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k a March Highlights and 2025 AoPS Online Class Information
jlacosta   0
Mar 2, 2025
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0 replies
jlacosta
Mar 2, 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
Three circles are concurrent
Twoisaprime   21
N 2 minutes ago by L13832
Source: RMM 2025 P5
Let triangle $ABC$ be an acute triangle with $AB<AC$ and let $H$ and $O$ be its orthocenter and circumcenter, respectively. Let $\Gamma$ be the circle $BOC$. The line $AO$ and the circle of radius $AO$ centered at $A$ cross $\Gamma$ at $A’$ and $F$, respectively. Prove that $\Gamma$ , the circle on diameter $AA’$ and circle $AFH$ are concurrent.
Proposed by Romania, Radu-Andrew Lecoiu
21 replies
2 viewing
Twoisaprime
Feb 13, 2025
L13832
2 minutes ago
IMO Shortlist 2011, Algebra 3
orl   45
N 9 minutes ago by Ilikeminecraft
Source: IMO Shortlist 2011, Algebra 3
Determine all pairs $(f,g)$ of functions from the set of real numbers to itself that satisfy \[g(f(x+y)) = f(x) + (2x + y)g(y)\] for all real numbers $x$ and $y$.

Proposed by Japan
45 replies
orl
Jul 11, 2012
Ilikeminecraft
9 minutes ago
Hard FE with positive reals
egxa   8
N 19 minutes ago by megarnie
Source: Turkey Olympic Revenge 2023 Shortlist A4
Find all functions $f:\mathbb{R^+}\to \mathbb{R^+}$ such that for all $x,y\in \mathbb{R^+}$
$f(xf(y)+y)=f(f(y))+yf(x)$
Proposed by Şevket Onur Yılmaz
8 replies
egxa
Jan 22, 2024
megarnie
19 minutes ago
Like Father Like Son... (or Like Grandson?)
AlperenINAN   1
N 24 minutes ago by hakN
Source: Turkey TST 2025 P4
Let $a,b,c$ be given pairwise coprime positive integers where $a>bc$. Let $m<n$ be positive integers. We call $m$ to be a grandson of $n$ if and only if, for all possible piles of stones whose total mass adds up to $n$ and consist of stones with masses $a,b,c$, it's possible to take some of the stones out from this pile in a way that in the end, we can obtain a new pile of stones with total mass of $m$. Find the greatest possible number that doesn't have any grandsons.
1 reply
+1 w
AlperenINAN
Today at 6:09 AM
hakN
24 minutes ago
Crazy number theory
MTA_2024   5
N 24 minutes ago by bjump
Find all couple $(p;q)$ of primes (greater than 5) such that : $$pq \mid (5^q-3^q)(5^p-3^p)$$
5 replies
MTA_2024
3 hours ago
bjump
24 minutes ago
hard number theory problem
Zavyk09   0
33 minutes ago
Source: forgotten
Find all couple $(x, y)$ of positive integers such that:
$$2^n + 3^n \mid x^n + y^n, \forall n \in \mathbb{N}^*$$
0 replies
Zavyk09
33 minutes ago
0 replies
The return of a legend inequality
giangtruong13   2
N 43 minutes ago by polishedhardwoodtable
Source: Legacy
Given that $0<a,b,c,d<1$.Prove that: $$ (1-a)(1-b)(1-c)(1-d) > 1-a-b-c-d $$
2 replies
giangtruong13
2 hours ago
polishedhardwoodtable
43 minutes ago
Slightly weird points which are not so weird
Pranav1056   9
N an hour ago by Retemoeg
Source: India TST 2023 Day 4 P1
Suppose an acute scalene triangle $ABC$ has incentre $I$ and incircle touching $BC$ at $D$. Let $Z$ be the antipode of $A$ in the circumcircle of $ABC$. Point $L$ is chosen on the internal angle bisector of $\angle BZC$ such that $AL = LI$. Let $M$ be the midpoint of arc $BZC$, and let $V$ be the midpoint of $ID$. Prove that $\angle IML = \angle DVM$
9 replies
Pranav1056
Jul 9, 2023
Retemoeg
an hour ago
2023 factors and perfect cube
proxima1681   4
N an hour ago by anudeep
Source: Indian Statistical Institute (ISI) UGB 2023 P4
