2
votes
1answer
26 views

Group divisibility question

I have the following question which I can't make sense of, here is the entire question: If $G$ is a group, $b\in G, o(b)=k$ and $b^n = e$, show that $k|n$ What is $o(b)$? Please help.
1
vote
2answers
32 views

GCD question involving coprimes

How can I prove that if $\gcd(a, b) = 1$, then $\gcd(ab, c) = \gcd(a, c) \times \gcd(b, c)$? By eea there exists $ax+by=1$ from $\gcd(a,b)=1$ so a and be are co-primes there also exists $dk=a$ and ...
0
votes
1answer
17 views

Proving $k$ is a common divisor of $a$ and $b$ iff $k\,|\gcd(a,b)$ — converse

I'm currently trying to solve the converse of this statement is true after proving the normal version is true. If $k$ is a common divisor of $a$ and $b$, then $k \,| \gcd(a, b)$ So far I know the ...
2
votes
1answer
17 views

Questions relating to gcd

Assume a, b and c are positive integers. 1) Suppose that a | b. Show that gcd(a, c) ≤ gcd(b, c). 2) Suppose that a ≤ b. Is it necessarily true that gcd(a, c) ≤ gcd(b, c)? I'm having trouble with ...
0
votes
1answer
32 views

Finding greatest common divisor between two polynomials.

I have the following past exam question: Calculate $\operatorname{gcd}(x^3 + 2x^2 + 2,2x^2 + 1)$ in $\mathbb{F}_3$ Now I haven't encountered this sort of gcd before(usually I am trying to solve ...
1
vote
3answers
47 views

If there is an $a\in\mathbb{Z}$ with $a^{n-1}\equiv 1\mod n$ but $a^{\frac{n-1}p}\not\equiv 1$ for all primes $p\mid n-1$, then $n$ is a prime

Let $n\in\mathbb{N}$ with $n\ge 3$ and $a\in\mathbb{Z}$ such that $$a^{n-1}\equiv1\text{ mod } n\;\;\;\wedge\;\;\;a^{\frac{n-1}{p}}\not\equiv1\text{ mod }n\;\;\;\forall p\in\mathbb{P}:p\mid n-1$$ ...
1
vote
0answers
50 views

Showing DO NOT exist GCD of $6$ and $2+2 \sqrt{-5}$ in $\Bbb Z[\sqrt{-5}]$.

Showing DO NOT exist gcd of $6$ and $2+2 \sqrt{-5}$ in $\Bbb Z[\sqrt{-5}]$. I tried it. Suppose $d$ is GCD of $6$ and $2+2 \sqrt(-5)$. then there exist $x,y \in \Bbb ...
0
votes
0answers
33 views

The elegant expression in terms of gcd and lcm - algebra - (2)

Definition: suppose a quantity $P$ is identified by $$ \frac{P}{k}\simeq \frac{P}{k}+1 $$ what we mean is that $$ P= 0\pmod{k}. $$ That means that when $P \to P+k$, then $$ \frac{P}{k}\to ...
2
votes
1answer
39 views

The elegant expression in terms of gcd and lcm - algebra

Given three positive integer numbers $k_1$, $k_2$, $k_3$, we may denote their greatest common divisor(gcd) by $\gcd(k_i,k_j)\equiv k_{ij}$ for gcd of a two pair of number $k_i,k_j$. ...
1
vote
2answers
71 views

Why define $\gcd(r,s)$ as a positive generator $d$ of the cyclic group $H=\{nr+ms|n,m\in\mathbb{Z}\}$?

