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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
May is an exciting month! National MATHCOUNTS is the second week of May in Washington D.C. and our Founder, Richard Rusczyk will be presenting a seminar, Preparing Strong Math Students for College and Careers, on May 11th.

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[*]May 21st, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 2 Math Jam, Problems 5 and 6, Canada/USA Mathcamp staff will discuss solutions to Problems 5 and 6 of the 2025 Mathcamp Qualifying Quiz![/list]
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0 replies
jlacosta
May 1, 2025
0 replies
k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
Inequality
Sadigly   3
N an hour ago by pooh123
Source: Azerbaijan Junior MO 2025 P5
For positive real numbers $x;y;z$ satisfying $0<x,y,z<2$, find the biggest value the following equation could acquire:


$$(2x-yz)(2y-zx)(2z-xy)$$
3 replies
Sadigly
Friday at 7:59 AM
pooh123
an hour ago
Calculus
youochange   2
N an hour ago by youochange
Find the area enclosed by the curves $e^x,e^{-x},x^2+y^2=1$
2 replies
youochange
Yesterday at 2:38 PM
youochange
an hour ago
A strong inequality problem
hn111009   0
an hour ago
Source: Somewhere
Let $a,b,c$ be the positive number satisfied $a^2+b^2+c^2=3.$ Find the minimum of $$P=\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}+\dfrac{3abc}{2(ab+bc+ca)}.$$
0 replies
hn111009
an hour ago
0 replies
help me please,thanks
tnhan.129   0
an hour ago
find f: R+ -> R such that:
f(x)/x + f(y)/y = (1/x + 1/y).f(sqrt(xy))
0 replies
tnhan.129
an hour ago
0 replies
Easy divisibility
a_507_bc   2
N an hour ago by TUAN2k8
Source: ARO Regional stage 2023 9.4~10.4
Let $a, b, c$ be positive integers such that no number divides some other number. If $ab-b+1 \mid abc+1$, prove that $c \geq b$.
2 replies
a_507_bc
Feb 16, 2023
TUAN2k8
an hour ago
Inspired by old results
sqing   0
an hour ago
Source: Own
Let $a,b,c,d$ be real numbers such that $a^2+b^2+c^2 =3$. Prove that$$\frac{9}{5}>(a-b)(b-c)(2a-1)(2c-1)\geq -16$$
0 replies
sqing
an hour ago
0 replies
integer functional equation
ABCDE   149
N an hour ago by ezpotd
Source: 2015 IMO Shortlist A2
Determine all functions $f:\mathbb{Z}\rightarrow\mathbb{Z}$ with the property that \[f(x-f(y))=f(f(x))-f(y)-1\]holds for all $x,y\in\mathbb{Z}$.
149 replies
ABCDE
Jul 7, 2016
ezpotd
an hour ago
A geometry problem involving 2 circles
Ujiandsd   0
an hour ago
Source: L
Point M is the midpoint of side BC of triangle ABC. The length of the radius of the outer circle of triangle ABM, triangle ACM
is 5 and 7 respectively find the distance between the center of their outer circles
0 replies
Ujiandsd
an hour ago
0 replies
Inequality, inequality, inequality...
Assassino9931   10
N 2 hours ago by sqing
Source: Al-Khwarizmi Junior International Olympiad 2025 P6
Let $a,b,c$ be real numbers such that \[ab^2+bc^2+ca^2=6\sqrt{3}+ac^2+cb^2+ba^2.\]Find the smallest possible value of $a^2 + b^2 + c^2$.