Let $n_1, n_2, \cdots , n_{51}$ be distinct natural numbers each of which has exactly $2023$ positive integer factors. For instance, $2^{2022}$ has exactly $2023$ positive integer factors $1,2, 2^{2}, 2^{3}, \cdots 2^{2021}, 2^{2022}$. Assume that no prime larger than $11$ divides any of the $n_{i}$'s. Show that there must be some perfect cube among the $n_{i}$'s.
4 replies
proxima1681
May 14, 2023
anudeep
an hour ago
circle geometry showing perpendicularity
Kyj9981   1
N an hour ago by Retemoeg
Two circles $\omega_1$ and $\omega_2$ intersect at points $A$ and $B$. A line through $B$ intersects $\omega_1$ and $\omega_2$ at points $C$ and $D$, respectively. Line $AD$ intersects $\omega_1$ at point $E \neq A$, and line $AC$ intersects $\omega_2$ at point $F \neq A$. If $O$ is the circumcenter of $\triangle AEF$, prove that $OB \perp CD$.
1 reply
Kyj9981
6 hours ago
Retemoeg
an hour ago
Hard problem
Tendo_Jakarta   5
N an hour ago by Tendo_Jakarta
Let the sequence \(x_{n}\) be such that
\[u_{1} = 1; \quad u_{n+1} = \dfrac{u_{1} + u_{2} +...+u_{n}}{n}+n-1 \quad \forall n \in \mathbb{N^{*}}\]and \(y_{n} =\dfrac{1}{u_{1}u_{2}} + \dfrac{1}{u_{3}u_{4}} + ... + \dfrac{1}{u_{2n-1}u_{2n}}  \quad \forall n \geq 1\). Find \(\lim_{n\rightarrow\infty}{y_{n}}\).
5 replies
Tendo_Jakarta
2 hours ago
Tendo_Jakarta
an hour ago
Oh no! Inequality again?
mathisreaI   108
N an hour ago by Maximilian113
Source: IMO 2022 Problem 2
Let $\mathbb{R}^+$ denote the set of positive real numbers. Find all functions $f: \mathbb{R}^+ \to \mathbb{R}^+$ such that for each $x \in \mathbb{R}^+$, there is exactly one $y \in \mathbb{R}^+$ satisfying $$xf(y)+yf(x) \leq 2$$
108 replies
mathisreaI
Jul 13, 2022
Maximilian113
an hour ago
Equation with powers
a_507_bc   5
N an hour ago by ali123456
Source: Serbia JBMO TST 2024 P1
Find all non-negative integers $x, y$ and primes $p$ such that $$3^x+p^2=7 \cdot 2^y.$$
5 replies
a_507_bc
May 25, 2024
ali123456
an hour ago
hermoso, raiz primitiva, orden??
holaquehace707070   0
an hour ago
Sea n un numero natural con mas de 2021 dıgitos donde ninguno de el-
los es 8 o 9. Suponga que n no tiene factores comunes con 2021. Demuestre que es posible
aumentar uno de los dıgitos de n en a lo mas 2 de modo que el numero resultante sea multiplo
de 2021.
0 replies
holaquehace707070
an hour ago
0 replies
Exponential + factorial diophantine
62861   33
N Nov 17, 2024 by megahertz13
Source: USA TSTST 2017, Problem 4, proposed by Mark Sellke
Find all nonnegative integer solutions to $2^a + 3^b + 5^c = n!$.

Proposed by Mark Sellke
33 replies
62861
Jun 29, 2017
megahertz13
Nov 17, 2024
Exponential + factorial diophantine
G H J
Source: USA TSTST 2017, Problem 4, proposed by Mark Sellke
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62861
3564 posts
#1 • 16 Y
Y by rightways, Davi-8191, Mathuzb, mathdragon2000, integrated_JRC, myh2910, OlympusHero, fidgetboss_4000, itslumi, son7, megarnie, centslordm, HWenslawski, Adventure10, Mango247, ItsBesi
Find all nonnegative integer solutions to $2^a + 3^b + 5^c = n!$.

Proposed by Mark Sellke
This post has been edited 1 time. Last edited by 62861, Jun 29, 2017, 8:51 PM
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v_Enhance
6858 posts
#2 • 23 Y
Y by laegolas, psa, MustafaKemal, Mathuzb, mathdragon2000, Mosquitall, integrated_JRC, myh2910, HolyMath, Kanep, Idio-logy, son7, v4913, sqrtX, tigerzhang, centslordm, guptaamitu1, HWenslawski, HamstPan38825, Adventure10, lian_the_noob12, Mango247, MS_asdfgzxcvb
For $n \le 4$, one can check the only solutions are: \begin{align*} 	2^2 + 3^0 + 5^0 &= 3! \\ 	2^1 + 3^1 + 5^0 &= 3! \\ 	2^4 + 3^1 + 5^1 &= 4!. \end{align*}Now we prove there are no solutions for $n \ge 5$.

