Renamed folder

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Mark 2023-05-05 15:06:56 -07:00
parent fad0f5b896
commit 51041aa804
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@ -1,63 +0,0 @@
% use [nosolutions] flag to hide solutions.
% use [solutions] flag to show solutions.
\documentclass[
solutions,
singlenumbering
]{../../resources/ormc_handout}
% Set to true to show knot measurements.
% Only used in tikzset.tex
\newif{\ifDebugKnot}
%\DebugKnottrue
\DebugKnotfalse
\input{tikzset.tex}
% Problems from "Why knot"
%
% Create largest crossing number with cord
% Human knot number: how many humans do you need to make the knot?
% Human knot number for trefoil composition?
% (looks like a wrap around center string)
%
% Figure-8 knot: mirror without letting go
%\usepackage{lua-visual-debug}
\begin{document}
\maketitle
<Advanced 2>
<Spring 2023>
{Knots and Braids}
{
Prepared by Mark on \today
}
\input{parts/0 intro.tex}
\input{parts/1 composition.tex}
\input{parts/2 links.tex}
\input{parts/3 sticks.tex}
\input{parts/4 braids.tex}
% Make sure the knot table is on an odd page
% so it may be removed in a double-sided
% handout.
\checkoddpage
\ifoddpage\else
\vspace*{\fill}
\begin{center}
{
\Large
\textbf{This page isn't empty.}
}
\end{center}
\vspace{\fill}
\pagebreak
\fi
\input{parts/table}
\end{document}

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@ -1,159 +0,0 @@
\section{Introduction}
\definition{}
To form a \textit{knot}, take a string, tie a knot, then join the ends. \par
You can also think of a knot as a path in three-dimensional space that doesn't intersect itself:
\vspace{2mm}
\begin{center}
\begin{minipage}[t]{0.3\textwidth}
\begin{center}
\begin{tikzpicture}[scale = 0.8, baseline=(p)]
\begin{knot}
\strand
(1,2) .. controls +(-45:1) and +(1,0) ..
(0, 0) .. controls +(-1,0) and +(-90 -45:1) ..
(-1,2);
\end{knot}
\coordinate (p) at (current bounding box.center);
\end{tikzpicture}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.3\textwidth}
\begin{center}
\begin{tikzpicture}[scale = 0.8, baseline=(p)]
% Knot is stupid and includes invisible handles in the tikz bounding box. This line crops the image to fix that.
\clip (-2,-1.7) rectangle + (4, 4);
\begin{knot}[
consider self intersections=true,
flip crossing = 2,
]
\strand
(1,2) .. controls +(-45:1) and +(120:-2.2) ..
(210:2) .. controls +(120:2.2) and +(60:2.2) ..
(-30:2) .. controls +(60:-2.2) and +(-90 -45:1) ..
(-1,2);
\end{knot}
\coordinate (p) at (current bounding box.center);
\end{tikzpicture}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.3\textwidth}
\begin{center}
\begin{tikzpicture}[scale = 0.8, baseline=(p)]
\clip (-2,-1.7) rectangle + (4, 4);
\begin{knot}[
consider self intersections=true,
flip crossing = 2,
]
\strand
(0,2) .. controls +(2.2,0) and +(120:-2.2) ..
(210:2) .. controls +(120:2.2) and +(60:2.2) ..
(-30:2) .. controls +(60:-2.2) and +(-2.2,0) ..
(0,2);
\end{knot}
\coordinate (p) at (current bounding box.center);
\end{tikzpicture}
\end{center}
\end{minipage}
\end{center}
If two knots may be deformed into each other without cutting, we say they are \textit{isomorphic}. \par
If two knots are isomorphic, they are essentially the same knot.
\definition{}
The simplest knot is the \textit{unknot}. It is show below on the left. \par
The simplest nontrivial knot is the \textit{trefoil} knot, shown to the right.
\begin{center}
\begin{minipage}[t]{0.48\textwidth}
\begin{center}
\begin{tikzpicture}[baseline=(p)]
\draw[circle] (0,0) circle (1);
\coordinate (p) at (current bounding box.center);
\end{tikzpicture}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.48\textwidth}
\begin{center}
\begin{tikzpicture}[baseline=(p), scale = 0.8]
\clip (-2,-1.7) rectangle + (4, 4);
\begin{knot}[
consider self intersections=true,
flip crossing = 2,
]
\strand
(0,2) .. controls +(2.2,0) and +(120:-2.2) ..
