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数学代写|抽象代数代写abstract algebra代考|Every subgroup of a cyclic group is cyclic
Proof. Let $G$ be a cyclic group with generator $a$, so that $G=\langle a\rangle$, and let $H$ be any subgroup of $G$. If $H$ is the trivial subgroup consisting only of $e$, then we are done since $H=\langle e\rangle$. If $H$ is nontrivial, then it contains some element different from the identity, which can then be written in the form $a^{n}$ for some integer $n \neq 0$. Since $a^{-n}=\left(a^{n}\right)^{-1}$ must also belong to $H$, we can assume that $H$ contains some power $a^{k}$ with $k>0$.
Let $m$ be the smallest positive integer such that $a^{m} \in H$. We claim that $H=$ $\left\langle a^{m}\right\rangle$. Since $a^{m} \in H$, we have $\left\langle a^{m}\right\rangle \subseteq H$, and so the main point is to show that each element of $H$ can be expressed as some power of $a^{m}$. Let $x \in H$. Then since $G=\langle a\rangle$, we have $x=a^{k}$ for some $k \in \mathbf{Z}$. By the division algorithm, $k=m q+r$ for $q, r \in \mathbf{Z}$ with $0 \leq r<m$. Then $x=a^{k}=a^{m q+r}=\left(a^{m}\right)^{q} a^{r}$. This shows that $a^{r}=\left(a^{m}\right)^{-q} x$ belongs to $H$ (since $a^{m}$ and $x$ belong to $H$ ). This contradicts the definition of $a^{m}$ as the smallest positive power of $a$ in $H$ unless $r=0$. Therefore $k=m q$ and $x=\left(a^{m}\right)^{q} \in\left\langle a^{m}\right\rangle$. We conclude that $H=\left\langle a^{m}\right\rangle$ and so $H$ is cyclic.
In our current terminology, Theorem 1.1.4 showed that every subgroup of $\mathbf{Z}$ is cyclic. This result can actually be used to give a very short proof of Theorem 3.5.1. Let $G=\langle a\rangle$ and let $H$ be a subgroup of $G$. Let $I=\left{n \in \mathbf{Z} \mid a^{n} \in H\right}$. It follows from the rules for exponents that $I$ is closed under addition and subtraction, so Theorem 1.1.4 implies that $I=m \mathbf{Z}$ for some integer $m$. We conclude that $H=\left\langle a^{m}\right\rangle$, and so $H$ is cyclic.
The next theorem shows that any cyclic group is isomorphic either to $\mathbf{Z}$ or to $\mathbf{Z}_{n}$. Thus any two infinite cyclic groups are isomorphic to each other. Furthermore, two finite cyclic groups are isomorphic if and only if they have the same order.
数学代写|抽象代数代写abstract algebra代考|Let G be a cyclic group
(a) If $G$ is infinite, then $G \cong \mathbf{Z}$.
(b) If $|G|=n$, then $G \cong \mathbf{Z}{n}$. Proof. (a) Let $G=\langle a\rangle$ be an infinite cyclic group. Define $\phi: \mathbf{Z} \rightarrow G$ by $\phi(m)=$ $a^{m}$, for all $m \in \mathbf{Z}$. The mapping $\phi$ is onto since $G=\langle a\rangle$, and Proposition 3.2.8 (a) shows that $\phi(m) \neq \phi(k)$ for $m \neq k$, so $\phi$ is also one-to-one. Finally, $\phi$ preserves the respective operations since $$ \phi(m+k)=a^{m+k}=a^{m} a^{k}=\phi(m) \phi(k) $$ This shows that $\phi$ is an isomorphism. (b) Let $G=\langle a\rangle$ be a finite cyclic group with $n$ elements. Define $\phi: \mathbf{Z}{n} \rightarrow G$ by $\phi([m])=a^{m}$, for all $[m] \in \mathbf{Z}_{n}$. In order to show that $\phi$ is a function, we must check that the formula we have given is well-defined. That is, we must show that if $k \equiv m(\bmod n)$, then $a^{k}=a^{m}$. This follows from Proposition 3.2.8 (c). Furthermore, if $\phi([k])=\phi([m])$, then the same proposition shows that $[k]=[m]$, and so $\phi$ is one-to-one. It is clear that $\phi$ is onto, since $G=\langle a\rangle$. Finally, $\phi$ preserves the respective operations since
$$
\phi([m]+[k])=a^{m+k}=a^{m} a^{k}=\phi([m]) \phi([k])
$$
This shows that $\phi$ is an isomorphism.
