This tag is for questions about divisibility, that is, determining when one thing is a multiple of another thing.

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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 ...
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18 views

$B$-powersmooth number divides $\mathrm{lcm}(1,2,3,\ldots B)$

Let $M$ be $B$-powersmooth (ie. all prime powers in $M$'s factorization are $\le B$). I want to prove that $M \mid \mathrm{lcm}(1,2,3,\ldots, B)$. I thought it would be easy to prove this using ...
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1answer
28 views

Remainder of trick-number divided by 9

How can I calculate the remainder of something like $199\cdot 741934^{1234}$ by 9?
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1answer
40 views

$nc_i\mid\prod_{i=1}^3(nc_i+1)-1$ iff $\exists c\in 6\mathbb{N}:c_i=ic$

Let $c_1<c_2<c_3$ be natural numbers and $$C_n=\prod_{i=1}^3(nc_i+1)-1\;\;\;\;\;(n\in\mathbb{N})$$ I want to show that it holds $$\forall n\forall i : nc_i\mid ...
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2answers
45 views

Question about kth root of a reduced ring element.

Let $n > 1$ be a positive integer. Let $k > 1$ be a positive integer. Define the reduced polynomial rings $f_n = \Bbb R[X_n]/(1+(X_n)^{n})$ How do we know if $(X_n)^{1/k}$ is an element of ...
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2answers
74 views

Prove that if $n^2 - 1$ is divisible by a prime number $p$ such that $n - 1$ is not divisible by $p$, then $n + 1$ is also divisible by $p$.

If this proposition is false, please give at least $3$ counter-examples, and try to modify the proposition so that it becomes true. If the proposition is true, please try to prove this even more ...
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40 views

divisibility question involving primes

I have a question concerning the following divisibility problem. For any prime $p$ we define set: $\mathtt{V_{p}}:=\Biggl\{F\in\Phi\Biggl|\begin{cases}p^2\nmid ...
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1answer
14 views

Show that if $a=bq+r $ and $d|a$ and $d |b $, then $d|a-bq $

Show that if $a=bq+r $ and $d|a$ and $d |b $, then $d|a-bq $ That is show that if $d $ divides $a $ and $a=bq+r $ then $d $ divides $a-bq $. Here $d |a $ means "d divides a", that is $ a=dk $ where ...
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3answers
156 views

Find positive integers $(x,n)$ such that $x^{n} + 2^{n} + 1$ is a divisor of $x^{n+1} +2^{n+1} +1$

Find all positive integers $(x,n)$ such that $x^{n} + 2^{n} + 1$ is a divisor of $x^{n+1} +2^{n+1} +1$ I encountered this question in one of my monthly assignments. Unfortunately, I don't know ...
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1answer
117 views

$\sum\limits_{k=0}^{n}\binom{2n+1}{2k+1}2^{3k}$ isn't divisible by 5

I have no idea Prove that for any $n$ natural number this sum $$\sum\limits_{k=0}^{n}\binom{2n+1}{2k+1}2^{3k}$$ isn't divisible by $5$. $\begin{array}{l} \left( {1 + x} \right)^{2n + 1} - ...
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54 views

Finding an asymptotic formula for $f(m,n)=\sum_{\substack{d\mid m \\ d\leq n}}1$?

$$f(m,n)=\sum_{\substack{d\mid m \\ d\leq n}}1$$ Here $n<m$ and $m$, $n$ are positive integers.
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2answers
113 views

Greatest common divisor with one parameter and condition

I have this homework question: Find $d = \gcd(10x+6, 3x+1) $ where $d > 5$ and $x$ is natural How can I solve it?
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1answer
71 views

Why does the number of divisors of a superior highly composite number is always a highly composite number up to 720720 ? (the only exception is 120)

I've calculated the number of divisors of every superior highly composite number up to $10^{27}$: http://oi59.tinypic.com/ndaijo.jpg The number of divisors of a superior highly composite number is ...
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1answer
126 views

Why isn't $1$ a superior highly composite number?

A superior highly composite number is a positive integer $n$ for which there is an $\epsilon>0$ such that $\dfrac{d(n)}{n^\epsilon} \geq \dfrac{d(k)}{k^\epsilon}$ for all $k>1$, where the ...
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1answer
75 views

Have you seen this theorem before? (GCD divides, neccessary & sufficient condition)

Conjecture. Let $a,b, c\in \Bbb{Z}, b \neq 0$, The following conditions are equivalent: (1) $d = \gcd(a,b)$ divides c. (2) There's a polynomial in $f \in \Bbb{Z}[X,Y]$ with $c$ constant term, such ...
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3answers
111 views

Prove that if $n+1$ is divisible by $m$ then $n^2-1$ is also divisible by $m$. [closed]

Prove that if $n+1$ is divisible by $m$ then $n^2-1$ is also divisible by $m$. And prove that if $n^2-1$ is divisible by $m$ then $n+1$ is also divisible by $m$.
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3answers
95 views

Is it true that $5^k \mid f(5^k)$?