(210:2) .. controls +(120:2.2) and +(60:2.2) ..
(-30:2) .. controls +(60:-2.2) and +(-2.2,0) ..
(0,2);
\end{knot}
\coordinate (p) at (current bounding box.center);
\end{tikzpicture}
\end{center}
\end{minipage}
\end{center}
\vfill
\pagebreak
\problem{}
Below are the only four knots with one crossing. \par
Show that every nontrivial knot more than two crossings. \par
\hint{There are four knots with two crossings. What are they?}
\begin{center}
\includegraphics[width=0.8\linewidth]{images/one crossing.png}
\end{center}
\begin{solution}
Draw them all. Each is isomorphic to the unknot.
\end{solution}
\vfill
\problem{}
Show that this is the unknot. \par
A wire or an extension cord may help.
\begin{center}
\includegraphics[width=0.35\linewidth]{images/big unknot.png}
\end{center}
\definition{}
As we said before, there are many ways to draw the same knot. \par
We call each drawing a \textit{projection}. Below are four projections of the \textit{figure-eight knot}.
\vspace{2mm}
\begin{center}
\includegraphics[width=0.8\linewidth]{images/figure eight many.png}
\end{center}
\vspace{2mm}
\problem{}
Convince yourself that these are equivalent. \par
Try to deform them into each other with a cord!
\vfill
\pagebreak

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@ -1,135 +0,0 @@
\section{Knot Composition}
Say we have two knots $A$ and $B$.
The knot $A \boxplus B$ is created by cutting $A$ and $B$ and joining their ends:
\begin{center}
\hfill
\begin{minipage}[t]{0.15\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/figure eight.png}
$A$
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.13\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/trefoil.png}
$B$
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.3\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/composition a.png}
$A \boxplus B$
\end{center}
\end{minipage}
\hfill~
\end{center}
We must be careful to avoid new crossings when composing knots:
\vspace{2mm}
\begin{center}
\includegraphics[width=0.45\linewidth]{images/composition b.png}
\end{center}
\vspace{2mm}
We say a knot is \textit{composite} if it can be obtained by composing two other knots. \par
We say a knot is \textit{prime} otherwise.
\problem{}
For any knot $K$, what is $K \boxplus \text{unknot}$?
\vfill
\problem{}
Use a pencil or a cord to compose the figure-eight knot with itself.
\vfill
\vfill
\pagebreak{}
\problem{}
The following knots are composite. \par
What are their prime components? \par
Try to make them with a cord. \par
\hint{Use the table at the back of this handout to decompose the second knot.}
\begin{center}
\hfill
\includegraphics[height=30mm]{images/decompose a.png}
\hfill
\includegraphics[height=30mm]{images/decompose b.png}
\hfill~\par
\vspace{4mm}
\end{center}
\begin{solution}
The first is easy, it's the trefoil composed with itself. \par
\vspace{2mm}
The second is knot $5_2$ composed with itself. \par
Note that the \say{three-crossing figure eight} is another projection of $5_2$. \par
The figure-eight knot is NOT a part of this composition. Look closely at its crossings.
\end{solution}
\vfill
\pagebreak
\definition{}
When we compose two knots, we may get different results. To fully understand this fact, we need to define knot \textit{orientation}.
\vspace{2mm}
An \textit{oriented knot} is created by defining a \say{direction of travel.} \par
There are two distinct ways to compose a pair of oriented knots:
\begin{center}
\hfill
\begin{minipage}[t]{0.25\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/orientation b.png}
Matching orientation
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.25\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/orientation c.png}
Inverse orientation
\end{center}
\end{minipage}
\hfill~
\end{center}
In this example, both compositions happen to give the same result. This is because the trefoil knot is \textit{invertible}: its direction can be reversed by deforming it. This is not true in general, as you will soon see.
\problem{}
Invert a directed trefoil.