This is in regards to definition 6.8, p. 62 from Fraleigh's "A first course in abstract algebra". 6.8 Definition Let $r$ and $s$ be two positive integers. The positive generator $d$ of the ...
2
votes
1answer
41 views

Show that $(x-a,x-b)=1$

Knowing that $K$ is a field, $a,b \in K$ different from each other,show that $x-a,x-b$ co-primes. We suppose that $\exists f(x) \in K(x)$ such that: $f(x)|x-a$ and $f(x)|x-b$ Then $\deg f(x) \leq ...
1
vote
1answer
37 views

GCD for multivariable polynomial ring

I'm reading Lectures on Modules and Rings by T. Y. Lam. It's on page 32 of the book, example 2.19A. It reads: (2.19A) Example. Let $k$ be a field. Then in the commutative polynomial ring $R = ...
2
votes
1answer
69 views

Relation between divisibility of polynomials in different rings, $h | f$ in $\mathbb{Z}[x], \mathbb{Z}/p^k\mathbb{Z}[x]$ and $\mathbb{F}_p[x]$

Let $p$ be a prime, $k$ a positive integer. Let $f,h \in \mathbb{Z}[x]$ be polynomials such that $h | f \mod p^k$ in $ (\mathbb{Z}/p^k\mathbb{Z})[x]$ $h \mod p$ is irreducible in $\mathbb{F}_p$ ...
7
votes
8answers
248 views

Proof of Divisibility of $n(n^2+20)$ by 48.

This is a question from Bangladesh National Math Olympiad 2013 - Junior Category that still haunts me a lot. I want to find an answer to this question. Please prove this. If $n$ is an even ...
0
votes
2answers
48 views

How can I prove this relation between gcd(a,b)?

I am stuck on starting this proof that involves gcd. Define $g_n=2^{2^n}+1$ and that $g_0g_1g_2...g_{n-1}=g_n-2$. Suppose that $a$ and $b$ are unequal positive integers. Prove that $gcd(g_a,g_b)=1$. ...
2
votes
1answer
44 views

gcd's in non-UFD rings

In a UFD ring we have that for coprime $a,b \in R$, i.e. $(a,b)=1$: $$ a|cb \Rightarrow a|c $$ Does this property hold for non-UFD rings? I think not but do not recall a standard ...
2
votes
1answer
134 views

Roots of $x^n - 1$ in an algebraically closed field of prime characteristic

Let $F$ be an algebraically closed field of characteristic $p$ , and let $n$ be a positive integer. Consider $ g := x^n - 1 \in F[x]$ Is it true that $ g$ has distinct roots in $F$ if and only if ...
0
votes
2answers
233 views

Writing a GCD of two numbers as a linear combination

I am working on GCD's in my Algebraic Structures class. I was told to find the GCD of 34 and 126. I did so using the Euclidean Algorithm and determined that it was two. I was then asked to write it ...
0
votes
1answer
42 views

Subring of Gaussian integers has no greatest common divisor property [duplicate]

Problem is: Produce elements a and b in the domain $R := \{x+2y\sqrt{-1} \mid x, y \in \mathbb{Z}\}$ having no gcd. How can produce this? Actually I use norm function, and brute force, but what ...
3
votes
1answer
103 views

Find two elements that don't have a gcd in a subring of Gaussian integers

Find two elements in the domain $R := \{ x + 2y \sqrt {-1} \mid x,y \in \mathbb{Z} \}$ that do not have a gcd. I have no idea how to start. But I know if we consider $R^\prime = \{ x + y \sqrt ...
10
votes
4answers
293 views

Prove: If $\gcd(a,b,c)=1$ then there exists $z$ such that $\gcd(az+b,c) = 1$

I can't crack this one. Prove: If $\gcd(a,b,c)=1$ then there exists $z$ such that $\gcd(az+b,c) = 1$ (the only constraint is that $a,b,c,z \in \mathbb{Z}$).
0
votes
3answers
38 views

When $\gcd{(ug, vg)}=g$, we have $\gcd{(ug, vg+1)}=1$?

I have a question about greatest common divisor. For an even integer $g$, assume that $\gcd{(ug, vg)}=g$, i.e., $\gcd{(u, v)}=1$. Then we have $\gcd{(ug, vg+1)}=1$? For example, let $g=2$, $u=8$, ...
0
votes
1answer
124 views

For natural numbers $a$ and $b$, show that $a \Bbb Z + b \Bbb Z = \gcd(a, b)\Bbb Z $

For natural numbers $a$ and $b$, show that $a \Bbb Z + b \Bbb Z = \gcd(a, b)\Bbb Z $ I just basically said that the gcd of a and b { written as C } obviously divides aZ and bZ, therefore it can be ...
1
vote
1answer
90 views

