diff --git a/Advanced/Symmetric Group/main.tex b/Advanced/Symmetric Group/main.tex index b9ca834..a9d789e 100755 --- a/Advanced/Symmetric Group/main.tex +++ b/Advanced/Symmetric Group/main.tex @@ -5,7 +5,7 @@ singlenumbering ]{../../resources/ormc_handout} \usepackage{../../resources/macros} - +\usetikzlibrary{calc} \uptitlel{Advanced 2} \uptitler{Winter 2023} @@ -14,14 +14,22 @@ +\def\line#1#2{ + \draw[line width = 0.3mm, ->, ocyan] + (#1) + -- ($(#1) + (0, -1)$) + -- ($(#2) + (0,1)$) + -- (#2); +} \begin{document} \maketitle \input{parts/0 intro} + \input{parts/1 cycle} + - % cycle notation % decomposition into transpositions % few more problems? diff --git a/Advanced/Symmetric Group/parts/0 intro.tex b/Advanced/Symmetric Group/parts/0 intro.tex index 9e1c32f..c70bc04 100644 --- a/Advanced/Symmetric Group/parts/0 intro.tex +++ b/Advanced/Symmetric Group/parts/0 intro.tex @@ -44,7 +44,7 @@ Why do we define permutations as a \textit{bijective} map? We can visualize permutations with a diagram we'll call the \say{braid.} -The arrows in the diagram denote the image of $f$ for each possible input. +The arrows in this diagram denote the image of $f$ for each possible input. Two examples are below: \vspace{2mm} @@ -60,10 +60,10 @@ Two examples are below: \node (4b) at (2, -2) {4}; \node (2b) at (3, -2) {2}; - \draw[line width = 0.3mm, ->, ocyan] (1a.south) -- (1a.south |- 0, -0.5) -- (1b.north |- 0,-1) -- (1b.north); - \draw[line width = 0.3mm, ->, ocyan] (2a.south) -- (2a.south |- 0, -0.5) -- (2b.north |- 0,-1) -- (2b.north); - \draw[line width = 0.3mm, ->, ocyan] (3a.south) -- (3a.south |- 0, -0.5) -- (3b.north |- 0,-1) -- (3b.north); - \draw[line width = 0.3mm, ->, ocyan] (4a.south) -- (4a.south |- 0, -0.5) -- (4b.north |- 0,-1) -- (4b.north); + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} \end{tikzpicture} \hfill \begin{tikzpicture}[scale=0.5] @@ -77,12 +77,14 @@ Two examples are below: \node (3b) at (2, -2) {3}; \node (4b) at (3, -2) {4}; - \draw[line width = 0.3mm, ->, ocyan] (1a.south) -- (1a.south |- 0, -0.5) -- (1b.north |- 0,-1) -- (1b.north); - \draw[line width = 0.3mm, ->, ocyan] (2a.south) -- (2a.south |- 0, -0.5) -- (2b.north |- 0,-1) -- (2b.north); - \draw[line width = 0.3mm, ->, ocyan] (3a.south) -- (3a.south |- 0, -0.5) -- (3b.north |- 0,-1) -- (3b.north); - \draw[line width = 0.3mm, ->, ocyan] (4a.south) -- (4a.south |- 0, -0.5) -- (4b.north |- 0,-1) -- (4b.north); + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} \end{tikzpicture} \hfill\null +\vspace{2mm} + Note that in all our examples thus far, the objects in our set have an implicit order. This is only for convenience. The elements of $\Omega$ are not ordered (it is a \textit{set}, after all), @@ -113,10 +115,10 @@ The rightmost diagram uses arbitrary, meaningless labels. \node (3b) at (2, -2) {3}; \node (4b) at (3, -2) {4}; - \draw[line width = 0.3mm, ->, ocyan] (1a.south) -- (1a.south |- 0, -0.5) -- (1b.north |- 0,-1) -- (1b.north); - \draw[line width = 0.3mm, ->, ocyan] (2a.south) -- (2a.south |- 0, -0.5) -- (2b.north |- 0,-1) -- (2b.north); - \draw[line width = 0.3mm, ->, ocyan] (3a.south) -- (3a.south |- 0, -0.5) -- (3b.north |- 0,-1) -- (3b.north); - \draw[line width = 0.3mm, ->, ocyan] (4a.south) -- (4a.south |- 0, -0.5) -- (4b.north |- 0,-1) -- (4b.north); + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} \end{tikzpicture} \hfill \begin{tikzpicture}[scale=0.5] @@ -130,10 +132,10 @@ The rightmost diagram uses arbitrary, meaningless labels. \node (3b) at (2, -2) {3}; \node (2b) at (3, -2) {2}; - \draw[line width = 0.3mm, ->, ocyan] (1a.south) -- (1a.south |- 0, -0.5) -- (1b.north |- 0,-1) -- (1b.north); - \draw[line width = 0.3mm, ->, ocyan] (2a.south) -- (2a.south |- 0, -0.5) -- (2b.north |- 0,-1) -- (2b.north); - \draw[line width = 0.3mm, ->, ocyan] (3a.south) -- (3a.south |- 0, -0.5) -- (3b.north |- 0,-1) -- (3b.north); - \draw[line width = 0.3mm, ->, ocyan] (4a.south) -- (4a.south |- 0, -0.5) -- (4b.north |- 0,-1) -- (4b.north); + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} \end{tikzpicture} \hfill \begin{tikzpicture}[scale=0.5] @@ -147,12 +149,14 @@ The rightmost diagram uses arbitrary, meaningless labels. \node (3b) at (2, -2) {$\circledcirc$}; \node (4b) at (3, -2) {$\boxdot$}; - \draw[line width = 0.3mm, ->, ocyan] (1a.south) -- (1a.south |- 0, -0.5) -- (1b.north |- 0,-1) -- (1b.north); - \draw[line width = 0.3mm, ->, ocyan] (2a.south) -- (2a.south |- 0, -0.5) -- (2b.north |- 0,-1) -- (2b.north); - \draw[line width = 0.3mm, ->, ocyan] (3a.south) -- (3a.south |- 0, -0.5) -- (3b.north |- 0,-1) -- (3b.north); - \draw[line width = 0.3mm, ->, ocyan] (4a.south) -- (4a.south |- 0, -0.5) -- (4b.north |- 0,-1) -- (4b.north); + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} \end{tikzpicture} \hfill\null +\vspace{2mm} + It shouldn't be hard to see that despite the different \say{output} order (2134 and 1432), \par the same permutation is depicted in all three diagrams. This example demonstrates two things: @@ -167,7 +171,6 @@ the same permutation is depicted in all three diagrams. This example demonstrate \vspace{2mm} -\vspace{1cm} Why, then, do we order our elements when we talk about permutations? As noted before, this is for convenience. If we assign a natural order to the elements of $\Omega$ (say, 1234), we can identify permutations by simply listing @@ -182,7 +185,7 @@ Draw braids for $[4123]$ and $[2341]$. Finally, note that permutations (as defined in \ref{permadef}) are \textit{not} \say{orderings of a certain set.} \par -They are defined as \textit{bijective maps}, which can be \textit{thought of} as orderings. \par +They are defined as \textit{bijective maps}, which can be written as orderings of a given array. \par Remember: permutations are verbs! \pagebreak \ No newline at end of file diff --git a/Advanced/Symmetric Group/parts/1 cycle.tex b/Advanced/Symmetric Group/parts/1 cycle.tex new file mode 100755 index 0000000..8166257 --- /dev/null +++ b/Advanced/Symmetric Group/parts/1 cycle.tex @@ -0,0 +1,387 @@ + +\section{Cycle Notation} + +\definition{} +The \textit{order} of a permutation $f$ is the smallest $n$ so that $f^n(x) = x$ for all $x$. \par +In other words, if we repeat this permutation $n$ times, we get back to where we started. + +\vspace{2mm} + +For example, consider $[2134]$. This permutation has order $2$, as we clearly see below: + +\begin{center} +\begin{tikzpicture}[scale=0.5] + \node (1a) at (0, 0.5) {1}; + \node (2a) at (1, 0.5) {2}; + \node (3a) at (2, 0.5) {3}; + \node (4a) at (3, 0.5) {4}; + + \node (2b) at (0, -2) {2}; + \node (1b) at (1, -2) {1}; + \node (3b) at (2, -2) {3}; + \node (4b) at (3, -2) {4}; + + \node (1c) at (0, -4.5) {1}; + \node (2c) at (1, -4.5) {2}; + \node (3c) at (2, -4.5) {3}; + \node (4c) at (3, -4.5) {4}; + + \line{1a}{1b} + \line{2a}{2b} + \line{3a}{3b} + \line{4a}{4b} + \line{1b}{1c} + \line{2b}{2c} + \line{3b}{3c} + \line{4b}{4c} +\end{tikzpicture} +\end{center} +Of course, swapping the first two elements twice results in the identity map. \par +$[2134]$ happens to be its own inverse. Also, the order of $[1234]$ is (of course) one. + + +\problem{} +What is the order of $[2314]$? \par +How about $[4321]$? \par +\note[Note]{You shouldn't need to draw any braids to solve this problem.