\vfill
\problem{}
The smallest non-invertible knot is $8_{17}$, shown below. \par
Compose $8_{17}$ with itself to obtain two different knots.
\begin{center}
\includegraphics[height=30mm]{knot table/8_17.png} \par
\vspace{2mm}
{\large Knot $8_{17}$}
\end{center}
\begin{solution}
\begin{center}
\includegraphics[width=0.8\linewidth]{images/noninvertible.png}
\end{center}
\end{solution}
\vfill
\pagebreak

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@ -1,77 +0,0 @@
\section{Links}
\definition{}
A \textit{link} is a set of knots intertwined with each other. \par
Just as with knots, we say that two links are \textit{isomorphic} if one can be deformed into the other.
\vspace{2mm}
The \textit{Whitehead link} is one of the simplest links we can produce. \par
It consists of two knots, so we say it is a \textit{link of two components}.
Two projections of the Whitehead link are shown below.
\begin{center}
\hfill
\begin{minipage}[t]{0.27\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/whitehead a.png}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.25\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/whitehead b.png}
\end{center}
\end{minipage}
\hfill~
\end{center}
\definition{}
The \textit{$n$-unlink} is the link that consists of $n$ disjoint unknots. \par
The 3-unlink is shown below:
\begin{center}
\begin{tikzpicture}
\draw[circle] (0,0) circle (0.7);
\draw[circle] (2,0) circle (0.7);
\draw[circle] (4,0) circle (0.7);
\end{tikzpicture}
\end{center}
\definition{}
We say a nontrivial link is \textit{Brunnian} if we get an $n$-unlink after removing any component.
\vspace{2mm}
The \textit{Borromean Rings} are a common example of this. If we were to cut any of the three rings, the other two would fall apart.
\begin{center}
\includegraphics[height=3cm]{images/borromean.png}
\end{center}
\vfill
\pagebreak
\problem{}
Find a Brunnian link with four components.
\vfill
\problem{}
Find a Brunnian link with $n$ components.
\begin{solution}
One of many possible solutions:
\begin{center}
\includegraphics[width=40mm]{images/brunnian.png}
\end{center}
\end{solution}
\vfill
\pagebreak

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@ -1,62 +0,0 @@
\section{Knots and Sticks}
\definition{}
The \textit{stick number} of a knot is the smallest number of \say{sticks} you must glue together to make the knot. An example of this is below.
\begin{center}
\includegraphics[width=30mm]{images/sticks.png}
\end{center}
\problem{}
Make a trefoil knot with sticks. \par
How many do you need?
\begin{solution}
\begin{center}
\includegraphics[width=20mm]{images/stick trefoil.png}
\end{center}
\end{solution}
\vfill
\problem{}
How many sticks do you need to make a figure-eight knot?
\begin{solution}
The figure-eight knot has stick number 7. \par
In fact, it is the only knot with stick number 7.
\end{solution}
\vfill
\pagebreak
\problem{}
Make the knot $5_1$ (refer to the knot table) with eight sticks.
\vfill
\problem{}
Show that the only nontrivial knot you can make with six sticks is the trefoil.
\vfill
\problem{}
Let $S(k)$ be the stick number of a knot $k$. \par
Show that $S(j \boxplus k) \leq s(j) + s(k) - 1$
\vfill
\problem{}
What is the stick number of $(\text{trefoil} \boxplus \text{trefoil})$?
\begin{solution}
You can make $(\text{trefoil} \boxplus \text{trefoil})$ with 8 sticks.
\begin{center}
\includegraphics[angle=90, width=40mm]{images/stick trefoil composition.png}
\end{center}
\end{solution}
\vfill
\pagebreak

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@ -1,204 +0,0 @@
\section{Braids}
\definition{}
A \textit{braid} is a set of $n$ strings with fixed ends. Two braids are equivalent if they may be deformed into each other without disconnecting the strings. Two braids are shown below:
\begin{center}
\begin{tikzpicture}
\pic[rotate=90, name prefix=braid] {
braid = {s_2^{-1} s_1^{-1} s_2 s_2 s_3^{-1} }
};
\braidbars{4}
\end{tikzpicture}
\hfill
\begin{tikzpicture}
\pic[rotate=90, name prefix=braid] {
braid = {s_2^{-1} s_3 s_2 s_1 s_1^{-1} s_2^{-1} s_3^{-1} s_1^{-1} s_2 s_2 s_3^{-1} }
};
\braidbars{4}
\end{tikzpicture}
\end{center}
\problem{}
Convince yourself that the braids above are equivalent.
