# Wild Arcs with simply connected complements: two examples.

As we saw in this previous post: this arc contains just one wild point; hence its complement is simply connected. But we can say more than that: this arc $K$ is “cellular”: that means that there is a collection of smooth 3-balls $B_1 \supset B_2 \supset B_3....$ where $\cap^{\infty}_{i=1}B_i = K$. That isn’t too hard so see: start by taking a small round 3-ball that encloses the wild point and taking a union of that ball with a tubular neighborhood of the rest of the arc outside of the ball. That first gadget is “almost” a ball; it might have a few handles which can be filled in. Then take a smaller ball around the wild point and repeat the process with a smaller tubular neighborhood…and repeat.

In fact, we can do this with ANY arc that has just one wild point.

Therefore $S^3 - K = S^3 - \cap^{\infty}_{i=1}B_i = \cup^{\infty}_{i=1}(S^3 - B_i)$ which is homeomorphic to $R^3$; hence this wild arc complement is homeomorphic to the complement of a tame arc.
Aside: this shows that arcs are NOT determined by their complements whereas smooth (or p. l. ) knots are.

But this arc is still wild; it has penetration index 3 near the wild point.

Now consider this example

This is like the wild Fox-Artin arc talked about in the previous post in this series, except that “one stitch has been missed. It’s complement is simply connected; the proof of this fact is suggested by this diagram:

The small loop represents a generator of the fundamental group; note how it bounds a disk in the complement of the arc.

However the complement is NOT $R^3$! There are different ways to see that; here is a fun one:

This shows $K$ with a smooth simple closed curve $J$ in its complement. Note: $J$ is compact (it is a smooth knot). Therefore there are two different ways to use $J$ to show that $S^3 - K$ is NOT homeomorphic to $R^3$:

1. In $R^3$, every compact set lies inside of some ball $B$. But one can show that there is no ball $B$ that contains $J$ and misses $K$
2. One can use algebraic topology: the fundamental group of $(S^3 -K)-J$ is NOT finitely generated; in $R^3$, the complement of any smooth knot is finitely generated.

This gives an example of an open, simply connected manifold that is not homemorphic to $R^3$.

The interested reader can consult the following references for more detail:

R. Daverman and G. Venema: Embeddings in Manifolds, American Mathematics Society Graduate Studies in Mathemtics, Vol. 106, 2009: Section 2.8
T. B. Rushing, Topological Embeddings, Academic Press, (Pure and Applied Mathematics, Volume 52) 1973, Section 2.4.

# Introduction to Wild Arcs: the interesting stuff that can happen

By “arc” I mean a topological embedding of the unit arc $[0,1]$ into $R^3$ or $S^3$.
If the embedding is chosen to be smooth or piecewise linear, there isn’t much to do since all such embeddings are ambient isotopic to a standard, boring linear line segment.

But if we even allow for as much as a single wild point, then weird things can happen.

It is probably best to procede with some examples.

1. Not all “wild looking” arcs are wild.

The arc in the upper left hand corner IS wild and the arc in the lower right hand corner (which is supposed to represent an infinite series of trefoil knots, each tied smaller and smaller and converging to a single point) is NOT wild.

I’ll sketch out a proof as to why it is tame: imagine a series of round 3-balls surrounding the “potentially bad” endpoint: arrange these balls so that they are concentric about the endpoint and there is one trefoil knot between the boundaries of each of these balls (as shown)

Denote these balls by $B_1, B_2, B_3.....$

Now for each trefoil between each $B_k, B_{k+1}$ there is an ambient isotopy which fixes ALL points outside of $B_k$ and inside $B_{K+1}$ and removes the trefoil knot; this is called the “lamp chord trick”:

The ONLY points that are moved are those between the boundaries of $B_k$ and $B_{k+1}$.

Denote this orientation preserving homeomorphism by $h_k$.

We can now define a homemorphism of 3-space by $h(x) = h_k(x)$ if $x \in B_k - int(B_{k+1})$ and $h(x) = x$ otherwise.
This takes this arc to a straight arc.

Note: if we took this “almost wild” arc and added even a slight arc “past” the endpoint, then the above procedure would fail. A wild arc that is the union of two tame arc is called mildly wild.

