diff --git a/Advanced/Group Theory/parts/01 groups.tex b/Advanced/Group Theory/parts/01 groups.tex index a58bdfb..121a7f2 100755 --- a/Advanced/Group Theory/parts/01 groups.tex +++ b/Advanced/Group Theory/parts/01 groups.tex @@ -6,7 +6,7 @@ A group must have the following properties: \\ \begin{enumerate} \item $G$ is closed under $\ast$. In other words, $a, b \in G \implies a \ast b \in G$. - \item $\ast$ is associative: $a \ast b = b \ast a$ + \item $\ast$ is associative: $(a \ast b) \ast c = a \ast (b \ast c)$ \item There is an \textit{identity} $\overline{0} \in G$, so that $a \ast \overline{0} = a$ for all $a \in G$. \item For any $a \in G$, there exists a $b \in G$ so that $a \ast b = \overline{0}$. $b$ is called the \textit{inverse} of $a$. \\ This element is written as $-a$ if our operator is addition and $a^{-1}$ otherwise. @@ -42,13 +42,59 @@ Make it one by modifying $\mathbb{R}$. \\ \vfill \problem{} -Can you construct a group that contains a single element? +What is the smallest group we can create? \begin{solution} Let $(G, \circledcirc)$ be our group, where $G = \{\star\}$ and $\circledcirc$ is defined by the identity $\star \circledcirc \star = \star$ Verifying that the trivial group is a group is trivial. \end{solution} +\vfill + +\problem{} +Let $G$ be the set of all bijections $A \to A$. \\ +Let $\circ$ be the usual composition operator. \\ +Is $(G, \circ)$ a group? + +\vfill +\pagebreak + +\problem{} +Show that a group has exactly one identity element. +\vfill + +\problem{} +Show that each element in a group has exactly one inverse. +\vfill + +\problem{} +Let $(G, \ast)$ be a group and $a, b, c \in G$. Show that... +\begin{itemize} + \item $a \ast b$ and $a \ast c \implies b = c$ + \item $b \ast a$ and $c \ast a \implies b = c$ +\end{itemize} +What does this mean intuitively? +\vfill + +\problem{} +Let $(G, \ast)$ be a finite group (i.e, $G$ has finitely many elements), and let $g \in G$. \\ +Show that $\exists~n \in Z^+$ so that $g^n = \overline{0}$ \\ +\hint{$g^n = g \ast g \ast ... \ast g$ $n$ times.} + +\vspace{2mm} + +The smallest such $n$ defines the \textit{order} of $(G, \ast)$. + +\vfill + +\problem{} +What is the order of 5 in $(\mathbb{Z}/25, +)$? \\ +What is the order of 2 in $((\mathbb{Z}/17)^\times, \times)$? \\ + +\vfill + +\problem{} +Show that if $G$ has four elements, $(G, \ast)$ is abelian. \vfill \pagebreak @@ -76,20 +122,12 @@ Recall your tables from \ref{modtables}: \\ \end{tabular} \end{center} -Convince yourself that $(\mathbb{Z}/4, +)$ and $( (\mathbb{Z}/5)^\times, \times )$ are the same group. \\ +Look at these tables and convince yourself that $(\mathbb{Z}/4, +)$ and $( (\mathbb{Z}/5)^\times, \times )$ are the same group. \\ We say that two such groups are \textit{isomorphic}. \vspace{2mm} -Intuitively, this means that $(\mathbb{Z}/4, +)$ and $( (\mathbb{Z}/5)^\times, \times )$ have the same algebraic structure. We can translate statements about addition in $\mathbb{Z}/4$ into statements about multiplication in $(\mathbb{Z}/5)^\times$ \\ +Intuitively, this means that these two groups have the same algebraic structure. We can translate statements about addition in $\mathbb{Z}/4$ into statements about multiplication in $(\mathbb{Z}/5)^\times$ \\ -\problem{} -Show that a group has exactly one identity element. -\vfill - -\problem{} -Show that each element in a group has exactly one inverse. - -\vfill \pagebreak