I guess if it is true that $5^k \mid f(5^k)$, where $f(n)$ denotes the $n$-th Fibonacci's number. I have tried to prove it by induction on $k$, but nothing. Have you got any ideas?
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1answer
28 views

Complex matrix division when only the amplitude of the vector to be divided is known.

Let A be a known complex matrix, B a complex vector, and C the complex vector to be solved. Imagine that we know that AC = B . Let assume that the number of lines of A and B are as many as needed. (In ...
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4answers
57 views

Help with groups

let $G$ be a finite group with $e$ Identity element and let $a$ and $b$ belong to $g$ prove that if: $\gcd(o(a),o(b)) =1$ then $\langle a \rangle \cap \langle b \rangle = \{ e \}$. if someone can ...
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78 views

If $3$ divides $q^3$, is it true that $3$ divides $q$?

I think this is true because of prime factorisations, i.e. If $3$ a factor of the prime factorisation of $q^3$, then $3$ is a factor of the prime factorisation of $q$. Therefore If $3$ divides ...
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1answer
44 views

Prove $\gcd(k, l) = d \Rightarrow \gcd(2^k - 1, 2^l - 1) = 2^d - 1$ [duplicate]

This is a problem for a graduate level discrete math class that I'm hoping to take next year (as a senior undergrad). The problem is as stated in the title: Given that $\gcd(k, l) = d$, prove that ...
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31 views

Let $f(x)\in K[x]$ ($K$ field). Prove that if $(f(x),f´(x))=1$ (greatest common divisor is 1) then $f(x)$ does not have multiple roots in $K$

Please I would like you to tell me if my proof is correct Let $f(x)\in K[x]$ ($K$ field). Prove that if $(f(x),f´(x))=1$ (greates common divisor is 1) then $f(x)$ does not have multiple roots in $K$ ...
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1answer
42 views

Is my proof right for this divisibility proof?

Prove For all integers $x$ if for all natural numbers $y$, $x$ does not divide $y$, then $x = 0$. I start by saying that $x\neq 0$ then $x\mid y$ there is exists an integer $d$ such that $xd=y$ if ...
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1answer
61 views

How do I work out the aspect ratio from the resolution by hand?

For $1024 \times 768$ I can see that $768/1024 = 0.75$, i.e. $\frac34$, so $4:3$ makes sense. How do I do it for other resolutions like $1920 \times 1080$ though?
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66 views

Proving a Pellian connection in the divisibility condition $(a^2+b^2+1) \mid 2(2ab+1)$

I'm trying to prove that all integer solutions $a > b \ge 0$ to the divisibility condition in the title, namely $$(a^2+b^2+1) \mid 2(2ab+1),$$ are given by ...
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14 views

Efficient way needed

Given N, M find the : GCD(1, 1) * GCD(1, 2) * … * GCD(1, M) * GCD(2, 1) * GCD(2, 2) * … * GCD(2, M) * … * GCD(N, 1) * GCD(N, 2) * … * GCD(N, M) modulo 10^9+7 Constraints: 1 <= N, M <= 2 * ...
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1answer
38 views

How to prove $D(n)<2n(\log\log n)$?

How to prove $D(n)<2n(\log\log{n})$ for all sufficiently large $n$ where $D(n)$ is the Divisor summatory function.
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1answer
68 views

Prove that a sequence of $11$ numbers always contains six numbers summing up to a multiple of $6$.

Prove that a sequence of $11$ numbers always contains six numbers summing up to a multiple of $6$. This is a problem from a selection to IMO 2014. I like thinking about this problem, it is ...
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1answer
60 views

product of greatest common divisor

Given two numbers $m$ and $n$ how can we calculate the gcd product of any two numbers i.e, $\operatorname{gcd p}(n,m)=\gcd(n,1)\gcd(n,2)\cdots\gcd(n,m)$ where gcd is the greatest common divisor? Can ...
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55 views

Prove the following : [duplicate]

Prove the following : $$ {{n}\choose{7}}-\left \lfloor{\frac{n}{7}}\right \rfloor $$ is divisible by 7.
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1answer
85 views

How many regulars do the primorials 223092870 and 6469693230 have?

Regulars = Divisors + Semidivisors http://global.britannica.com/EBchecked/topic/496213/regular-number So for example: 6 has 5 regulars: 1, 2, 3, 4, 6. 8 has 4 regulars: 1, 2, 4, 8. 9 has 3 ...
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67 views

Among the superior highly composite numbers, which are the most divisor dense numbers?