Binh Luan and Nhan Xet, Vietnam
10 replies
Assassino9931
Yesterday at 9:38 AM
sqing
2 hours ago
Grid with rooks
a_507_bc   3
N 2 hours ago by TUAN2k8
Source: ARO Regional stage 2022 9.3
Given is a positive integer $n$. There are $2n$ mutually non-attacking rooks placed on a grid $2n \times 2n$. The grid is splitted into two connected parts, symmetric with respect to the center of the grid. What is the largest number of rooks that could lie in the same part?
3 replies
a_507_bc
Feb 16, 2023
TUAN2k8
2 hours ago
IMO Shortlist 2013, Number Theory #3
lyukhson   47
N 2 hours ago by cursed_tangent1434
Source: IMO Shortlist 2013, Number Theory #3
Prove that there exist infinitely many positive integers $n$ such that the largest prime divisor of $n^4 + n^2 + 1$ is equal to the largest prime divisor of $(n+1)^4 + (n+1)^2 +1$.
47 replies
lyukhson
Jul 10, 2014
cursed_tangent1434
2 hours ago
Darboux cubic
srirampanchapakesan   1
N 2 hours ago by srirampanchapakesan
Source: Own
Let P be a point on the Darboux cubic (or the McCay Cubic ) of triangle ABC.

P1P2P3 is the circumcevian or pedal triangle of P wrt ABC.

Prove that P also lie on the Darboux cubic ( or the McCay Cubic) of P1P2P3 .
1 reply
srirampanchapakesan
May 7, 2025
srirampanchapakesan
2 hours ago
IMO Shortlist 2011, Algebra 2
orl   43
N 2 hours ago by ezpotd
Source: IMO Shortlist 2011, Algebra 2
Determine all sequences $(x_1,x_2,\ldots,x_{2011})$ of positive integers, such that for every positive integer $n$ there exists an integer $a$ with \[\sum^{2011}_{j=1} j  x^n_j = a^{n+1} + 1\]

Proposed by Warut Suksompong, Thailand
43 replies
1 viewing
orl
Jul 11, 2012
ezpotd
2 hours ago
Sequence inequality
BR1F1SZ   1
N 2 hours ago by IndoMathXdZ
Source: 2025 Francophone MO Seniors P1
Let $a_1, a_2, a_3, \ldots$ be a sequence of positive integers satisfying the following property: for all positive integers $k < \ell$, for all distinct integers $m_1, m_2, \ldots, m_k$ and for all distinct integers $n_1, n_2, \ldots, n_\ell$,
\[
a_{m_1} + a_{m_2} + \cdots + a_{m_k} \leqslant a_{n_1} + a_{n_2} + \cdots + a_{n_\ell}.
\]Prove that there exist two integers $N$ and $b$ such that $a_n = b$ for all $n \geqslant N$.
1 reply
BR1F1SZ
4 hours ago
IndoMathXdZ
2 hours ago
A property of divisors
rightways   13
N Apr 25, 2025 by Ilikeminecraft
Source: Kazakhstan NMO 2016, P1
Prove that one can arrange all positive divisors of any given positive integer around a circle so that for any two neighboring numbers one is divisible by another.
13 replies
rightways
Mar 17, 2016
Ilikeminecraft
Apr 25, 2025
A property of divisors
G H J
G H BBookmark kLocked kLocked NReply
Source: Kazakhstan NMO 2016, P1
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rightways
868 posts
#1 • 3 Y
Y by Adventure10, Mango247, ItsBesi
Prove that one can arrange all positive divisors of any given positive integer around a circle so that for any two neighboring numbers one is divisible by another.
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AdithyaBhaskar
652 posts
#2 • 2 Y
Y by Adventure10, Mango247
This problem is quite old. For instance, a special case can be found in the Ireland Team Training materials.
So I come to a solution.
This result trivially holds if the number, $n$, is a prime power, so assume otherwise. Now let $n =ap$ where $a,p$ are positive integers and $p$ is prime. We induct, assuming this result to be true for $a.$ Create the circle for $a$ and call it $\mathcal{C}.$ Further, by $k\mathcal{C}$ we denote the circle obtained by multiplying each entry of $\mathcal{C}$ by $k.$ Then, consider the two circles $\mathcal{C}$ and $p\mathcal{C}.$ Let $a,b$ be two successive entries in $\mathcal{C}.$ Then cut $\mathcal{C}$ in between $a,b$ and $p\mathcal{C}$ between $pa,pb.$ Now attach the two figures obtained so that $pa$ is joined to $a$ and $b$ is joined to $pb.$ Clearly this arrangement suffices by induction.
We can choose the base case as any prime power, for which any arrangement holds. By induction, this result holds for all $n \in \mathbb{N}.$
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dolphinday
1324 posts
#3 • 1 Y
Y by sami1618
OTIS stronger version wrote:
Let $n\geq 2$ be a positive integer.
Prove that all divisors of $n$ can be written as a sequence $d_1,\dots,d_k$ such that
for any $1\leq i<k$ one of $\frac{d_i}{d_{i+1}}$ and $\frac{d_{i+1}}{d_i}$ is a prime number.