If $\gcd(f(x), g(x)) = 1$, then $\gcd(h(x)f(x), g(x)) = \gcd(h(x), g(x))$

This is not homework, but I would just like a hint. The question asks Let $f(x), g(x), h(x) \in F[x]$ (where $F$ is a field), and $\gcd(f(x), g(x)) = 1$. Show that $\gcd(f(x)h(x), g(x)) = ...
1
vote
2answers
209 views

Rabin's test for polynomial irreducibility over $\mathbb{F}_2$

I know that $f(x) = x^{169}+x^2+1$ is a reducible polynomial over $\mathbb{F}_2$. I want to show this using Rabin's irreduciblity test. First, I then have to check if $f$ is a divisor of ...
2
votes
2answers
73 views

Prooving by absurd that $d \nmid 4,5,10, 20$

What I'm trying to solve is the following: Given that $(a:b) = 2$, proove that $(a^2 + 2b^2+10:20) = 2$. So, basically, I think that what I need to do is to show that if $d = a^2 + 2b^2 + 10$, then ...
3
votes
0answers
76 views

Find the fundamental group and the Alexander polynomial

I would like to find the Alexander polynomial of the link $L$, described below. Let $K(q,r)$ be the $(q,r)$-torus knot embedded on a torus $V$. Inside the torus $V$, consider a smaller solid torus ...
1
vote
2answers
150 views

Division algorithm: What is the meaning of the propositions?

I face trouble with understanding certain propositions in the division algorithm. For example, Let $a,b \in \Bbb Z, b>0$. Let $S = \{a - qb: q \in \Bbb Z, a - qb \geq 0\}$. To ...
3
votes
1answer
95 views

What can we say about the order of a group given the order of two elements?

If I know that a group of finite order has two elements $a$ and $b$ s.t. their orders are $6$ and $10$, respectively. What statements can be made regarding the order of the group? I know by ...
5
votes
1answer
50 views

Kernel of the evaluation map on a power series ring

Let $R$ be a commutative ring with unity and $r \in R$ a nilpotent element. Is it true that if $f \in R[[\epsilon]]$ satisfies $f(r) = 0$, then $(\epsilon - r) | f$ in $R[[\epsilon]]$? I tried solving ...
3
votes
2answers
2k views

Proof of $\gcd(a,b)=ax+by$

Here is my proof of $\gcd(a,b)=ax+by$ for $a, b, x, y \in \mathbb{Z}$. Am I doing something wrong? Are there easier proofs? $a,b \in \mathbb{Z}, g=\gcd(a,b)$ and suppose $g \neq ax + by$. Let $c$ be ...
3
votes
3answers
401 views

How to prove that $z\cdot\text{gcd}(a,b)=\text{gcd}(za,zb)$

I need to prove that $z \cdot \text{gcd}(a,b)=\text{gcd}(za,zb)$. I tried a lot, for example, looking at set of common divisors of the two sides, but I can't conclude anything from that. Can you ...
4
votes
2answers
537 views

Showing that gcd does not exist for $3(1+\sqrt{-5})$ and $3(1-\sqrt{-5})$ in $\mathbb Z[\sqrt{-5}]$.

An exercise asks me to show that $3(1+\sqrt{-5})$ and $3(1-\sqrt{-5})$ have no greatest common divisor in $\mathbb Z[\sqrt{-5}]$. I think I have to find two maximal common divisors which are not ...
1
vote
6answers
635 views

Proof that $\mathbb{Z}$ has no zero divisors

Everyone knows the rules of zero divisors like $$\forall \alpha,\beta\in\mathbb{R}\;:\;\alpha\cdot\beta = 0\Rightarrow\alpha=0\vee \beta=0.$$ But how can I prove it for $\mathbb{Z}$? My first try was ...
5
votes
2answers
369 views

Is Gauss's lemma valid for polynomials with coefficients in a GCD domain?

Wikipedia's proof of Gauss's lemma requires this theorem: If $(C \mid S\cdot T) \land \lnot \operatorname{invertible}(C)$, $C$ has a non-invertible divisor in common with at least one of $S$ and ...