Ring - 2
2020-2νκΈ°, λνμμ βνλλμ1β μμ μ λ£κ³ 곡λΆν λ°λ₯Ό μ 리ν κΈμ λλ€. μ§μ μ μΈμ λ νμμ λλ€ :)
μ°λ¦¬κ° Groupμ λΆλ₯νλ―μ΄ Ringμ λΆλ₯ν΄λ³΄μ!
Keyword.
- Unity & unit
- division ring
- field & skew field
- Quaternion
- zero-divisor
- Integral Domain; μ μ
Definition. Zero ring
μμκ° νλ λΏμΈ Ring.
Definition. Unity & unit
Let $R$ is a ring, THEN
- Unity: a multiplicative identity.
- unit: κ³±μ λν μμμ΄ μ‘΄μ¬νλ μμ
Example.
- $\mathbb{Z}$μμ Unitiy: $1$
- $\mathbb{Z}$μμ unit: $\{1, -1\}$
Theorem.
If the multiplicative identity exist, it is unique.
proof.
βadditive identityμ μ μΌμ±β μ¦λͺ κ³Ό λΉμ·ν λ§₯λ½μΌλ‘ νλ©΄ λ¨.
Example.
$\mathbb{Z}_{rs} \cong \mathbb{Z}_r \times \mathbb{Z}_s$ is ring, IF $(r, s) = 1$.
HW.
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.
μ¦, $R\setminus\{0\}$ is a group w.r.t. multiplication.
Field & skew-field
Definition. field & skew field
Division Ringμ΄ κ³±μ λν΄ κ°νμΈμ§ μ¬λΆμ λ°λΌ
- Commutative division ring
- Field
- κ³±μ μ λν΄ κ΅°μ μ΄λ£¨λ©΄μ, κ·Έκ²μ΄ κ°νκ΅°
- non-Commutative division ring
- skew Field
- κ³±μ μ λν΄ κ΅°μ μ΄λ£¨μ§λ§, κ°νμ΄ μλ
Example. $2\mathbb{Z}$
$2\mathbb{Z}$ is a commutative ring without unity.
μλνλ©΄, $1\notin 2\mathbb{Z}$λΌμ
Example. $\mathbb{Q}$
$\mathbb{Q}$ is a division ring, also commutative ring.
λ°λΌμ $\mathbb{Q}$λ Fieldλ€!!
λ§μ°¬κ°μ§λ‘ $\mathbb{R}$, $\mathbb{C}$λ Fieldμ!
Quternion
Example. quaternion set
The set of quaternions is a skew field.
Definition. Quaternion $Q$
- λ§μ μ component-wise additionμΌλ‘ μ μ
-
κ³±μ μ quaterion units $\mathbf{i}$, $\mathbf{j}$, $\mathbf{k}$μ λν κ·μΉμ λ°λΌ μν
- κ³±μ μ λν μμ: $\dfrac{w - x\mathbf{i} - y\mathbf{j} - z\mathbf{k}}{w^2 + x^2 + y^2 + z^2}$
- κ³±μ μ λν νλ±μ: $1 = (1, 0, 0 ,0)$
- $q\cdot\bar{q} = w^2 + x^2 + y^2 + z^2$
βQuaternionμ 3μ°¨μ νμ μμ μ¬μ©νλ€!β
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°λΌμ $3$, $4$λ $\mathbb{Z}_{12}$μμ zero-divisorμ.
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 $d:=(m, n) \ne 1$; 곡μ½μκ° 1μ΄ μλ, μ¦ $n$κ³Ό μλ‘μκ° μλ $m$μ μ ννλ€.
THEN,
\[m\left(\frac{n}{d}\right) = \left(\frac{m}{d}\right)n \equiv 0\]μ¦, $m$κ³Ό $\frac{n}{d}$κ° zero-divisorλ€. $\blacksquare$
Corollary.
IF $p$ is a prime, THEN $\mathbb{Z}_p$ has no zero-divisor.
proof.
$\mathbb{Z}_p$μ μμλ λͺ¨λ $p$μ μλ‘μμ΄λ―λ‘, zero-divisorκ° μ‘΄μ¬νμ§ μλλ€.
Theorem.
IF $p$ is a prime, THEN $\mathbb{Z}_p$ is a field.
μ¦, λͺ¨λ non-zero elementκ° multiplicative inverseλ₯Ό κ°μ§λ€λ λ§!!
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 Identityμ μν΄, $ax + pq = 1$μ΄ λλλ‘ νλ $x, y$κ° μ‘΄μ¬νλ€.
μμ μμμ module $p$λ₯Ό μ·¨νλ©΄,
\[\begin{aligned} ax + pq &= 1 \\ (ax + pq) \; (\textrm{mod } p) &\equiv 1 \\ ax \; (\textrm{mod } p) &\equiv 1 \\ \end{aligned}\]μ΄λ, $x$κ° λ°λ‘ $a$μ multiplicative inverseλ€!
$\mathbb{Z}_p$μ λͺ¨λ μμμ λν΄ multiplicative inverseκ° νμ μ‘΄μ¬νλ―λ‘, $\mathbb{Z}_p$λ division ringμ΄λ€.
$\mathbb{Z}_p$λ commutative ringμ΄κΈ°λ νλ―λ‘, $\mathbb{Z}_p$λ fieldμ΄λ€. $\blacksquare$
Integral Domain
Definition. Integral Domain; μ μ
An Integral Domain is a commutative ring with Unitiy, and without zero-divisors.
Example.
Homework.
$\mathbb{Z}_n$ is an integral domain, IFF $n$ is prime.
Theorem. Bezoutβs 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.
2λ² λͺ μ λ§ μ¦λͺ ν΄λ³΄μ.
$(n, m) = d$μΈ $n$, $m$λ₯Ό gcdμΈ $d$λ‘ λλμ΄ λ³΄μ. κ·Έλ¬λ©΄
$\left( \frac{n}{d}, \frac{m}{d} \right) = 1$μ΄ λλ€.
λ°λΌμ 1λ² λͺ μ μ μν΄ Bezoutβs Identityκ° μ‘΄μ¬νλ€.
\[\frac{n}{d} x + \frac{m}{d} y = 1\]μλ³μ $d$λ₯Ό κ³±νλ©΄
\[nx + my = d\]μΈ μμ μ»λλ€. $\blacksquare$
Definition. Characteristics of Ring; νμ νμ
If there exist $n \in \mathbb{N}$ s.t. $na \equiv 0$ ($a \in R$),
then call the smallest $n$ as a βCharacteristic of a Ringβ.
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 $a \in R$,
\[\begin{aligned} n \cdot a &= n \cdot (1 \cdot a) \\ &= (n \cdot 1) \cdot a \\ &= 0 \cdot a \\ &= 0 \end{aligned}\]