We can show that there is a bijection between finding a sequence of divisors and finding a path(from vertex to adjacent vertex) on an lattice $n$-dimensional hyperbox(on the coordinate plane) so that all lattice points are covered by the path. Note that vertex Here $n$ is the number of distinct prime divisors of the positive integer in question which we will call $\mathcal{P}$. Note that our hyperbox has dimensions $(e_1 + 1) \times (e_2 + 1) \times \dots \times (e_n + 1)$ where $\mathcal{P} = \prod_i^n p_i^{e_i}$. Note that point $(x_1, x_2, x_3, \dots, x_n)$ corresponds to $\prod_i^n x_i^{p_i}$. WLOG our path starts at the origin on the coordinate plane which corresponds to $1$ in terms of a divisors.
We will use induction to prove this claim. Our base case of $n = 1$ is trivially true. Then we can easily induct upwards since we can connect our path between two $n-1$ hypercube graphs at the corners to form a block of $n-1$ hypercube graphs with length $e_n + 1$, so our induction is done.
This post has been edited 2 times. Last edited by dolphinday, May 29, 2024, 7:53 PM
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ItsBesi
146 posts
#4
Y by
OTIS version
We will prove it by using induction.

$\textbf{Base case:}$ $n$-has only one prime factor $\iff n=p^{\alpha}$ then we can form the sequence as follows:
$1,p,p^2, \dots , p^{\alpha}$.

$\textbf{Inductive hypothesis:}$ Assume it's true for $n$-having $\ell$ prime factors.

$\textbf{Inductive step:}$ Now we will prove that works for $n$ having $\ell+1$ prime factors.
$\iff n=q^{\beta} \cdot t$ where $t$ has $\ell$ prime factors and let $a_1,a_2, \dots, a_k$ be the sequence of $t$.
So now to prove that $n=q^{\beta} \cdot t$ works we form the sequence as follows:
$a_1,a_2, \dots, a_k , q \cdot a_1, q \cdot a_2, \dots, q \cdot a_k \cdot q^2 \cdot a_1, q^2 \cdot a_2, \dots , q^2 \cdot a_k ,\cdots \cdots q^{\beta} \cdot a_1 , q^{\beta} \cdot a_2, \dots, q^{\beta} \cdot a_k.$

Hence we are done $\blacksquare$.
This post has been edited 1 time. Last edited by ItsBesi, Oct 1, 2024, 1:01 PM
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balllightning37
389 posts
#5
Y by
The Otis version:

Let the prime factorization of $n$ be $p^aq^br^c\dots$. Now, consider a coordinate lattice from $(1,1,1,...)$ to $(a,b,c,...)$ so that we have a one-to-one correspondence from factors of $n$ to points on the lattice. Notice that the given condition means that the lattice points corresponding to two adjacent terms of the sequence are adjacent themselves.

Thus, the question is just asking if all such coordinate lattices have a Hamiltonian path. This is pretty easy induction on the dimension of the lattice.

remark
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AshAuktober
1005 posts
#6
Y by
Otis version:
Induction of the number of prime factors of n works
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Maximilian113
575 posts
#7
Y by
We claim that there exists a sequence with either $d_1=1, d_k=n$ or $d_k=1, d_1=n.$ We show this by inducting on $\alpha,$ the number of prime factors that $n$ has.

