2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)

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2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)

μš°λ¦¬κ°€ Group을 λΆ„λ₯˜ν–ˆλ“―이 Ring을 λΆ„λ₯˜ν•΄λ³΄μž!

Keyword.

  • Unity & unit
  • division ring
  • field & skew field
  • Quaternion
  • zero-divisor
  • Integral Domain; μ •μ—­

Definition. Zero ring

\[\{ 0 \}\]

μ›μ†Œκ°€ ν•˜λ‚˜ 뿐인 Ring.


Definition. Unity & unit

Let $R$ is a ring, THEN

  • Unity: a multiplicative identity.
  • unit: 곱에 λŒ€ν•œ 역원이 μ‘΄μž¬ν•˜λŠ” μ›μ†Œ


Example.

  • $\mathbb{Z}$μ—μ„œ Unitiy: $1$
  • $\mathbb{Z}$μ—μ„œ unit: $\{1, -1\}$



Theorem.

If the multiplicative identity exist, it is unique.


proof.

β€œadditive identity의 μœ μΌμ„±β€ 증λͺ…κ³Ό λΉ„μŠ·ν•œ λ§₯락으둜 ν•˜λ©΄ 됨.


Example.

$\mathbb{Z}_{rs} \cong \mathbb{Z}_r \times \mathbb{Z}_s$ is ring, IF $(r, s) = 1$.

HW.

\[\begin{aligned} \phi : \mathbb{Z}_{rs} &\longrightarrow \mathbb{Z}_r \times \mathbb{Z}_s \\ n &\longmapsto n(1, 1) \end{aligned}\]

THEN $\phi$ is a ring isomorphism.



Division Ring

Definition. division ring

Let $R$ be a ring with unity,

$R$ is a division ring, IF every non-zero element has a mutliplicative inverse.

즉, $R\setminus\{0\}$ is a group w.r.t. multiplication.


Field & skew-field

Definition. field & skew field

Division Ring이 곱에 λŒ€ν•΄ κ°€ν™˜μΈμ§€ 여뢀에 따라

  • Commutative division ring
    • Field
    • κ³±μ…ˆμ— λŒ€ν•΄ ꡰ을 μ΄λ£¨λ©΄μ„œ, 그것이 κ°€ν™˜κ΅°
  • non-Commutative division ring
    • skew Field
    • κ³±μ…ˆμ— λŒ€ν•΄ ꡰ을 μ΄λ£¨μ§€λ§Œ, κ°€ν™˜μ΄ μ•„λ‹˜


Example. $2\mathbb{Z}$

$2\mathbb{Z}$ is a commutative ring without unity.

μ™œλƒν•˜λ©΄, $1\notin 2\mathbb{Z}$λΌμ„œ


Example. $\mathbb{Q}$

$\mathbb{Q}$ is a division ring, also commutative ring.

λ”°λΌμ„œ $\mathbb{Q}$λŠ” Fieldλ‹€!!

λ§ˆμ°¬κ°€μ§€λ‘œ $\mathbb{R}$, $\mathbb{C}$도 Fieldμž„!



Quternion

Example. quaternion set

The set of quaternions is a skew field.


Definition. Quaternion $Q$

\[Q = \{w + x\mathbf{i} + y\mathbf{j} + z\mathbf{k} \mid \mathbf{i}^2 = \mathbf{j}^2 = \mathbf{k}^2 = \mathbf{i}\mathbf{j}\mathbf{k} = -1\}\]
  • λ§μ…ˆμ€ component-wise addition으둜 μ •μ˜
  • κ³±μ…ˆμ€ quaterion units $\mathbf{i}$, $\mathbf{j}$, $\mathbf{k}$에 λŒ€ν•œ κ·œμΉ™μ— 따라 μ‹œν–‰

  • κ³±μ…ˆμ— λŒ€ν•œ 역원: $\dfrac{w - x\mathbf{i} - y\mathbf{j} - z\mathbf{k}}{w^2 + x^2 + y^2 + z^2}$
  • κ³±μ…ˆμ— λŒ€ν•œ 항등원: $1 = (1, 0, 0 ,0)$
  • $q\cdot\bar{q} = w^2 + x^2 + y^2 + z^2$


β€œQuaternion은 3차원 νšŒμ „μ—μ„œ μ‚¬μš©ν•œλ‹€!”



zero-divisor

Definition. zero-divisor

Let $R$ is a ring, for $a, b \in R$,

IF $ab=0$, THEN $a$ and $b$ is called a zero-divisor.


Example.

In $\mathbb{Z}_{12}$,

\[3 \cdot 4 \equiv 12 \equiv 0\]

λ”°λΌμ„œ $3$, $4$λŠ” $\mathbb{Z}_{12}$μ—μ„œ zero-divisorμž„.



Theorem.

In $\mathbb{Z}_n$, the zero-divisors are precisely the elements which are not relatively prime to $n$.


proof.