A tricky way to do this is to take modulo $120$, since \begin{align*} 	2^a \pmod{120} &\in \{ 1, 2, 4, 8, 16, 32, 64 \} \\ 	3^b \pmod{120} &\in \{ 1, 3, 9, 27, 81 \} \\ 	5^c \pmod{120} &\in \{ 1, 5, 25 \} \end{align*}and by inspection one notes that no three elements have vanishing sum modulo $120$.

I expect most solutions to instead use casework. Here is one possible approach with cases (with $n \ge 5$). First, we analyze the cases where $a < 3$:
  • $a=0$: No solutions for parity reasons.
  • $a=1$: since $3^b + 5^c \equiv 6 \pmod 8$, we find $b$ even and $c$ odd (hence $c \neq 0$). Now looking modulo $5$ gives that $3^b + 5^c \equiv 3 \pmod 5$,
  • $a=2$: From $3^b + 5^c \equiv 4 \pmod 8$, we find $b$ is odd and $c$ is even. Now looking modulo $5$ gives a contradiction, even if $c = 0$, since $3^b \in \{2,3 \pmod 5\}$ but $3^b + 5^c \equiv 1 \pmod 5$.
Henceforth assume $a \ge 3$. Next, by taking modulo $8$ we have $3^b+5^c \equiv 0 \pmod 8$, which forces both $b$ and $c$ to be odd (in particular, $b,c > 0$). We now have \begin{align*} 	2^a + 5^c &\equiv 0 \pmod 3 \\ 	2^a + 3^b &\equiv 0 \pmod 5. \end{align*}The first equation implies $a$ is even, but the second equation requires $a$ to be odd, contradiction. Hence no solutions with $n \ge 5$.
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jeff10
1117 posts
#3 • 3 Y
Y by centslordm, Adventure10, Mango247
Solution
This post has been edited 2 times. Last edited by jeff10, Jun 30, 2017, 12:06 AM
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sunfishho
447 posts
#5 • 1 Y
Y by Adventure10
v_Enhance wrote:
A tricky way to do this is to take modulo $120$, since \begin{align*} 	2^a \pmod{120} &\in \{ 1, 2, 4, 8, 16, 32, 64 \} \\ 	3^b \pmod{120} &\in \{ 1, 3, 9, 27, 81 \} \\ 	5^c \pmod{120} &\in \{ 1, 5, 25 \} \end{align*}and by inspection one notes that no three elements have vanishing sum modulo $120$.
Just wondering, what is the motivation for picking 120?
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mathdragon2000
2458 posts
#6 • 3 Y
Y by sunfishho, Adventure10, Mango247
I'm not the one you're asking, but...
Motivation?
This post has been edited 1 time. Last edited by mathdragon2000, Nov 27, 2017, 2:11 AM
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inxang
157 posts
#7 • 2 Y
Y by Adventure10, Mango247
CantonMathGuy wrote:
Find all nonnegative integer solutions to $2^a + 3^b + 5^c = n!$.

Proposed by Mark Sellke

Help me!

Solve integers equation:

$2^x+5^y-7^z=0$
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ubermensch
820 posts
#9 • 1 Y
Y by Adventure10
Nice diophantine, but I thought it was on the easier side for a TST... I found a rather roundabout way to do it(at least compared to the $mod$ $120$ solution here)-
Sketch
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AlastorMoody
2125 posts
#10 • 2 Y
Y by SHREYAS333, Adventure10
Solution
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Math-wiz
6107 posts
#12
Y by
CantonMathGuy wrote:
Find all nonnegative integer solutions to $2^a + 3^b + 5^c = n!$.

Proposed by Mark Sellke

solution
This post has been edited 1 time. Last edited by Math-wiz, Mar 15, 2020, 5:48 AM
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kvs
620 posts
#13 • 1 Y
Y by Mango247
I claim the only solutions are $\boxed{(a,b,c,n) = (1,1,0,3), (4,1,1,4), (2,0,0,3)}$. One can easily check that all of these work.