The base case is trivial. Now, suppose that our proposition holds for $\alpha=\ell$ for some positive integer $\ell.$ Then suppose $n$ has $\ell+1$ prime factors. Let $p$ be a prime factor of $n$ such that $v_p(n)=x.$ Also, let $n=p^xm.$ Then by our inductive hypothesis, there exists a sequence from $1$ to $m.$ Then, we can move to $mp,$ and using our inductive hypothesis again we can get to $p.$ Continuing, we will eventually end at either $p^x$ or $mp^x=n.$ (Visualize this as a path through lattice points in a $\ell$-dimensional space.) Hence, this provides a valid path, so we are done. QED
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blueprimes
353 posts
#8
Y by
OTIS wrote:
Let $n\geq 2$ be a positive integer. Prove that all divisors of $n$ can be written as a sequence $d_1,\dots,d_k$ such that for any $1\leq i<k$ one of $\frac{d_i}{d_{i+1}}$ and $\frac{d_{i+1}}{d_i}$ is a prime number.

For a sequence $S$ let $S^{*}$ be the reverse of $S$ and $m \times S$ for a number $m$ as the sequence $S$ when corresponding elements are multiplied by $m$.

Now suppose $n$ has $\ell$ distinct prime factors, we induct on $\ell$. For the base case of $\ell = 1$, let $n = p^a$ where $p$ is prime. The sequence $1, p, p^2, \dots, p_a$ clearly works. Now assume that the claim holds for an arbitrary $\ell \ge 1$. Then consider the prime factorization $n = p_1^{e_1} p_2^{e_2} \dots p_{\ell + 1}^{e_{\ell + 1}}$. By the inductive hypothesis, there exists some sequence of divisors $T$ of $p_1^{e_1} p_2^{e_2} \dots p_{\ell}^{e_{\ell}}$ that satisfies the conditions of the problem. Then the composite sequence
\[ T, p_{\ell + 1} \times T^{*}, p_{\ell + 1}^2 \times T, p_{\ell + 1}^3 \times T^{*} \dots \]works, so our induction is complete. We are done.
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de-Kirschbaum
199 posts
#9
Y by
We will induct on the number of prime factors of $n$. If $n$ has $m=1$ prime factors then clearly the sequence
$$1,p,p^2,\ldots,p^{e}$$works.

Then let us assume that this works $m=l$. Consider $m=l+1$, so $n=p_1^{e_1}p_2^{e_2}\cdots p_{l+1}^{e_{l+1}}$. We will divide the sequence into $e_{l+1}+1$ blocks of length $(e_1+1)\cdots(e_l+1)$. For the first block we will use the good construction for $m=l$, $$1, \ldots , p_1^{e_1}\cdots p_l^{e_l}$$For the second block we will flip this sequence and multiply each term by $p_{l+1}$,

$$p_1^{e_1}\cdots p_l^{e_l} p_{l+1}, \ldots , p_{l+1}$$
For the third block we will flip this sequence again and multiply each term by $p_{l+1}$,

$$p_{l+1}^2, \ldots, p_1^{e_1}\cdots p_l^{e_l} p_{l+1}^2$$and we can easily verify that we get a satisfactory construction following this method. Thus by induction we are done.
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gladIasked
648 posts
#10
Y by
Proceed with induction on the number of distinct primes that divide $n$. When exactly one prime divides $n$, we can write out the divisors as follows: $1, p, p^2, \dots, p^{k-1}$.

Now, let the prime factorization of $n$ be $p_1^{e_1}p_2^{e_2}\dots p_i^{e_i}$. Let $m=p_1^{e_1}p_2^{e_2}\dots p_{i-1}^{e_{i-1}}$, so $n=mp_i^{e_i}$. We are clearly able to write a sequence starting with $mp_i^{e_i}$ and ending with $dp_i^{e_i}$ where $d\mid m$ by induction (this sequence covers all divisors of $n$ with $\nu_{p_i}(n)=e_i$). Then, we write $dp_i^{e_{i-1}}$ and (by induction) ``build back up" to $mp_i^{e_i-1}$ (this covers all divisors of $n$ with $\nu_{p_i}=e_i-1$). Then, we write $mp_i^{e_i-2}$ and build down to $dp_i^{e_i-2}$, covering all divisors of $n$ with $\nu_{p_i}=e_i-2$. We can continue this ``building up" and ``building down" process until we cover all divisors of $n$, completing our induction. $\blacksquare$
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akliu
1800 posts
#11
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I actually solved this question fairly quickly because it relates to a question from a math camp's application that I did this year. Consider a single prime $p_1$, and list out the sequence as follows: $1$, $p_1$, $p_1^2$, $\dots$, $p_1^{\nu_{p_1}(n)}$. For each value $k$ in the sequence, replace the number with another sequence $k$, $kp_2$, $kp_2^2$, $\dots$, $kp_2^{\nu_{p_2}(n)}$, and alternate this between values of $k$ such that adjacent values are multiplied by the same sequence in either increasing or decreasing order. For example, for $n = 18$, we start out with the sequence $1$, $2$. Then, we include the prime $3$, and we get $1$, $3$, $9$, $18$, $6$, $2$. Do this for all primes, and we have a sequence that goes over all the divisors of $n$ where each step is either dividing or multiplying by a prime number; the sequence is split into groups of varying values of $\nu_p$, and each of those gorups has groups of varying values of $\nu_q$, and so on throughout all desired primes.