2. An arc that has only one wild point has a simply connected complement, even though the arc is wild.
An example of this is the arc in the upper left and picture. It turns out that if one encloses the “potentially bad” endpoint of the arc by a smooth 2-sphere, that sphere will intersect that arc in at least 3 points. That seems “obvious” but requires proof. And the proof, while not requiring sophisticated techniques (e. g. esoteric algebraic or differential topology), isn’t easy. I know from experience. 🙂

The idea of a sphere surrounding a wild point having to hit the arc in a set number of points is called the “penetration index” of the arc. More on that for a subsequent post; a good reference is: 24), : W. R. Alford and B. J. Ball, Some almost polyhedral wild arcs, Duke Math. J. 30 (1963), 33-38. MR 26 # 1858.

Note: some wild arcs have a wild point with an infinite penetration index.

On the other hand, any closed loop in the complement of the arc bounds a disk that misses the arc. Here is a sketch as to why:

Let $\alpha$ be a closed loop (say, a smooth or piecewise linear one) that misses the arc. Two compact sets in a metric space that miss must remain at least some set distance $\epsilon$ from each other. So enclose the endpoint of the arc by a ball of radius $\frac{\epsilon}{2}$ and it is easy to see that $\alpha$ bounds a disk that misses that ball. But that disk might hit the arc in a finite number of points. But that is no problem because those points are tame points of the arc; they are outside of the ball that contains the only wild point.

That part of the arc can be enclosed by a tube (called a tubular neighborhood of the arc) that runs to the non-wild endpoint of the arc. That neighborhood can be used to construct “caps” that run around the tame endpoint of the arc; in that way the original disk that $\alpha$ bounded can be replaced by a new disk which has “feelers” which push past the tame endpoint of the arc.

Now this proof works for an arc whose wild point is at an endpoint; the same idea works if the solitary wild point is not an end point.

3. There are arcs whose complement is NOT simply connected. Here is an example of one:

Note: the fundamental group of the arc complement maps non-trivially onto a subgroup of the alternating group $A_5$ (the same one you studied in your abstract algebra class).

A good reference for this:

Fox, Ralph H.; Artin, Emil (1948), “Some wild cells and spheres in three-dimensional space”, Annals of Mathematics. Second Series 49: 979–990, ISSN 0003-486X, JSTOR 1969408, MR 0027512

Note: this arc has two wild points.

4. It is possible for two disjoint wild arcs to “link”: that is, be situated in a way so that the arcs are disjoint but also so that there is no sphere which encloses one arc but not the other.

Example: image two copies of the Fox-Artin arc shown above, but in the middle “stitch”, link the two middle strands.

Now if there were a 2-sphere $S$ separating the two arcs, it would have to miss each arc (and therefore the 4 wild points) by some distance $2\epsilon$. So surround each of the 4 wild points with a smooth sphere of radius, say, $\epsilon$. Then one can obtain two disjoint graphs; each of these graphs is equal to the respective wild arc outside of the ball bounded by the sphere, and inside the balls we can just connect the non-wild parts of the arcs (the parts outside of the balls) by smooth arcs inside of the balls; hence in a sense, we have approximated the two disjoint wild arcs by two disjoint smooth graphs which are equal to the wild arcs outside of a small neighborhood of the wild points.

It is easy to see (via standard algebraic topology or by classical knot theory techniques) that these two smooth graphs are NOT splittable. Hence the two wild arcs weren’t splittable either.

Note: we have just skimmed the surface of wild arc theory; there is a lot out there.

A good place to start would be the Fox-Artin paper mentioned above along with the two books:

R. Daverman and G. Venema: Embeddings in Manifolds, American Mathematics Society Graduate Studies in Mathemtics, Vol. 106, 2009: Section 2.8
T. B. Rushing, Topological Embeddings, Academic Press, (Pure and Applied Mathematics, Volume 52) 1973, Section 2.4. The edition I have is out of print; I linked to the newer one.

Note: arcs can be very pathologically embedded in higher dimensions. That doesn’t make sense to me, but check out this paper by D. Wright: (link is to an open access PDF copy)

Abstract. Let A and B be arcs in E^3, Euclidean 3-space. Then A can be
“slipped” off B; i.e., there exists a homeomorphism of ^E3 onto itself,
arbitrarily close to the identity, such that h(A) n B = 0. The purpose of
this note is to show that arcs in E^n (n > 4) do not always enjoy this
property. The examples depend heavily on a recent result of McMillan.

