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Edited to simplify and clarify the argument
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Lee Mosher
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I just ran across this question, and thought I would give a precise version of the proof Ilya suggested. I believe I learned this proof in Richie Miller's topology course, Michigan State University, 1977 or so.

Choose a triangulation of the surface $S$, equipped with the simplicial metric. Choose a maximal one-ended subforestsubtree $F$$T$ of the dual 1-skeleton $S^{(1)}$. By a ``one-ended subforest'' I mean a subgraph each of whose components is a one-ended tree. The subforestsubtree $F$$T$ contains every dual $0$-cell, that is, the barycenter of every 2-simplex. Also, $F$$T$ contains dual 1-cells crossing certain $1$-simplices. For each component $T$ of $F$ letLet $U_T$$U$ be the union of the open 2-simplices and open 1-simplices that contain a point of $T$. The metric completion of $U_T$$U$, denoted $\bar U_T$$\bar U$, is a closed disc with one boundary point removed, and so there is a deformation retraction from $\bar U_T$$\bar U$ onto its boundary $\partial \bar U_T$$\partial \bar U$. These deformation retractions piece togetherAttaching $\bar U$ to give$S - U$ in the obvious way to form the surface $S$, the deformation retraction $\bar U \to \partial\bar U$ induces a deformation retraction fromof $S$ onto $S - \cup_T U_T$$S-U$, wnich is a subcomplex of the 1-skeleton.

By the way, the subforestsubtree $F \subset S^{(1)}$$T \subset S^{(1)}$ can be constructed by an explicit process. Enumerate the dual $0$-cells $v_1,v_2,\ldots \in S^{(1)}$. Construct one-ended subforestssubtrees $F_1,F_2,\ldots \subset S^{(1)}$$T_1,T_2,\ldots \subset S^{(1)}$ as follows. $F_1$$T_1$ is any proper ray based at $v_1$. If $v_n \in F_{n-1}$$v_n \in T_{n-1}$ then $F_n = F_{n-1}$$T_n = T_{n-1}$. If $v_n$ is in a finite component $T$ of $S^{(1)} - F_{n-1}$$v_n \not\in T_{n-1}$, choose an dual 1-cell $e$ connecting alet $0$-cell$T_n$ be the union of $T$ to a point of$T_{n-1}$ with any arc $F_{n-1}$,$\alpha \subset S^{(1)}$ having one endpoint at $v_n$ and letintersecting $F_n = F_{n-1} \cup T \cup e$$T_{n-1}$ in its opposite endpoint. IfEach $v_n$$T_n$ is in an infinite component $T$ of $S^{(1)} - F_{n-1}$a one-ended tree by induction, let $F_n$ beand since the unionradius $r$ neighborhood of $F_{n-1}$ with an infinite ray$v_1$ in $T$. Noticing that the inclusion$T_n$ stabilizes as $F_{n-1} \subset F_n$ is injective on the component set$n \to \infty$, it follows that the union of the $F_n$$T = \cup_n T_n$ is a one-ended subforestsubtree of $S^{(1)}$, and it is maximal because it contains each $v_i$.

I think this proof generalizes to any dimension, to give the theorem that Igor Belegradek refers to.

--- Edited to simplify and clarify the argument ---

I just ran across this question, and thought I would give a precise version of the proof Ilya suggested. I believe I learned this proof in Richie Miller's topology course, Michigan State University, 1977 or so.

Choose a triangulation of the surface $S$, equipped with the simplicial metric. Choose a maximal one-ended subforest $F$ of the dual 1-skeleton $S^{(1)}$. By a ``one-ended subforest'' I mean a subgraph each of whose components is a one-ended tree. The subforest $F$ contains every dual $0$-cell, that is, the barycenter of every 2-simplex. Also, $F$ contains dual 1-cells crossing certain $1$-simplices. For each component $T$ of $F$ let $U_T$ be the union of the open 2-simplices and open 1-simplices that contain a point of $T$. The metric completion of $U_T$, denoted $\bar U_T$, is a closed disc with one boundary point removed, and so there is a deformation retraction from $\bar U_T$ onto its boundary $\partial \bar U_T$. These deformation retractions piece together to give a deformation retraction from $S$ onto $S - \cup_T U_T$, wnich is a subcomplex of the 1-skeleton.

By the way, the subforest $F \subset S^{(1)}$ can be constructed by an explicit process. Enumerate the dual $0$-cells $v_1,v_2,\ldots \in S^{(1)}$. Construct one-ended subforests $F_1,F_2,\ldots \subset S^{(1)}$ as follows. $F_1$ is any proper ray based at $v_1$. If $v_n \in F_{n-1}$ then $F_n = F_{n-1}$. If $v_n$ is in a finite component $T$ of $S^{(1)} - F_{n-1}$, choose an dual 1-cell $e$ connecting a $0$-cell of $T$ to a point of $F_{n-1}$, and let $F_n = F_{n-1} \cup T \cup e$. If $v_n$ is in an infinite component $T$ of $S^{(1)} - F_{n-1}$, let $F_n$ be the union of $F_{n-1}$ with an infinite ray in $T$. Noticing that the inclusion $F_{n-1} \subset F_n$ is injective on the component set, it follows that the union of the $F_n$ is a one-ended subforest of $S^{(1)}$, and it is maximal because it contains each $v_i$.