Let non-zero $m \in \mathbb{Z}_n$ and $d:=(m, n) \ne 1$; κ³΅μ•½μˆ˜κ°€ 1이 μ•„λ‹Œ, 즉 $n$κ³Ό μ„œλ‘œμ†Œκ°€ μ•„λ‹Œ $m$을 μ„ νƒν•œλ‹€.

THEN,

\[m\left(\frac{n}{d}\right) = \left(\frac{m}{d}\right)n \equiv 0\]

즉, $m$κ³Ό $\frac{n}{d}$κ°€ zero-divisorλ‹€. $\blacksquare$


Corollary.

IF $p$ is a prime, THEN $\mathbb{Z}_p$ has no zero-divisor.

proof.

$\mathbb{Z}_p$의 μ›μ†ŒλŠ” λͺ¨λ‘ $p$와 μ„œλ‘œμ†Œμ΄λ―€λ‘œ, zero-divisorκ°€ μ‘΄μž¬ν•˜μ§€ μ•ŠλŠ”λ‹€.



Theorem.

IF $p$ is a prime, THEN $\mathbb{Z}_p$ is a field.

즉, λͺ¨λ“  non-zero elementκ°€ multiplicative inverseλ₯Ό κ°€μ§„λ‹€λŠ” 말!!


proof.

Let non-zero $a\in\mathbb{Z}_p$, then $1 \le a \le p-1$.

Since $p$ is a prime, $(a, p) = 1$,

BΓ©zout's Identity에 μ˜ν•΄, $ax + pq = 1$이 λ˜λ„λ‘ ν•˜λŠ” $x, y$κ°€ μ‘΄μž¬ν•œλ‹€.

μœ„μ˜ μ‹μ—μ„œ module $p$λ₯Ό μ·¨ν•˜λ©΄,

\[\begin{aligned} ax + pq &= 1 \\ (ax + pq) \; (\textrm{mod } p) &\equiv 1 \\ ax \; (\textrm{mod } p) &\equiv 1 \\ \end{aligned}\]

μ΄λ•Œ, $x$κ°€ λ°”λ‘œ $a$의 multiplicative inverseλ‹€!

$\mathbb{Z}_p$의 λͺ¨λ“  μ›μ†Œμ— λŒ€ν•΄ multiplicative inverseκ°€ 항상 μ‘΄μž¬ν•˜λ―€λ‘œ, $\mathbb{Z}_p$λŠ” division ring이닀.

$\mathbb{Z}_p$λŠ” commutative ring이기도 ν•˜λ―€λ‘œ, $\mathbb{Z}_p$λŠ” field이닀. $\blacksquare$



Integral Domain

Definition. Integral Domain; μ •μ—­

An Integral Domain is a commutative ring with Unitiy, and without zero-divisors.


Example.

\[\mathbb{Z}, \quad \mathbb{Z}_p\]


Homework.

$\mathbb{Z}_n$ is an integral domain, IFF $n$ is prime.



Theorem. Bezout’s Identity

For $n, m \in \mathbb{Z}$,

1. if $(n, m) = 1$, then $\exists x, y \in \mathbb{Z}$ s.t. $nx + my = 1$.

2. if $(n, m) = d$, then $\exists x, y \in \mathbb{Z}$ s.t. $nx + my = d$.

proof.

2번 λͺ…μ œλ§Œ 증λͺ…ν•΄λ³΄μž.

$(n, m) = d$인 $n$, $m$λ₯Ό gcd인 $d$둜 λ‚˜λˆ„μ–΄ 보자. 그러면

$\left( \frac{n}{d}, \frac{m}{d} \right) = 1$이 λœλ‹€.

λ”°λΌμ„œ 1번 λͺ…μ œμ— μ˜ν•΄ Bezout’s Identityκ°€ μ‘΄μž¬ν•œλ‹€.

\[\frac{n}{d} x + \frac{m}{d} y = 1\]

양변에 $d$λ₯Ό κ³±ν•˜λ©΄

\[nx + my = d\]

인 식을 μ–»λŠ”λ‹€. $\blacksquare$



Definition. Characteristics of Ring; ν™˜μ˜ ν‘œμˆ˜

If there exist $n \in \mathbb{N}$ s.t. $na \equiv 0$ ($a \in R$),

then call the smallest $n$ as a β€œCharacteristic of a Ring”.

β€» If there is no such $n$, then we say Char- of a Ring = 0.


Example.

Char- of $\mathbb{Z}_{10}$ = 10;

($\textrm{Char}(\mathbb{Z}_{10}) = 10$)

Char- of $\mathbb{Z}$, $\mathbb{Q}$ = 0


Theorem.

Let $R$ be a ring with unity 1,

then $n \cdot 1 = 0 \iff n \cdot a = 0 \quad \forall \; a \in R$.

proof.

($\impliedby$) Clear

($\implies$) Supp. $n\cdot1 = 0$

Let $a \in R$,

\[\begin{aligned} n \cdot a &= n \cdot (1 \cdot a) \\ &= (n \cdot 1) \cdot a \\ &= 0 \cdot a \\ &= 0 \end{aligned}\]