1. Introduction. If X and Y are subsets of E^n, Euclidean «-space, we say
that X can be slipped off Y in E ” if for each e > 0 there is an (epsilon)-homeomorphism h: E” -» E” such that h{X) n Y = 0; otherwise, we say X
cannot be slipped off Y. Results of Armentrout [1] and McMillan [5] show that
if A and B are arcs in E^3, then A can be slipped off B. We show that this is
false in higher dimensions by proving the following
Theorem. There exist cellular arcs A and B in E^n (n > 4) such that A
cannot be slipped off B.

This strikes me as very non-intuitive; you’d think that higher dimensional space would provide “more room” for maneuver but it also provides more room to introduce pathologies as well.

To see the beast of an arc that Wright is talking about, here is D. McMillan’s article (open access PDF) about an arc that is so pathological, it doesn’t have a neighborhood inside a manifold that embedds in any $S^n$. Of course, this neighborhood cannot be a manifold because of the Whitney embedding theorem.

# What is Knot Theory Anyway?

A knot is an embedding of the circle $\{(x,y)|x^2 + y^2 =1 \}$ into 3-space. By “3-space” we usually mean $R^3$ or $S^3$, which is the 3-sphere, which can be thought of as $R^3$ with a point added at infinity. $S^3$ is sometimes preferred because it is a compact space.

Note: sometimes we focus on the image of the embedding itself (i. e., the geometric object) and sometimes we focus on the map, which includes information about orientation.

Example: If one has $t \in [0, 2\pi),$ then $f(t) = (x(t), y(t), z(t)), x(t) = (2+cos(3t))cos(2t), y(t) = (2+cos(3t))sin(3t), z(t) = sin(3t)$ is a knot. Here are two different MATLAB plots of the image:

The second is a projection of the image of the trefoil onto the $x, y$ plane. If we endow such a projection with “crossing information”, we call the image a diagram for the knot.

Here, the broken line indicates that the strand passes under another strand.

It is custom to insist on “regular” projections, which means that:

1. All “singularities” (points on the diagram which correspond to more than one point of the knot) are double points (there are no points where 3 or more strands of the knot’s projection meet)
2. All crossings are “honest” crossings; that is there are no “tangents” (places where the projection “kisses” another strand).

Note: one can think of a diagram as a “shadow” of the knot on a plane, provided one adds crossing information at all double points.

Now not all knots possess a diagram, but it is a known fact that all smooth knots (knots that arise from differentiable embeddings) and all picewise linear knots (knots whose image consists of a finite number of straight line segments glued end to end) have a projection.

Most of knot theory research deals with smooth or piecewise linear embeddings of the circle into $S^3$ or $R^3$. There is knot theory of similar embeddings into other 3-manifolds, embeddings of $S^2$ into $S^4$ (higher dimensional knot theory) or embeddings of graphs into $S^3$.

Also, link theory deals with multiple knots together.

The above shows the Borromean Rings, which are three linked knots, no two of which are linked to each other. This is a famous 3-component link.

This blog will mostly focus on the following:
1. non-smooth (and non-piecewise linear) embeddings of the circle into $S^3$.

These two diagrams are of non-smooth (and non-p. l.) knots; we call these wild knots. Notice how the stitches and arcs get smaller and converge to a point? That point is called a wild point. I will give a precise definition later; for right now we’ll tell you that it is impossible to assign a tangent vector to those points in some well defined way.

2. An arc is the image of $[0,1]$ into 3 space. The mathematics of smooth (or p. l.) arcs in 3-space is pretty boring. Every smooth or p. l. arc “can be straightened in space” into a straight, boring arc.
On the other hand, the mathematics of wild arcs (think: non-smooth/p. l. ) is every interesting.

The above arc has two wild points (the end points) and can NOT be straightened out in space into a straight arc. We’ll make this concept clear a bit later in another post).

3. Straight lines (a copy of the real line) into open 3-manifolds; we will insist that the “two infinities” of the line go to the “infinities” in the manifold.

In the above, the reader is invited to think of the “line” being embedded in the space $D^2 \times R$ where $D^2$ is the standard 2-disk. Think of an infinitely long solid tube or cylinder (like a long pipe).
I will call this Proper Knot Theory; the term “proper” is a technical term, which I will explain here: a continuous map $f:X \rightarrow Y$ is said to be proper if for all compact sets $C \subset Y, f^{-1}(C)$ is compact. Here is an example of a non-proper embedding: consider $f: R \rightarrow R$ given by $f(x) = arctan(x)$. The inverse image of $[0, \frac{\pi}{2}]$ is not compact.