抽象代数代写
数学代写|抽象代数代写ABSTRACT ALGEBRA代考|EVERY SUBGROUP OF A CYCLIC GROUP IS CYCLIC
证明。让G是一个有发生器的循环群一种, 以便G=⟨一种⟩, 然后让H是任何子群G. 如果H是仅由以下组成的平凡子群和, 那么我们就完成了H=⟨和⟩. 如果H是非平凡的,则它包含一些与身份不同的元素,然后可以写成一种n对于某个整数n≠0. 自从一种−n=(一种n)−1也必须属于H, 我们可以假设H包含一些力量一种到和到>0.
让米是最小的正整数,使得一种米∈H. 我们声称H= ⟨一种米⟩. 自从一种米∈H, 我们有⟨一种米⟩⊆H,所以重点是表明H可以表示为一种米. 让X∈H. 那么自从G=⟨一种⟩, 我们有X=一种到对于一些到∈从. 通过除法算法,到=米q+r为了q,r∈从和0≤r<米. 然后X=一种到=一种米q+r=(一种米)q一种r. 这表明一种r=(一种米)−qX属于H s一世nC和$一种米$一种nd$X$b和一世这nG吨这$H$. 这与定义相矛盾一种米作为最小的正功率一种在H除非r=0. 所以到=米q和X=(一种米)q∈⟨一种米⟩. 我们得出结论H=⟨一种米⟩所以H是循环的。
在我们目前的术语中,定理 1.1.4 表明,每个子群从是循环的。这个结果实际上可以用来对定理 3.5.1 给出一个非常简短的证明。让G=⟨一种⟩然后让H成为一个子群G. 让我=\left{n \in \mathbf{Z} \mid a^{n} \in H\right}我=\left{n \in \mathbf{Z} \mid a^{n} \in H\right}. 从指数规则可以看出一世在加减法下是封闭的,所以定理 1.1.4 意味着一世=米从对于某个整数米. 我们得出结论H=⟨一种米⟩, 所以H是循环的。
下一个定理表明任何循环群都是同构的从或者从n. 因此,任何两个无限循环群彼此同构。此外,两个有限循环群是同构的当且仅当它们具有相同的阶。
数学代写|抽象代数代写ABSTRACT ALGEBRA代考|LET G BE A CYCLIC GROUP
一种如果G是无限的,那么G≅从.
(a) If $G$ is infinite, then $G \cong \mathbf{Z}$.
(b) If $|G|=n$, then $G \cong \mathbf{Z}{n}$. Proof. (a) Let $G=\langle a\rangle$ be an infinite cyclic group. Define $\phi: \mathbf{Z} \rightarrow G$ by $\phi(m)=$ $a^{m}$, for all $m \in \mathbf{Z}$. The mapping $\phi$ is onto since $G=\langle a\rangle$, and Proposition 3.2.8 (a) shows that $\phi(m) \neq \phi(k)$ for $m \neq k$, so $\phi$ is also one-to-one. Finally, $\phi$ preserves the respective operations since $$ \phi(m+k)=a^{m+k}=a^{m} a^{k}=\phi(m) \phi(k) $$ This shows that $\phi$ is an isomorphism. (b) Let $G=\langle a\rangle$ be a finite cyclic group with $n$ elements. Define $\phi: \mathbf{Z}{n} \rightarrow G$ by $\phi([m])=a^{m}$, for all $[m] \in \mathbf{Z}_{n}$. In order to show that $\phi$ is a function, we must check that the formula we have given is well-defined. That is, we must show that if $k \equiv m(\bmod n)$, then $a^{k}=a^{m}$. This follows from Proposition 3.2.8 (c). Furthermore, if $\phi([k])=\phi([m])$, then the same proposition shows that $[k]=[m]$, and so $\phi$ is one-to-one. It is clear that $\phi$ is onto, since $G=\langle a\rangle$. Finally, $\phi$ preserves the respective operations since
$$
\phi([m]+[k])=a^{m+k}=a^{m} a^{k}=\phi([m]) \phi([k])
$$是同构。
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