\vfill
\pagebreak
\definition{}
A braid can be \textit{closed} by conecting its ends:
\begin{center}
\begin{tikzpicture}
\pic[rotate=90, name prefix=braid] {
braid = {s_2^{-1} s_1^{-1} s_2 s_2 s_3^{-1} }
};
\closebraid{4}
\widebraidbars{4}
\end{tikzpicture}
\end{center}
\problem{}
When will a closed braid form a knot? \par
When will a closed braid form a link?
\vfill
\problem{}
Draw a braid that creates a $3$-unlink when closed.
\vfill
\pagebreak
\problem{}<braidify>
Draw the following knots as closed braids.
\begin{center}
\hfill
\begin{minipage}[t]{0.13\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/trefoil.png}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.15\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/closed braid a.png}
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.15\textwidth}
\begin{center}
\includegraphics[width=\linewidth]{images/closed braid b.png}
\end{center}
\end{minipage}
\hfill~
\end{center}
\vfill
\pagebreak
\problem{}
We can describe the projection of a braid by listing which strings cross over and under each other as we move along the braid. \par
\vspace{2mm}
For example, consider a three-string braid. If the first string crosses over the second, we'll call that a $1$ crossing. If first string crosses \textbf{under} the second, we'll call that a $-1$ crossing.
\begin{center}
\hfill
\begin{minipage}[t]{0.2\textwidth}
\begin{center}
\begin{tikzpicture}
\pic[
name prefix = braid,
braid/number of strands = 3
] {
braid = {s_1}
};
\end{tikzpicture} \par
\texttt{1} crossing
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.2\textwidth}
\begin{center}
\begin{tikzpicture}
\pic[
name prefix = braid,
braid/number of strands = 3
] {
braid = {s_1^{-1}}
};
\end{tikzpicture} \par
\texttt{-1} crossing
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.2\textwidth}
\begin{center}
\begin{tikzpicture}
\pic[
name prefix = braid,
braid/number of strands = 3
] {
braid = {s_2}
};
\end{tikzpicture} \par
\texttt{2} crossing
\end{center}
\end{minipage}
\hfill
\begin{minipage}[t]{0.2\textwidth}
\begin{center}
\begin{tikzpicture}
\pic[
name prefix = braid,
braid/number of strands = 3
] {
braid = {s_2^{-1}}
};
\end{tikzpicture} \par
\texttt{-2} crossing
\end{center}
\end{minipage}
\hfill~
\end{center}
\problem{}
Verify that the following is a $[1, 2, 1, -2, 1, 2]$ braid. \par
Read the braid left to right, with the bottom string numbered $1$.
\begin{center}
\begin{tikzpicture}
\pic[
rotate = 90,
name prefix = braid,
braid/number of strands = 3
] {
% When we rotate a braid
braid = {s_1 s_2 s_1 s_2^{-1} s_1 s_2}
};
\labelbraidstart{3}
\end{tikzpicture}
\end{center}
\vfill
\problem{}
Draw the five-string braid defined by $[1, 3, 4, -3, 2, 4]$
\begin{solution}
\begin{center}
\begin{tikzpicture}
\pic[
rotate = 90,
name prefix = braid,
braid/number of strands = 5
] {
braid = {s_1 s_3 s_4 s_3^{-1} s_2 s_4}
};
\labelbraidstart{5}
\labelbraidend{5}
\end{tikzpicture}
\end{center}
\end{solution}
\vfill
\pagebreak
\problem{}
Identify the knot generated by the 4-string braid $[(-1, 2, 3)^2,~(3)^3]$ \par
\hint{$[(1, 2)^2, 3] = [1, 2, 1, 2, 3]$}
\hint{This knot has 6 crossings. Use the knot table.}
\vfill
\problem{}
Show that the $n$-string braid $[(1, 2, ..., n-1)^m]$ forms a knot iff $m$ and $n$ are coprime.
\vfill
\pagebreak

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@ -1,42 +0,0 @@
\section{Table of Prime Knots}
A knot's \textit{crossing number} is the minimal number of crossings its projection must contain. \par
Finding a knot's crossing number is a fairly difficult problem.