} + + +\vfill + +\problem{Bonus} +Show that all permutations (on a finite set) have a well-defined order. \par +In other words, show that there is always an integer $n$ so that $f^n(x) = x$. + +\vfill +\pagebreak + + +As you may have noticed, the square-bracket notation we've been using thus far is a bit unwieldy. +Permutations are verbs---but we've been referring to them using a noun (namely, their output when +applied to an ordered sequence of numbers). Our notation fails to capture the meaning of the +underlying object. + +\vspace{2mm} + +Think about it: is the permutation $[1234]$ different than the permutation $[12345]$? \par +Indeed, these permutations operate on different sets---but they are both the identity! \par +What should we do if we want to talk about the identity on $\{1, 2, ..., 10\}$? + +\vspace{2mm} + +We need something better. + + + + +\definition{} +Any permutation is composed of a number of \textit{cycles}. \par + +For example, consider the permutation $[2134]$, which consists of one two-cycle: $1 \to 2 \to 1$ \par +\note[Note]{$3 \to 3$ and $4 \to 4$ are also cycles, but we'll ignore them. One-cycles aren't aren't interesting.} + +\begin{center} +\begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + + \draw[line width = 0.3mm, ->, ocyan] + (1) + -- ($(1) + (0,-1)$) + -- ($(2) + (0,-1)$) + -- (2); + + \draw[line width = 0.3mm, ->, ocyan] + (2) + -- ($(2) + (0, 1)$) + -- ($(1) + (0, 1)$) + -- (1); +\end{tikzpicture} +\end{center} + + +The permutation $[431265]$ is a bit more interesting---it contains of two cycles: \par +($1 \to 3 \to 2 \to 4 \to 1$ and $5 \to 6 \to 5$) + + +\begin{center} +\begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + \node (5) at (4, 0) {5}; + \node (6) at (5, 0) {6}; + + \draw[line width = 0.3mm, ->, ocyan] + (3) + -- ($(3) + (0,-1)$) + -- ($(2) + (0,-1)$) + -- (2); + + \draw[line width = 0.3mm, ->, ocyan] + (2) + -- ($(2) + (0,1.5)$) + -- ($(4) + (0,1.5)$) + -- (4); + + \draw[line width = 0.3mm, ->, ocyan] + (4) + -- ($(4) + (0,-1.5)$) + -- ($(1) + (0,-1.5)$) + -- (1); + + \draw[line width = 0.3mm, ->, ocyan] + (1) + -- ($(1) + (0,1)$) + -- ($(3) + (0,1)$) + -- (3); + + \draw[line width = 0.3mm, ->, ogreen] + (5) + -- ($(5) + (0,-1)$) + -- ($(6) + (0,-1)$) + -- (6); + + \draw[line width = 0.3mm, ->, ogreen] + (6) + -- ($(6) + (0,1)$) + -- ($(5) + (0,1)$) + -- (5); + +\end{tikzpicture} +\end{center} + + + +\problem{} +Find all cycles in $[5342761]$. + +\begin{solution} + \begin{center} + \begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + \node (5) at (4, 0) {5}; + \node (6) at (5, 0) {6}; + \node (7) at (6, 0) {7}; + + \draw[line width = 0.3mm, ->, ocyan] + (1) + -- ($(1) + (0,2)$) + -- ($(7) + (0,2)$) + -- (7); + + \draw[line width = 0.3mm, ->, ocyan] + (7) + -- ($(7) + (0,-1.5)$) + -- ($(5) + (0,-1.5)$) + -- (5); + + \draw[line width = 0.3mm, ->, ocyan] + (5) + -- ($(5) + (0,1.5)$) + -- ($(1) + (0.5,1.5)$) + -- ($(1) + (0.5,-1)$) + -- ($(1) + (0,-1)$) + -- (1); + + \draw[line width = 0.3mm, ->, ogreen] + (2) + -- ($(2) + (0,-1.5)$) + -- ($(4) + (0,-1.5)$) + -- (4); + + \draw[line width = 0.3mm, ->, ogreen] + (4) + -- ($(4) + (0,1)$) + -- ($(3) + (0,1)$) + -- (3); + + \draw[line width = 