We now prove there are no other solutions. First, we show that, if $n \geq 5, a \geq 3, b \geq 1$, there are no solutions. First, take the equation mod $4$ to get $3^b+5^c \equiv 0 \pmod{4}$, or $(-1)^b \equiv -1 \pmod{4}$, implying $b$ is odd. Taking the equation mod $8$ gives $3^b+(-3)^c \equiv 3 + (-3)^c \equiv 0 \pmod{8}$, implying $c$ is odd as well. Taking the equation mod $3$ gives $2^a+2^c \equiv 0 \pmod{3}$, implying $a$ is even since $c$ is odd. Finally, taking the equation mod $15$ gives $2^a \equiv \{1,4\} \pmod{15}, 3^b \equiv \{3,12\} \pmod{15}, 5^c \equiv 5 \pmod{5}$ (from the parities above). One can easily check that no combination of these residues sums to $0$ mod $15$.

If $n < 5$, a finite case check gives the solutions listed above. From now, we assume $n \geq 5$. First, assume $b < 1$, or $b=0$. Then, $2^a+5^c+1 = n!$. If $c = 0$, then $2^a+2=n!$, which is impossible since $2^a+2$ is never divisible by $8$. Thus, $c \geq 1$. Taking the equation mod $5$ gives $2^a +1  \equiv 0 \pmod{5}$, so $a \equiv 2 \pmod{4}$. However, mod $4$ gives $2 \equiv 0 \pmod{4}$, contradiction.

If $n \geq 5$ and $a < 3$, then we do casework on the value of $a$. If $a = 0$, we get $3^b+5^c+1$ is even, which is impossible. If $a=1$, we get $3^b+5^c+2 =n!$. Taking mod $4$ gives $(-1)^b+3 \equiv 0 \pmod{4}$, so $b$ is even. Taking mod $8$ gives $5^c + 3 \equiv 0 \pmod{8}$, so $c$ is odd. However, taking mod $5$ gives $3^b+2 \equiv 0 \pmod{5}$, so $b$ must be odd, contradiction. If $a=2$, then $3^b+5^c+4 = n!$. As before, mod $4$ gives $b$ odd, so mod $8$ gives $5^c+7 \equiv 0 \pmod{8}$, which is impossible. Thus, we have exhausted all cases and the above solutions are the only ones.

Remarks
This post has been edited 1 time. Last edited by kvs, Jun 8, 2020, 4:42 AM
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GeronimoStilton
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#14 • 1 Y
Y by naman12
This was an amusing exercise in bashing.

Consider some solution $(a,b,c,n)$. Remark that $2^a+3^b+5^c \ge 1+1+1 = 3$, so $n\ge 3$. Additionally, remark $2\mid n!-3^b-5^c$, so $a\ge 1$. Let $a=k+1$.

Claim: We have $n\le 4$.

Solution: Suppose otherwise.

Consider the case $a=1$. Rearranging yields $3^b+5^c=n!-2$. Take modulo $4$ to get $3^b+1\equiv 2\pmod{4}$, hence $b$ is even. In the case $c\ge 1$, we could take modulo $5$ to get $3^b\equiv 3\pmod{5}$, which can't occur if $b$ is even. Hence, we have $c=0$ and $3^b=n!-3$. This has no solution if $n=5$, so we may assume $n\ge 6$. Then, $3^b\equiv n!-3\equiv 6\pmod{9}$, which is absurd.

Now, we may assume $a\ge 2$. Take modulo $4$ to get $3^b+1\equiv 0\pmod{4}$, so $b$ is odd. Take modulo $5$ to get $2^a+3^b\equiv 0\pmod{5}$. Since $b$ is odd, $3^b\equiv 2,3\pmod{5}$, implying that $2^a\equiv 2,3\pmod{5}$ and $a$ is odd as well. In particular, this implies $a\ge 3$. Take modulo $8$ to get $3+5^c\equiv 0\pmod{8}$, which implies $c$ is odd. Take modulo $3$ to get $2+5\equiv 0\pmod{3}$, which is false. Hence, there are no solutions for $n\ge 5$, and we are done. $\fbox{}$

Now, we have the bound $3\le n\le 4$.

Case 1: We have $n=3$. Then, we have $6=2^a+3^b+5^c \ge 1+1+5^c=2+5^c$. This implies $c=0$, so $5=2^a+3^b \ge 2^a+1$. This implies $1\le a\le 2$. Taking $a=1$ gives $(a,b,c,n) = (1,1,0,3)$ and taking $a=2$ gives $(2,0,0,3)$. These constitute all possibilities in this scenario.