I think this proof generalizes to any dimension, to give the theorem that Igor refers to.

I just ran across this question, and thought I would give a precise version of the proof Ilya suggested. I believe I learned this proof in Richie Miller's topology course, Michigan State University, 1977 or so.

Choose a triangulation of the surface $S$, equipped with the simplicial metric. Choose a maximal one-ended subtree $T$ of the dual 1-skeleton $S^{(1)}$. The subtree $T$ contains every dual $0$-cell, that is, the barycenter of every 2-simplex. Also, $T$ contains dual 1-cells crossing certain $1$-simplices. Let $U$ be the union of the open 2-simplices and open 1-simplices that contain a point of $T$. The metric completion of $U$, denoted $\bar U$, is a closed disc with one boundary point removed, and so there is a deformation retraction from $\bar U$ onto its boundary $\partial \bar U$. Attaching $\bar U$ to $S - U$ in the obvious way to form the surface $S$, the deformation retraction $\bar U \to \partial\bar U$ induces a deformation retraction of $S$ onto $S-U$, wnich is a subcomplex of the 1-skeleton.

By the way, the subtree $T \subset S^{(1)}$ can be constructed by an explicit process. Enumerate the dual $0$-cells $v_1,v_2,\ldots \in S^{(1)}$. Construct one-ended subtrees $T_1,T_2,\ldots \subset S^{(1)}$ as follows. $T_1$ is any proper ray based at $v_1$. If $v_n \in T_{n-1}$ then $T_n = T_{n-1}$. If $v_n \not\in T_{n-1}$, let $T_n$ be the union of $T_{n-1}$ with any arc $\alpha \subset S^{(1)}$ having one endpoint at $v_n$ and intersecting $T_{n-1}$ in its opposite endpoint. Each $T_n$ is a one-ended tree by induction, and since the radius $r$ neighborhood of $v_1$ in $T_n$ stabilizes as $n \to \infty$, it follows that $T = \cup_n T_n$ is a one-ended subtree of $S^{(1)}$, and it is maximal because it contains each $v_i$.

I think this proof generalizes to any dimension, to give the theorem that Igor Belegradek refers to.

--- Edited to simplify and clarify the argument ---

Source Link
Lee Mosher
  • 15.4k
  • 2
  • 42
  • 81

I just ran across this question, and thought I would give a precise version of the proof Ilya suggested. I believe I learned this proof in Richie Miller's topology course, Michigan State University, 1977 or so.

Choose a triangulation of the surface $S$, equipped with the simplicial metric. Choose a maximal one-ended subforest $F$ of the dual 1-skeleton $S^{(1)}$. By a ``one-ended subforest'' I mean a subgraph each of whose components is a one-ended tree. The subforest $F$ contains every dual $0$-cell, that is, the barycenter of every 2-simplex. Also, $F$ contains dual 1-cells crossing certain $1$-simplices. For each component $T$ of $F$ let $U_T$ be the union of the open 2-simplices and open 1-simplices that contain a point of $T$. The metric completion of $U_T$, denoted $\bar U_T$, is a closed disc with one boundary point removed, and so there is a deformation retraction from $\bar U_T$ onto its boundary $\partial \bar U_T$. These deformation retractions piece together to give a deformation retraction from $S$ onto $S - \cup_T U_T$, wnich is a subcomplex of the 1-skeleton.

By the way, the subforest $F \subset S^{(1)}$ can be constructed by an explicit process. Enumerate the dual $0$-cells $v_1,v_2,\ldots \in S^{(1)}$. Construct one-ended subforests $F_1,F_2,\ldots \subset S^{(1)}$ as follows. $F_1$ is any proper ray based at $v_1$. If $v_n \in F_{n-1}$ then $F_n = F_{n-1}$. If $v_n$ is in a finite component $T$ of $S^{(1)} - F_{n-1}$, choose an dual 1-cell $e$ connecting a $0$-cell of $T$ to a point of $F_{n-1}$, and let $F_n = F_{n-1} \cup T \cup e$. If $v_n$ is in an infinite component $T$ of $S^{(1)} - F_{n-1}$, let $F_n$ be the union of $F_{n-1}$ with an infinite ray in $T$. Noticing that the inclusion $F_{n-1} \subset F_n$ is injective on the component set, it follows that the union of the $F_n$ is a one-ended subforest of $S^{(1)}$, and it is maximal because it contains each $v_i$.

I think this proof generalizes to any dimension, to give the theorem that Igor refers to.