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\begin{document}

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\section*{Warsaw circle}

\hypertarget{context}{}\subsubsection*{{Context}}\label{context}

\hypertarget{topology}{}\paragraph*{{Topology}}\label{topology}

[[!include topology - contents]]

\hypertarget{contents}{}\section*{{Contents}}\label{contents}

\noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak
\noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak
\noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak
\noindent\hyperlink{a_borsuk_shape_map_}{A (Borsuk) shape map $f\colon S^1 \to S_W$}\dotfill \pageref*{a_borsuk_shape_map_} \linebreak
\noindent\hyperlink{from_a_ech_point_of_view}{From a ech point of view}\dotfill \pageref*{from_a_ech_point_of_view} \linebreak
\noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak


\hypertarget{idea}{}\subsection*{{Idea}}\label{idea}

The \textbf{Warsaw circle} is a [[topological space]] that serves to illustrate some of the ideas of [[shape theory]].

A topological space may have very little separating it from `[[manifold]]ness', yet a `singularity' can cause havoc! The simple example, here, is known as the \textbf{Warsaw Circle} as it was studied extensively by K. Borsuk and his Polish collaborators, see the book (\hyperlink{Borsuk75}{Borsuk 75}).

\hypertarget{definition}{}\subsection*{{Definition}}\label{definition}

The \textbf{Warsaw circle} $S_W$ is the [[subset]] of the [[plane]], $\mathbb{R}^2$, specified by

\begin{displaymath}
\{(x,\sin\left(\frac{1}{x}\right)) \mid -\frac{1}{2\pi} \lt x \leq \frac{1}{2\pi}, x \neq 0\}\cup \{(0,y) | -1 \leq y \leq 1\} \cup C,
\end{displaymath}
where $C$ is an arc in $\mathbb{R}^2$ joining $(\frac{1}{2\pi}, 0)$ and $(-\frac{1}{2\pi}, 0)$, disjoint from the other two subsets specified above except at its endpoints.

It looks something like [[warsaw.pdf|this:file]]:



\textbf{Note} There is a variant version $S_W'$ with no $(x,\sin(\frac{1}{x}))$-bit for the $x\lt 0$ and the curve $C$ joins $(0,0)$ to $(\frac{1}{2\pi}, 0)$. The discussion adapts very easily to that. For this version, there is a surjective continuous map $\mathbb{R} \to S_W'$. See eg \href{https://en.wikipedia.org/wiki/Shape_theory_%28mathematics%29#/media/File:Warsaw_Circle.png}{Wikipedia} for a picture.

\hypertarget{properties}{}\subsection*{{Properties}}\label{properties}

The Warsaw circle is a [[compact space|compact]] [[metric space]], but is not [[locally connected space|locally connected]] along the line corresponding to $\{(0,y) \mid -1 \leq y \leq 1\}$, so is not a [[manifold]], nor for that matter a [[polyhedron]]. It is [[connected space|connected]], but not [[pathwise connected space|pathwise connected]] as no path can get out from the `line'. (The variant version noted above \emph{is} pathwise connected.) We note

\begin{itemize}%
\item $\pi_0(S_W)$ is two points;


\item $\pi_1(S_W)$ is trivial at any base point.



\end{itemize}
There is a simple continuous map from $S^0$, the 0-circle, $\{-1,1\}$, to $S_W$ which is a [[weak homotopy equivalence]]. (For instance define $f(-1) = (0,0)$ and $f(1)$ to be any point in the outer $sin(1/x)$ part of the space, it does not matter which one.) This is not a [[homotopy equivalence]]. (In fact it is instructive to look at maps from $S_W$ to $S^0$! It does not take long.)

A striking thing about the picture is that it `clearly' divides the plane into two components, an inside and an outside, and has a definite sense of being `almost' a circle. It has a line of singularities, but otherwise \ldots{} .

If we consider, not just $S_W$ as a compact metric space, but as a subspace of the plane, then we can take small [[open neighbourhood]]s of $S_W$, to be definite take

\begin{displaymath}
N_{\frac{1}{n}}(S_W) = \{ \underline{x}\in \mathbb{R}^2 | d(\underline{x},S_W) \lt \frac{1}{n}\}.
\end{displaymath}
This looks like an [[annulus]] with a thickenning at one small section. It has the [[homotopy type]] of a [[circle]]. If $N \gt n$, $N_{\frac{1}{N}}(S_W)\subset N_{\frac{1}{n}}(S_W)$, of course, (we will write $i^N_n$ for this map, and this is a homotopy equivalence. The Warsaw circle, $S_W$, is clearly the intersection of all these almost annular neighbourhoods. (Note, also clearly, that the complements of these neighbourhoods are gradually occupying more and more of the two components of $\mathbb{R}^2- S_W$.)

