixed point. This shows that for every self map f on $S^{2n}$, $f^{2}$ has a fixed point. –  Ali Taghavi Jan 1 at 13:23

Nice question! Here's what I can show.

Let $X$ be a smooth closed manifold. Then:

(1) If $\chi(X) = 0$, then $X$ is not $n$-POP for any $n$.

(2) If $\chi(X) \neq 0$ and $X$ is orientable, then $X$ is $\text{lcm}(1, 2, ... n)$-POP with respect to maps $f : X \to X$ of nonzero degree, where $n = \text{max}(b_0 + b_2 + ..., b_1 + b_3 + ...)$ (where $b_i$ is the $i^{th}$ Betti number of $X$).

Proof of 1. We will use the converse of the Poincaré-Hopf theorem: if $\chi(X) = 0$, then $X$ admits a nonvanishing vector field. Let $\varphi(t)$ denote the flow of this vector field. Let $t_{0}>0$ be small enough so that $\varphi(t_0)$ has no fixed points. Such $t_{0}$ exists, because there is a positive uniform lower bound for the period of all periodic orbits.(As a consequence of the flow box theorem, around regular points of a vector field). For a given $n \in \mathbb{N}$, let $f = \varphi \left( \frac{t_0}{n} \right)$. Then $f^n$ has no fixed points, hence $X$ is not $n$-POP. $\Box$

(I strongly suspect that in this case $X$ is not POP either; it seems like we should be able to consider a small flow of a sufficiently generic nonvanishing vector field. But I don't know how to finish this argument.)

Proof of 2. We will need the following two observations.

Lemma 1: Let $f_0, f_1$ be linear operators acting on two finite-dimensional vector spaces $V_0, V_1$. If $\text{tr}(f_0^k) = \text{tr}(f_1^k)$ for $k$ between $1$ and $\text{max}(\dim V_0, \dim V_1)$, then $f_0$ and $f_1$ have the same nonzero eigenvalues with the same multiplicities.

Proof. The above condition implies, using the Newton-Girard identities, that $f_0$ and $f_1$ have the same characteristic polynomial up to factors of $t$. $\Box$

Lemma 2: Let $X$ be an $n$-dimensional smooth closed oriented manifold and let $f : X \to X$ be a map of nonzero degree. Then every eigenvalue of $f$ acting on cohomology (with complex coefficients) is nonzero.

Proof. Let $e_1, ..., e_d$ be a basis of generalized eigenvectors for the action of $f$ on $H^k(X, \mathbb{C})$. By Poincaré duality the cup product $H^k \otimes H^{n-k} \to H^n$ is nondegenerate, so we can find a dual basis $e_1^{\ast}, ..., e_d^{\ast}$ of $H^{n-k}(X, \mathbb{C})$. Since $f$ acts by a nonzero scalar, namely $\deg f$, on $e_i \smile e_i^{\ast}$ for all $i$, the generalized eigenvalue of $e_i$ must also be nonzero. $\Box$

Now back to the proof of 2. With hypotheses as above, let $f_0$ denote the map induced by $f$ on the direct sum $V_0$ of the even-dimensional complex cohomology of $X$ and let $f_1$ denote the map induced by $f$ on the direct sum $V_1$ of the odd-dimensional complex cohomology of $X$, so that the Lefschetz trace of $f^k$ can be written

$$L(f^k) = \text{tr}(f_0^k) - \text{tr}(f_1^k).$$

By Lemma 2, the eigenvalues of $f_0$ and $f_1$ are all nonzero, so if $f_0$ and $f_1$ have the same nonzero eigenvalues then in particular $\dim V_0 = \dim V_1$. By the contrapositive of Lemma 1, if $\chi(X) = \dim V_0 - \dim V_1 \neq 0$, then there exists some $k$ between $1$ and $n = \text{max}(\dim V_0, \dim V_1)$ such that $L(f^k) \neq 0$, hence, by the Lefschetz fixed point theorem, such that $f^k$ has a fixed point. In particular, $f^{\text{lcm}(1, 2, ... n)}$ has a fixed point. $\Box$

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For $1$, it is sufficient to check that a sufficiently generic vector field has countably many loops. Then there is a real number that is not a rational multiple of the period of any loop. –  Will Sawin Jan 2 at 3:18
@Will: cool. How would I check that? My differential geometry is quite poor. –  Qiaochu Yuan Jan 2 at 4:11
For $\dim X=3$ you can always equip a contact 1-form $\lambda$ and speak about the Reeb vector field $R$, satisfying $\lambda(R)=1$ and $d\lambda(R,\cdot)=0$. "Loops" here are called Reeb orbits (and these exist by Taubes' proof of the Weinstein conjecture), and for generic $\lambda$ all Reeb orbits are "cut out transversely", and in particular are isolated. For example, the Hopf fibration depicts the Reeb flow on $(S^3,\lambda=\sum x_idy_i-y_idx_i)$ whose orbits are the $S^1$-fibers, and perturbing this contact form will break that foliation. –  Chris Gerig Jan 3 at 21:08
For anyone who's still thinking about this, this seems silly but I can't think of an example of a endomap of degree $0$ without fixed points. All of the examples of maps of degree $0$ I can think of are projection maps $M \times N \to M \times \{ n \}$, which fix $n \in N$. Any ideas? –  Qiaochu Yuan Jan 4 at 7:47
More generally, assume that $M$ is a manifold and $f$ is a map on $M$ without fixed point. Fix a point $y_{0}\in M$, Then $F:M\times M \to M\times M$ with $F=(f,y_{0})$ is another example –  Ali Taghavi Jan 6 at 10:28