I’m searching for the most divisor dense natural numbers. Firstly we have the highly composite numbers: 1, 2, 4, 6, 12, 24, 36, 48, 60, 120, 180, 240, 360, 720, 840, 1260, 1680, 2520, 5040, … But ...
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71 views

Is x/x equal to 1

My question is whether $x/x$ is always equal to 1. I am mostly intersted in real numbers and particularly wonder whether $x/x$ is defined at $x=0$. On one hand the division should simplify to 1, on ...
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116 views

Asymptotic divisor function / primorials

Let $p_n\#\equiv\prod_{k=1}^{n}p_k$ (primorial), and $\sigma(n)=\sum_{d|n}^{}d$ (divisor function). Could someone please tell me what the general asymptotic of $\dfrac{\sigma(p_n\#)}{p_n\#}$ is? It ...
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1answer
27 views

For how many values of $a,b,c\in(1,2\ldots,p-1)$ does $p$ | $({a^2}-bc)$ where $p$ is an odd prime number

In a mock test for an entrance exam I am preparing for came the following question: Let $p$ be an odd prime number and $T_p$ be the following set of matrices $$ T_p= \left( ...
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26 views

Divisibility Problem: How can I solve this?

Suppose that $a,b,q,r$ are any integers such that $b > 0$ and $a = bq + r$, with $0\le r<b$, and suppose $b|a$. Must it be the case that $r = 0$? Justify your answer. Can anyone please let me ...
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1answer
43 views

Verify my proof on elementary number theory

I've tried to prove this theorem, which is very simple, but is a kind of practice for me. Let $a,b$ be two positive integers. Therefore, if $a+b$ is a composite number, $frac(\frac{a}{l}) + ...
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54 views

Vieta jumping with non-monic polynomials

I have recently discovered Vieta jumping as a problem-solving technique. In order to teach myself about it, I have located most (all of?) the standard references, both here on MSE and "out there" (via ...
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36 views

Moving up the Y axis the lengh of the hypotenuse of a right triangle

If i have a right triangle ABC with B being the right triangle and length AB = 50 and length BC = 50. Based on the Cartesian coordinate system if i wanted to move up the Y axis the length of the ...
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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 ...
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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 $k_i,k_j$. ...
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26 views

$(x+b)^3\mid P(x)+a$ and $(x-a)^3\mid P(x)-a$

$a,b\in\mathbb{C}$, $b!=0$ I need to find all the polynomials $P$ of degree $5$ verifying: $ \begin{cases}(x+b)^3\mid P(x)+a\\ (x-b)^3\mid P(x)-a\end{cases} $ PS : there was en error, i fixed it ...
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86 views

How does $n!^2$ divide $(2n)!$? [duplicate]

How can I show that $(n!)^2$ divides $(2n)!$, where $n$ is a natural number? So far I've noticed that we can rewrite $\dfrac{(2n)!}{(n)!^2}$ as a combination and we know that combinations are always ...
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1answer
44 views

Properties of Integers

A theorem presented in my discrete math book. Let $d$ be the smallest positive integer of the form $ax + by$. Then $d = \gcd(a,b)$, where gcd means greatest common divisor. I don't understand ...
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59 views

How to show $(n-1)^3n^3(n+1)^3$ is divisible by 7 and 9?

Yeah it looks like a basic, really elementary question, but i'm having hard time with it. First i tried to show that it's divisible by 9 $$(n-1)^3n^3(n+1)^3 = ((n+1)(n-1))^3n^3 = (n^2-1)^3n^3 = ...
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1answer
31 views

Number Theory Divisibility Question

(From Math Challenge II Number Theory packet) Given that $a,b,n$ are positive integers. Assume that for any positive integer $k\neq b, (k-b)\mid(k^n-a)$, the which of the following must be true? ...
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17 views

WordProblem on factors and remainder theorem

Mr.Chaalu while travelling by Ferry queen has travelled the distance one Kilometer more, than the fare he paid per km. Initially he had total amount of Rs.350/- in his wallet. Now he is only left with ...
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1answer
57 views

Prove that $n$ is divisible by $6$

Problem: Let $x^2+mx+n$ and $x^2+mx-n$ give integer roots where $(m,n)$ are integers. Show that $n$ is divisible by $6$ My attempt: Since the roots are integers then the discriminants of both the ...
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2answers
63 views

How to prove that at least one of $a,b,c,d$ is not divisible by $ad-bc$ if $ad-bc>1$?

we have $ad-bc >1$ is it true that at least one of $a,b,c,d$ is not divisible by $ad-bc$ ? Thanks in advance. Example: $a=2$ , $b = 1$, $c = 2$, $d = 2$, $ad-bc = 2$ so $b$ is not divisible by ...
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51 views

If a natural number $x$ is divisible by $3$

Is the sentence If a natural number $x$ is divisible by $3$ then, if it is not divisible by $3$ then it is divisible by $5$ true or false?