Equivalence Classes for Knots
In most of knot theory, what is studied is NOT the knots themselves but their “equivalence classes”. For example: the first example of the knot we have had a very specific function to define it. However, if we were to say, take a strand of the knot and move it a little, we’d get a different embedding, but mathematically we’d want to think of it as being “the same as” the original embedding. This makes the subject much more doable. Besides, knot theory is studied mostly because it impacts the study of the topology of 3-manifolds: such spaces are modified by doing operations (called “surgery”) which are often defined as being done along some embedded circle: a knot. In many cases, the objected obtained doesn’t differ “topologically” if the surgery knot is changed by some “motion of space”.

The same principle often applies if a scientist is, say, studying a knotted molecule or DNA strand.

So we need to state the equivalence classes.

Classical Knot Theory (the kind most often done)
Note: sometimes oriented knots are studied (the diagrams have arrows) and sometimes the unoriented knots are studied (no arrows). Sometimes this makes a difference as we shall see later.

The above is an example of an oriented knot diagram.

The most common equivalence class used:
Given two knots (or links) in three space, say, $K_1, K_2$; we say that $K_1$ is equivalent to $K_2$ if there is a map called an “ambient isotopy” that connects the two. More particularly there exists $F: S^3 \times [0,1] \rightarrow S^3$ where:
1. $F(-,t)$ is a homemomorphism of $S^3$ for all $t \in [0,1]$.
2. $F(K,0) = K_1$ and $F(K,1) = K_2$ for some $K \subset S^3$, $K$ homeomorphic to the circle.
The above is just a fancy way of saying that we can “deform space” to turn $K_1$ into $K_2$; almost never do we worry about finding, say, a formula for $F$.

It turns out that this definition is equivalent to the following simpler definition: $K_1, K_2$ are equivalent knots if there is some orientation preserving homeomorphism $f: S^3 \rightarrow S^3$ such that $f(K_1) = K_2$. Needless to say, this is easier to state, but one loses the sense of taking a knotted piece of string and playing with it (which is what you are doing in the first definition).

There is also another type of equivalence that is used: two knots $K_1, K_2$ can be declared to be equivalent if there is a homeomorphism (possibly non-orientation preserving) $f: S^3 \rightarrow S^3$ such that $f(K_1) = K_2$.

If $f$ is orientation reversing and $f(K_1) = K_2$ then $K_1$ and $K_2$ are called mirror images.

So, classical knot theory (the kind most often studied) boils down to four different kinds:
1. oriented knots; mirror images considered equivalent.
2. oriented knots; mirror images NOT automatically considered equivalent.
3. non-oriented knots, mirror images considered equivalent.
4. non-oriented knots, mirror images not automatically considered equivalent.

A knot that is different from a knot with the same image but with a different orientation (arrow direction) is said to be non-invertible.
A knot that is different from its mirror image is said to be chiral.

The trefoil knot: is chiral but invertible (you can reverse the arrows by an orientation preserving homeomorphism)
The figure 8 knot: is NOT chiral and is invertible.

Non-invertible knots exist; here is an example: ($8_{17}$)

The astute reader might wonder: “hey, you didn’t say anything about your isotopy or homeomorphism being smooth, piecewise linear or merely topological”. It turns out that in classical knot theory, this is a settled foundational question and therefore unimportant (here and here).

However this issue does appear in other kinds of knot theory, including those we will be discussing.

Wild knots
A knot (link or arc) is said to be tame if it is equivalent to a smooth (or p. l.) knot (equivalence class of choice). If it isn’t, it is called wild.
Note: it isn’t always immediately obvious if an arc is wild or tame; for example, the arc in the upper left hand corner is wild (wild point is the left end point) whereas the the lower right arc (which has separate trefoil knots converging to an endpoint) is actually tame!

We will discuss this later; note that the “infinite trefoil” arc is just on the edge of being wild; were we to add on, say, a straight segment at the left hand endpoint and extend it any finite distance at all, the arc would become wild. That appears to make no sense at all (at first glance) but in a later post I will provide a proof.

We will study wild knots of various kinds; note: it is possible for a knot to be wild at ALL of its points. We’ll get to this in a later post; if you can’t wait, here is an example: consider the following picture, which is supposed to represent a nested series of solid tori, (think: a bagel or doughnut) which are nested inside one another. If we intersect all of these knotted up tori, we end up with a very ill behaved wild knot in 3-space; this knot is wild at all of its points:

I am running out of steam; so in our next installment I’ll talk about different types of equivalence classes for knots in 3 space and for lines (proper knots) in open 3-manifolds.