We have a inverse system ([[pro-object]]) of topological spaces all of which have the [[homotopy type]] of a [[polyhedron]], \ldots{} in fact always the same polyhedron, $S^1$. Note that by our use of a specific [[cofinal diagrams|cofinal]] family of neighbourhoods of $S_W$, indexed by the [[natural number]]s, we have an \emph{inverse sequence}. That was a choice and we could have chosen differently or not at all. The ability to pick a sequence of neighbourhoods is related to the fact that we are considering a \textbf{compact metric space}.

Another point to note is that not only is each of the neighbourhoods [[homotopic]] to $S^1$, but the inclusion maps making up the `bonds' of the inverse sequence, are homotopy equivalence. This is a particularity of $S_W$ and other examples, such as the [[solenoid]]s need not have this `stability' property. The Warsaw circle is an example of what is called a [[stable space]].

\hypertarget{a_borsuk_shape_map_}{}\subsubsection*{{A (Borsuk) shape map $f\colon S^1 \to S_W$}}\label{a_borsuk_shape_map_}

There is a sequence of maps, $\{f_n : S^1 \to N_{\frac{1}{n}}(S_W)\mid n\in \mathbb{N}\}$, so that for each pair, $(n,N)$, with $N\gt n$, there is a homotopy $f_n \sim i^N_n f_N$. This makes a (Borsuk) [[shape map]] from the [[circle]] to the Warsaw circle. Each $f_n$ is in fact a [[homotopy equivalence]] and we can use a choice of homotopy inverses to get another shape map $g : S_W\to S^1$ and these make up a \textbf{shape equivalence}.

(A more detailed description of shape maps and shape equivalences in the Borsuk version of shape theory, is given in the entry [[Borsuk shape theory]]. The version given here skates over some points. It is, in fact, near the ANR-systems approach to shape.)

\hypertarget{from_a_ech_point_of_view}{}\subsubsection*{{From a ech point of view}}\label{from_a_ech_point_of_view}

To get $\check{C}(S_W,-)$ ech nerve complex of $S_W$, (see [[Čech methods]]), we can calculate $\check{C}(S_W,\alpha)$ for an arbitrary [[open cover]] $\alpha$ of $S_W$, but we need not do that (in fact that is a silly thing to do!). We first note that $S_W$ is [[compact space|compact]] so we need only consider finite open covers, as these form a [[cofinal subcategory]] of the category of all open covers. (`Cofinal subcategory' means that its inclusion into the bigger category is a [[cofinal functor]].)

Next we look at any finite open cover and note that it has a refinement in the form of open balls of radius $\frac{1}{n}$, in other words we can restrict to (well chosen) such covers, giving a countable family of open covers that have to be worked with.

For such open covers the nerve will look a bit like [[circle.pdf|this:file]].

There may be fine detail in the rectangle depending on the choice of cover, but that detail will disappear as one passes to finer and finer scales. (New holes may occur, but again going finer those disappear.) Cofinally it looks like a space obtained by adding in a thin rectangle transverse to a circle at one small segment. For different open coverings, the only difference will be where the region of attachment (marked $*$) will occur and the relative thinness of the rectangle. The line of singularities given by the interval $[-1,1]$ on the $y$-axis cannot be `observed', of course. If one passes to finer and finer covers, most of the curve does not change appreciably. It just gets subdivided, but the part near $*$ will lengthen, `spawning' a very large number of new vertices.

There are two important points to note:

\begin{itemize}%
\item (essentially) each $\check{C}(S_W,\alpha)$ has the homotopy type of a circle


\item the transition maps, $C(S_W,\alpha)\to C(S_W,\beta)$, will be (cofinally) homotopy equivalences.



\end{itemize}
(With a bit more care in the choice of the covers these can be made exact statements, not just `essentially' or cofinally true.)

We note that there are obvious maps of pro-objects $\check{C}(S^1,-)\to \check{C}(S_W,-)$, and back again. These give an isomorphism in $pro-Ho(sSets)$. This is the ech homotopy versions of the observations made for Borsuk's shape above.

\hypertarget{references}{}\subsection*{{References}}\label{references}

\begin{itemize}%
\item [[K. Borsuk]], \emph{Theory of Shape}, Monografie Matematyczne Tom 59,Warszawa (1975)

\end{itemize}
[[!redirects Warsaw circle]] [[!redirects Warsaw Circle]]



\end{document}