\vspace{1mm}
This table contains the a 20 smallest prime knots, ordered by crossing number. \par
Mirror images are not included, even if the mirror image produces a nonisomorphic knot.
\vfill
% Images are from the appendix of the Knot book.
{
\def\w{24mm}
\newcounter{knotcounter}
\foreach \a in {%
{3_1},{4_1},{5_1},{5_2},%
{6_1},{6_2},{6_3},{7_1},%
{7_2},{7_3},{7_4},{7_5},%
{7_6},{7_7},{8_1},{8_2},%
{8_3},{8_4},{8_5},{8_6}%
}{
\stepcounter{knotcounter}
\hfill
\begin{minipage}{\w}
\begin{center}
\includegraphics[width=\linewidth]{knot table/\a.png} \par
\vspace{2mm}
{\huge $\a$}
\end{center}
\end{minipage}
\ifnum\value{knotcounter}=4
\hfill~\par
\vspace{4mm}
\setcounter{knotcounter}{0}
\fi
}
}
\vfill
\pagebreak

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@ -1,106 +0,0 @@
\usetikzlibrary{
knots,
hobby,
braids,
decorations.pathreplacing,
shapes.geometric,
calc
}
\ifDebugKnot
\tikzset{
knot diagram/draft mode = crossings,
knot diagram/only when rendering/.style = {
show curve endpoints,
%show curve controls
}
}
\fi
\tikzset{
circle/.style = {
line width = 0.8mm,
},
knot diagram/every strand/.append style={
line width = 0.8mm,
black
},
show curve controls/.style={
postaction=decorate,
decoration={
show path construction,
curveto code={
\draw[blue, dashed]
(\tikzinputsegmentfirst) -- (\tikzinputsegmentsupporta)
node [at end, draw, solid, red, inner sep=2pt]{}
;
\draw[blue, dashed]
(\tikzinputsegmentsupportb) -- (\tikzinputsegmentlast)
node [at start, draw, solid, red, inner sep=2pt]{}
node [at end, fill, red, ellipse, inner sep=2pt]{}
;
}
}
},
show curve endpoints/.style={
postaction=decorate,
decoration={
show path construction,
curveto code={
\node [fill, blue, ellipse, inner sep=2pt] at (\tikzinputsegmentlast) {};
}
}
},
braid/gap = 0.2,
braid/width = 5mm,
braid/height = -8mm,
braid/control factor = 0.75,
braid/nudge factor = 0.05,
braid/every strand/.style = {
line width = 0.7mm
},
bar/.style = {
% Should match braid line width
line width = 0.7mm,
fill = black
}
}
% Braid bar macro
% Argument: number of strands
\newcommand{\braidbars}[1]{
\draw[bar] ([xshift=-0.7mm]braid-1-s) rectangle (braid-#1-s);
\draw[bar] ([xshift=-0.7mm]braid-rev-1-e) rectangle (braid-rev-#1-e);
}
\newcommand{\widebraidbars}[1]{
\draw[bar] ([xshift=-0.7mm,yshift=-2mm]braid-1-s) rectangle ([yshift=2mm]braid-#1-s);
\draw[bar] ([xshift=-0.7mm,yshift=-2mm]braid-rev-1-e) rectangle ([yshift=2mm]braid-rev-#1-e);
}
% Closed braid loops
% Argument: number of strands
\newcommand{\closebraid}[1]{
\foreach \x in {1, ..., #1} {
\draw[braid/every strand, rounded corners = 4mm]
(braid-\x-s) --
([shift=(west:5*\x mm)]braid-\x-s) --
([shift=(west:5*\x mm),shift=(south:10*\x mm)]braid-\x-s) --
([shift=(east:5*\x mm),shift=(south:10*\x mm)]braid-rev-\x-e) --
([shift=(east:5*\x mm)]braid-rev-\x-e) --
(braid-rev-\x-e)
;
}
}
\newcommand{\labelbraidstart}[1]{
\foreach \x in {1, ..., #1} {
\node at ([xshift=-2mm]braid-\x-s) {$\x$};
}
}
\newcommand{\labelbraidend}[1]{
\foreach \x in {1, ..., #1} {
\node at ([xshift=2mm]braid-\x-e) {$\x$};
}
}