An example for $3$ primes for the reader's clarity: $n = 60$ turns into $1$, $2$, $4$. We alternate the sequences of the powers of $3$, and arrive at $1, 3$, $6, 2$, $4, 12$. Finally, adding the sequences of $5$, we get the final sequence: $1, 5, 15, 3, 6, 30, 10, 2, 4, 20, 60, 12$.

A short bonus: It is possible to loop the sequence back to $1$ if and only if $n$ is not a perfect square, or a perfect power of a prime. In particular, the example with $n = p^aq^b$ can be drawn as a grid as such:
\begin{align*}
    1, p, p^2, \dots\\
    q, pq, p^2q, \dots\\
    \dots \\
    q^b, q^bp, q^bp^2,\dots
\end{align*}
Finding a "prime dividing path" on this grid such that the path covers all the numbers on the grid bijects to trying to find a Hamiltonian cycle through a rectangular grid of dimensions $(a+1)(b+1)$.
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clarkculus
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#12 • 2 Y
Y by akliu, centslordm
Induct on the number of distinct prime factors $k$ of $n$. The base case $k=1$ or $n=p_1^{e_1}$ for primes $p_1$ works with the sequence $\overset\rightarrow{X_1}=\{1,p_1,p_1^2,\dots,p_1^{e_1}\}$.

Let $\overset\leftarrow{X_k}$ be the sequence $\overset\rightarrow{X_k}$ with all elements in the opposite order. If $A\oplus B$ concatenates sequences $A$ and $B$, it then follows that
\[\overset\rightarrow{X_{k+1}}=\overset\rightarrow{X_k}\oplus(p_{k+1}\cdot\overset\leftarrow{X_k})\oplus(p_{k+1}^2\cdot\overset\rightarrow{X_k})\oplus(p_{k+1}^3\cdot\overset\leftarrow{X_k})\dots\]is a sequence satisfying the problem statement, which completes the inductive step.
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akliu
1800 posts
#13
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Nice formalization! I didn't think about writing it that way.
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Ilikeminecraft
627 posts
#14
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Notice that all powers of primes work, and 1 work. Also notice that any number that is of the form $pq$ work, where $p, q$ are primes, because of the sequence $1, p, pq, q.$

Let $k$ be the number of primes dividing $n.$ Assume that $n\geq 2.$ I claim that there exists a sequence satisfying the problem such that the first element is 1, and the last element is the product of at most $k - 1$ primes. We prove this with induction on number of divisors. The idea is:

Now, assume that for $n < k,$ our statement is true. We will prove that $n = k$ works. If $k$ is prime, we have $1, k$ works. If $k$ isn't prime, let a prime dividing $k$ be $p.$ Let the sequence of $\frac{k}{p}$ be $a = \{1, s_2, s_3, \ldots, s_a\}.$ Let the sequence of $\frac{k}{p^{\nu_p(k)}}$ be $b = \{1, s_2', \ldots, s_{\frac{k}{p^{\nu_p(k)}}}'\}.$ We know these both exist since our hypothesis. I claim that $\{1, s_2p, s_3p, \ldots, s_a p, \frac{k}{p^{\nu_p(k)}s_1'}, \ldots\}$ works.
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