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Moishe Kohan
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Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. I claim that $F_g$ is a convex subset of $T$. Indeed, take two points $p, q\in F_g$. Then the Teichmuller geodesic segment $pq$ between $p, q$ is mapped via $g$ to a path $c$ in $T$ of length $\le d(p,q)$ (since $g: T\to T$ is a holomorphic map, it weakly decreases the Kobayashi-Teichmuller distance). But $c$ connects $p$ and $q$, hence, its length $=d(p,q)$, i.e. $c$ is a geodesic segment equal to $pq$; thus, $pq$ has to be fixed by $g$ pointwise. Thus, $pq\subset F_g$. (Most likely, $F_g$ is a submanifold but we do not need this.) Convexity of $F_g$ implies that it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible and locally contractible finite-dimensional locally compact metrizable space (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvarietyand locally contractible subset, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. I claim that $F_g$ is a convex subset of $T$. Indeed, take two points $p, q\in F_g$. Then the Teichmuller geodesic segment $pq$ between $p, q$ is mapped via $g$ to a path $c$ in $T$ of length $\le d(p,q)$ (since $g: T\to T$ is a holomorphic map, it weakly decreases the Kobayashi-Teichmuller distance). But $c$ connects $p$ and $q$, hence, its length $=d(p,q)$, i.e. $c$ is a geodesic segment equal to $pq$; thus, $pq$ has to be fixed by $g$ pointwise. Thus, $pq\subset F_g$. (Most likely, $F_g$ is a submanifold but we do not need this.) Convexity of $F_g$ implies that it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible and locally contractible finite-dimensional locally compact metrizable space (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvariety, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. I claim that $F_g$ is a convex subset of $T$. Indeed, take two points $p, q\in F_g$. Then the Teichmuller geodesic segment $pq$ between $p, q$ is mapped via $g$ to a path $c$ in $T$ of length $\le d(p,q)$ (since $g: T\to T$ is a holomorphic map, it weakly decreases the Kobayashi-Teichmuller distance). But $c$ connects $p$ and $q$, hence, its length $=d(p,q)$, i.e. $c$ is a geodesic segment equal to $pq$; thus, $pq$ has to be fixed by $g$ pointwise. Thus, $pq\subset F_g$. (Most likely, $F_g$ is a submanifold but we do not need this.) Convexity of $F_g$ implies that it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible and locally contractible finite-dimensional locally compact metrizable space (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible and locally contractible subset, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

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Moishe Kohan
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Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. The subset I claim that $F_g$ is a convex complex submanifoldsubset of $T$. Indeed, take two points (I did not check carefully that it$p, q\in F_g$. Then the Teichmuller geodesic segment $pq$ between $p, q$ is mapped via $g$ to a submanifold,path $c$ in $T$ of length $\le d(p,q)$ (since $g: T\to T$ is a prioriholomorphic map, it is only an analytic subvarietyweakly decreases the Kobayashi-Teichmuller distance). Convexity should imply that itBut $c$ connects $p$ and $q$, hence, its length $=d(p,q)$, i.e. $c$ is a submanifoldgeodesic segment equal to $pq$; thus, $pq$ has to be fixed by $g$ pointwise. In the worst caseThus, a subvariety suffices for the proof$pq\subset F_g$. (Most likely, since all I need$F_g$ is that it admits a triangulationsubmanifold but we do not need this.) In particular,Convexity of $F_g$ implies that it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and    $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible and locally contractible finite-dimensional CW complex locally compact metrizable space (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvariety, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. The subset $F_g$ is a convex complex submanifold of $T$. (I did not check carefully that it is a submanifold, a priori, it is only an analytic subvariety. Convexity should imply that it is a submanifold. In the worst case, a subvariety suffices for the proof, since all I need is that it admits a triangulation.) In particular, it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and  $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible finite-dimensional CW complex (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvariety, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. I claim that $F_g$ is a convex subset of $T$. Indeed, take two points $p, q\in F_g$. Then the Teichmuller geodesic segment $pq$ between $p, q$ is mapped via $g$ to a path $c$ in $T$ of length $\le d(p,q)$ (since $g: T\to T$ is a holomorphic map, it weakly decreases the Kobayashi-Teichmuller distance). But $c$ connects $p$ and $q$, hence, its length $=d(p,q)$, i.e. $c$ is a geodesic segment equal to $pq$; thus, $pq$ has to be fixed by $g$ pointwise. Thus, $pq\subset F_g$. (Most likely, $F_g$ is a submanifold but we do not need this.) Convexity of $F_g$ implies that it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and  $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible and locally contractible finite-dimensional locally compact metrizable space (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvariety, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.

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Moishe Kohan
  • 12.2k
  • 1
  • 36
  • 58

Let $T$ denote the Teichmuller space of a hyperbolic Riemann surface of finite conformal type. Suppose that $f: T\to T$ is a holomorphic map which has a periodic orbit $Z$, i.e. a finite invariant subset on which $f$ acts as an order $n$ cyclic permutation. Set $g=f^n$, then $g$ fixes $Z$ pointwise. Since $f$ and $g$ commute, $f(F_g)\subset F_g$, where $F_g$ is the fixed-point set of $g$ in $T$. It is nonempty since $Z\subset F_g$. The subset $F_g$ is a convex complex submanifold of $T$. (I did not check carefully that it is a submanifold, a priori, it is only an analytic subvariety. Convexity should imply that it is a submanifold. In the worst case, a subvariety suffices for the proof, since all I need is that it admits a triangulation.) In particular, it is contractible. The restriction of $f$ to $F_g$ is a periodic homeomorphism of order $n$. It is now a classical argument going back to Nielsen that $f$ has a fixed point in $F_g$. It goes as follows. Set $F:=F_g$ and $h:=f|_F$. Suppose first that $n$ is prime. A finite cyclic group of prime order cannot act freely on a contractible finite-dimensional CW complex (since finite nontrivial groups have infinite cohomological dimension). Hence, some nontrivial element of $\langle h\rangle$ fixes a point in $F$. But, since $n$ is prime, this element generates the cyclic group $\langle h\rangle$. The general case is proven by induction on the number of prime factors of $n$. Suppose that $n=pq$ where $p$ is prime. Then $h^q$ has order $p$ and, hence, has nonempty fixed-point set $F'=F_{h^q}$ in $F$. But $F'$ is again convex, hence, a contractible submanifold/subvariety, invariant under the action of $h$. Thus, we reduced the problem to finding a fixed point of $h^p$ in $F'$. The order of $h^p$ is $q<n$ and we continue inductively.