### 数学代写|表示论代写Representation theory代考|MTH4107

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 数学代写|表示论代写Representation theory代考|Definition and Examples

We start by recalling the definition of a ring: A ring is a non-empty set $R$ together with an addition $+: R \times R \rightarrow R,(r, s) \mapsto r+s$ and a multiplication $:: R \times R \rightarrow R$, $(r, s) \mapsto r \cdot s$ such that the following axioms are satisfied for all $r, s, t \in R$ :
(R1) (Associativity of $+) r+(s+t)=(r+s)+t$.
(R2) (Zero element) There exists an element $0_{R} \in R$ such that $r+0_{R}=r=0_{R}+r$.
(R3) (Additive inverses) For every $r \in R$ there is an element $-r \in R$ such that $r+(-r)=0_{R}$
(R4) (Commutativity of $+) r+s=s+r$.
(R5) (Distributivity) $r \cdot(s+t)=r \cdot s+r \cdot t$ and $(r+s) \cdot t=r \cdot t+s \cdot t$.
(R6) (Associativity of $\cdot) r \cdot(s \cdot t)=(r \cdot s) \cdot t$.
(R7) (Identity element) There is an element $1_{R} \in R \backslash{0}$ such that $1_{R} \cdot r=r=r \cdot 1_{R}$
Moreover, a ring $R$ is called commutative if $r \cdot s=s \cdot r$ for all $r, s \in R$.
As usual, the multiplication in a ring is often just written as $r s$ instead of $r \cdot s$; we will follow this convention from now on.

Note that axioms ( $\mathrm{R} 1)-(\mathrm{R} 4)$ say that $(R,+)$ is an abelian group. We assume by Axiom (R7) that all rings have an identity element; usually we will just write 1 for $1_{R}$. Axiom (R7) also implies that $1_{R}$ is not the zero element. In particular, a ring has at least two elements.
We list some common examples of rings.
(1) The integers $\mathbb{Z}$ form a ring. Every field is also a ring, such as the rational numbers $\mathbb{Q}$, the real numbers $\mathbb{R}$, the complex numbers $\mathbb{C}$, or the residue classes $\mathbb{Z}{p}$ of integers modulo $p$ where $p$ is a prime number. (2) The $n \times n$-matrices $M{n}(K)$, with entries in a field $K$, form a ring with respect to matrix addition and matrix multiplication.
(3) The ring $K[X]$ of polynomials over a field $K$ where $X$ is a variable. Similarly, the ring of polynomials in two or more variables, such as $K[X, Y]$.

Examples (2) and (3) are not just rings but also vector spaces. There are many more rings which are vector spaces, and this has led to the definition of an algebra.

## 数学代写|表示论代写Representation theory代考|Division Algebras

A commutative ring is a field precisely when every non-zero element has an inverse with respect to multiplication. More generally, there are algebras in which every non-zero element has an inverse, and they need not be commutative.

Definition 1.7. An algebra $A$ (over a field $K$ ) is called a division algebra if every non-zero element $a \in A$ is invertible, that is, there exists an element $b \in A$ such that $a b=1_{A}=b a$. If so, we write $b=a^{-1}$. Note that if $A$ is finite-dimensional and $a b=1_{A}$ then it follows that $b a=1_{A}$; see Exercise $1.8$.

Division algebras occur naturally, we will see this later. Clearly, every field is a division algebra. There is a famous example of a division algebra which is not a field, this was discovered by Hamilton.

Example 1.8. The algebra $\mathbb{H}$ of quaternions is the 4-dimensional algebra over $\mathbb{R}$ with basis elements $1, i, j, k$ and with multiplication defined by
$$i^{2}=j^{2}=k^{2}=-1$$
and
$$i j=k, j i=-k, j k=i, k j=-i, k i=j, i k=-j$$
and extending to linear combinations. That is, an arbitrary element of $\mathbb{H}$ has the form $a+b i+c j+d k$ with $a, b, c, d \in \mathbb{R}$, and the product of two elements in $\mathbb{H}$ is given by
\begin{aligned} &\left(a_{1}+b_{1} i+c_{1} j+d_{1} k\right) \cdot\left(a_{2}+b_{2} i+c_{2} j+d_{2} k\right)= \ &\left(a_{1} a_{2}-b_{1} b_{2}-c_{1} c_{2}-d_{1} d_{2}\right)+\left(a_{1} b_{2}+b_{1} a_{2}+c_{1} d_{2}-d_{1} c_{2}\right) i \ &+\left(a_{1} c_{2}-b_{1} d_{2}+c_{1} a_{2}+d_{1} b_{2}\right) j+\left(a_{1} d_{2}+b_{1} c_{2}-c_{1} b_{2}+d_{1} a_{2}\right) k \end{aligned}
It might be useful to check this formula, see Exercise $1.11$.
One can check directly that the multiplication in $\mathrm{H}$ is associative, and that it satisfies the distributive law. But this will follow more easily later from a different construction of $\mathbb{H}$, see Example $1.27$.

## 数学代写|表示论代写Representation theory代考|Definition and Examples

(R1) (结合性 $+) r+(s+t)=(r+s)+t$.
(R2) (零元素) 存在一个元素 $0_{R} \in R$ 这样 $r+0_{R}=r=0_{R}+r$.
(R3) (加法逆) 对于每个 $r \in R$ 有一个元素 $-r \in R$ 这样 $r+(-r)=0_{R}$
(R4) (交换律 $+) r+s=s+r$.
(R5) (分配性) $r \cdot(s+t)=r \cdot s+r \cdot t$ 和 $(r+s) \cdot t=r \cdot t+s \cdot t$.
(R6) (结合性.) $r \cdot(s \cdot t)=(r \cdot s) \cdot t$.
(R7) (标识元素) 有一个元素 $1_{R} \in R \backslash 0$ 这样 $1_{R} \cdot r=r=r \cdot 1_{R}$

(1) 整数 $\mathbb{Z}$ 形成一个环。每个域也是一个环，比如有理数 $\mathbb{Q}$ ，实数 $\mathbb{R}$ ，复数 $\mathbb{C}$ ，或剩余类 $\mathbb{Z} p$ 整数模 $p$ 在哪里 $p$ 是一个素 数。(2) $n \times n$-矩阵 $M n(K)$ ，在字段中包含条目 $K$ ，关于矩阵加法和矩阵乘法形成一个环。
(3) 戒指 $K[X]$ 域上的多项式 $K$ 在哪里 $X$ 是一个变量。类似地，两个或多个变量中的多项式环，例如 $K[X, Y]$.

## 数学代写|表示论代写Representation theory代考|Division Algebras

$$i^{2}=j^{2}=k^{2}=-1$$

$$i j=k, j i=-k, j k=i, k j=-i, k i=j, i k=-j$$

$$\left(a_{1}+b_{1} i+c_{1} j+d_{1} k\right) \cdot\left(a_{2}+b_{2} i+c_{2} j+d_{2} k\right)=\quad\left(a_{1} a_{2}-b_{1} b_{2}-c_{1} c_{2}-d_{1} d_{2}\right)+\left(a_{1} b_{2}+b_{1} a_{2}\right.$$

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