# equivalent formulation of the tube lemma

Let us recall the thesis of the tube lemma. Assume, that $X$ and $Y$ are topological spaces  .

(TL) If $U\subseteq X\times Y$ is open (in product topology) and if $x\in X$ is such that $x\times Y\subseteq U$, then there exists an open neighbourhood $V\subseteq X$ of $x$ such that $V\times Y\subseteq U$.

(CP) The projection $\pi:X\times Y\to X$ given by $\pi(x,y)=x$ is a closed map.

The following theorem relates these two statements:

Proof. ,,$\Rightarrow$” Let $F\subseteq X\times Y$ be a closed set  and let $U=(X\times Y)\backslash F$ be its open complement  . We will show, that $\pi(F)$ is closed, by showing that $V=X\backslash\pi(F)$ is open. So assume, that $x\in V$. Obviously

 $\big{(}\pi^{-1}(x)=x\times Y\big{)}\cap F=\emptyset.$

Therefore $x\times Y\subseteq U$ and by (TL) there exists open neighbourhood $V^{\prime}\subseteq X$ of $x$ such that $V^{\prime}\times Y\subseteq U$. It easily follows, that $V^{\prime}\subseteq V$ and it is open, so (since $x$ was chosen arbitrary) $V$ is open.

,,$\Leftarrow$” Let $U\subseteq X\times Y$ be an open subset such that $x\times Y\subseteq U$ for some $x\in X$. Let $F=(X\times Y)\backslash U$. Then $F$ is closed and by (CP) we have that $\pi(F)\subseteq X$ is closed. Also $x\not\in\pi(F)$ and thus $V=X\backslash\pi(F)$ is an open neighbourhood of $x$. It can be easily checked, that $V\times Y\subseteq U$, which completes     the proof. $\square$

Remark. The theorem doesn’t state that any of statements is true. It is well known (see tha parent object), that if both $X$ and $Y$ are Hausdorff  with $Y$ compact  , then both are true. On the other hand, for example for $X=Y=\mathbb{R}$, where $\mathbb{R}$ denotes reals with standard topology, they are both false. For example consider

 $F=\{(x,y)\in\mathbb{R}^{2}\ |\ xy=1\}.$

Of course $F$ is closed, but $\pi(F)=\mathbb{R}\backslash\{0\}$ is not closed, so the (CP) is false.

Title equivalent formulation of the tube lemma EquivalentFormulationOfTheTubeLemma 2013-03-22 19:15:18 2013-03-22 19:15:18 joking (16130) joking (16130) 4 joking (16130) Theorem msc 54D30