0.3mm, ->, ogreen] + (3) + -- ($(3) + (0,-1)$) + -- ($(2) + (0.5,-1)$) + -- ($(2) + (0.5,1)$) + -- ($(2) + (0,1)$) + -- (2); + \end{tikzpicture} + \end{center} +\end{solution} + +\vfill + + +\problem{} +What permutation (on five objects) consists of the cycles $3 \to 5 \to 3$ and $1 \to 2 \to 4 \to 1$? + + +\begin{solution} + \begin{center} + \begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + \node (5) at (4, 0) {5}; + + \draw[line width = 0.3mm, ->, ocyan] + (3) + -- ($(3) + (0,1)$) + -- ($(5) + (0,1)$) + -- (5); + + \draw[line width = 0.3mm, ->, ocyan] + (5) + -- ($(5) + (0,-1)$) + -- ($(3) + (0,-1)$) + -- (3); + + \draw[line width = 0.3mm, ->, ogreen] + (1) + -- ($(1) + (0,-1)$) + -- ($(2) + (0,-1)$) + -- (2); + + \draw[line width = 0.3mm, ->, ogreen] + (2) + -- ($(2) + (0,1.5)$) + -- ($(4) + (0,1.5)$) + -- (4); + + \draw[line width = 0.3mm, ->, ogreen] + (4) + -- ($(4) + (0,-1.5)$) + -- ($(1) + (0.5,-1.5)$) + -- ($(1) + (0.5,1)$) + -- ($(1) + (0,1)$) + -- (1); + \end{tikzpicture} + + This is $[41523]$ + \end{center} +\end{solution} + + + +\vfill +\pagebreak + +\definition{} +We now have a solution to our problem of notation. +Instead of referring to permutations using their output, we will refer to them using their \textit{cycles}. + +\vspace{2mm} + +For example, we'll write $[2134]$ as $(12)$, which denotes the cycle $1 \to 2 \to 1$: + + +\begin{center} +\begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + + \draw[line width = 0.3mm, ->, ocyan] + (1) + -- ($(1) + (0,-1)$) + -- ($(2) + (0,-1)$) + -- (2); + + \draw[line width = 0.3mm, ->, ocyan] + (2) + -- ($(2) + (0, 1)$) + -- ($(1) + (0, 1)$) + -- (1); +\end{tikzpicture} +\end{center} + + + +As another example, $[431265]$ is $(1324)(56)$ in cycle notation. \par +Note that we write $[431265]$ as a \textit{composition} of two cycles: \par +applying the permutation $[431265]$ is the same as applying $(1324)$, then applying $(56)$. + +\begin{center} +\begin{tikzpicture}[scale=0.5] + \node (1) at (0, 0) {1}; + \node (2) at (1, 0) {2}; + \node (3) at (2, 0) {3}; + \node (4) at (3, 0) {4}; + \node (5) at (4, 0) {5}; + \node (6) at (5, 0) {6}; + + \draw[line width = 0.3mm, ->, ocyan] + (3) + -- ($(3) + (0,-1)$) + -- ($(2) + (0,-1)$) + -- (2); + + \draw[line width = 0.3mm, ->, ocyan] + (2) + -- ($(2) + (0,1.5)$) + -- ($(4) + (0,1.5)$) + -- (4); + + \draw[line width = 0.3mm, ->, ocyan] + (4) + -- ($(4) + (0,-1.5)$) + -- ($(1) + (0,-1.5)$) + -- (1); + + \draw[line width = 0.3mm, ->, ocyan] + (1) + -- ($(1) + (0,1)$) + -- ($(3) + (0,1)$) + -- (3); + + \draw[line width = 0.3mm, ->, ogreen] + (5) + -- ($(5) + (0,-1)$) + -- ($(6) + (0,-1)$) + -- (6); + + \draw[line width = 0.3mm, ->, ogreen] + (6) + -- ($(6) + (0,1)$) + -- ($(5) + (0,1)$) + -- (5); + +\end{tikzpicture} +\end{center} + + +\problem{} +Convince yourself that disjoint cycles commute. \par +That is, $(1324)(56) = (56)(1324)$ since $(1324)$ and $(56)$ do not overlap. + +\problem{} +Write the following in square-bracket notation. +\begin{itemize} + \item $(12)$ \tab~\tab on a set of 2 elements + \item $(12)(435)$ \tab on a set of 5 elements + \vspace{2mm} + \item $(321)$ \tab~\tab on a set of 3 elements + \item $(321)$ \tab~\tab on a set of 6 elements + \vspace{2mm} + \item $(1234)$ \tab on a set of 4 elements + \item $(3412)$ \tab on a set of 4 elements +\end{itemize} + +\vfill + +\problem{} +Write the following in square-bracket notation. +Be careful. +\begin{itemize} + \item $(13)(243)$ \tab on a set of 4 elements + \item $(243)(13)$ \tab on a set of 4 elements +\end{itemize} + +\vfill +\pagebreak \ No newline at end of file