Case 2: We have $n=4$. Then, we have $24=2^a+3^b+5^c \ge 1+1+5^c=2+5^c$. This implies $c\le 1$, so $c\in \{0,1\}$.

Subcase 1: We have $c=0$. Rearrange to get $23=2^a+3^b \ge 1+3^b$. This implies $b\le 2$. Taking $b=2$ gives no solution, taking $b=1$ gives no solution, and taking $b=0$ gives no solution, hence there are no solutions in this case.

Subcase 2: We have $c=1$. Rearrange to get $19=2^a+3^b\ge 1+3^b$. This implies $b\le 2$. Taking $b=2$ gives no solution, taking $b=1$ gives $a=4$, and taking $b=0$ gives no solution, hence we have the solution $(a,b,c,n) = (4,1,1,4)$.

We have considered all cases, so the solutions are $(a,b,c,n) = (1,1,0,3), (2,0,0,3),(4,1,1,4)$.
This post has been edited 1 time. Last edited by GeronimoStilton, Jul 19, 2020, 4:51 PM
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dchenmathcounts
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#15
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This is just low brain. The solutions are $(2,0,0,3),(1,1,0,3),(4,1,1,4).$ We show $n\geq 5$ has no solutions.

By mod $4,$ $3^b+5^c\equiv 3^b+1\equiv 0\pmod{4},$ so $b\equiv 1\pmod{2}.$ Thus by mod $8,$ $c\equiv 1\pmod{2}$ as well. By mod $5,$ $2^a+3^b\equiv 0\pmod{5},$ implying $a\equiv b\pmod{4},$ or $a\equiv b\equiv 1\pmod{2}.$ But by mod $3,$ $2^a+5^c\equiv 2^a+2^c\equiv 0\pmod{3},$ implying that $c\equiv 0\pmod{2},$ which is a contradiction.
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fidgetboss_4000
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#16
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Wow, yet another “oly NT problem” trivialized just by taking mods of small numbers
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IAmTheHazard
5000 posts
#17 • 2 Y
Y by son7, centslordm
For $n \leq 4$, we can manually check everything, finding that the only solutions are $(a,b,c,n)=(2,0,0,3),(1,1,0,3),(4,1,1,4)$.

We now eliminate any $n \geq 5$.
First take $\pmod{3}$. We see that $(-1)^a+(-1)^c \equiv 0 \pmod{3}$, so $a$ and $c$ are opposite parity.
Taking $\pmod{4}$ gives the following:
If $a=0$ then the LHS is odd, but for $n \geq 5$, $n!$ is even: contradiction
If $a=1$, then the LHS becomes $3+(-1)^b$ and the RHS is $0$, so we get $b$ even. However, we also get that $c$ is even from the opposite parity condition. But then, taking $\pmod{8}$ gets that the LHS is $4$, while the RHS is $0$ because $n \geq 5$: contradiction.
So we have $a \geq 2$. then the LHS becomes $1+(-1)^b$ and the RHS is $0$, so $b$ is odd.
Now consider $\pmod{8}$.
If $a=2$, then $c$ is odd, so $5^c \equiv 5 \pmod{8}$. Since we know $b$ is odd, we see that $3^b \equiv 3 \pmod{8}$, so the LHS is equal to $4$ but the RHS must be $0$: contradiction.
So $a \geq 3$. Then we get $5^c + 3^b \equiv 5^c+3 \pmod{8}$. Since the RHS is $0$, we have $5^c \equiv 5 \pmod{8}$, which implies that $c$ is odd and $a$ is even.

Therefore, $a$ is even, $b$ is odd, and $c$ is odd. Let $a=2x,b=2y+1,c=2z+1$. Then the condition rewrites as:
$$4^x+3\cdot 9^y+5\cdot 25^z = n!$$Now, taking $\pmod{5}$, we see that $(-1)^x+3\cdot (-1)^y \equiv 0 \pmod{5}$. However, we can easily verify that this has no solutions. Thus any $n \geq 5$ fails. $\blacksquare$
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jeteagle
480 posts
#19 • 1 Y
Y by kevinmathz
Solved this when I was sleeping lel.

When $n \le 4$, we see that the solutions are $(a, b, c, n) = (2, 0, 0, 3), (1, 1, 0, 3), (4, 1, 1, 4).$

We will now prove that any $n \ge 5$ won't have any solutions. Notice how $n! \equiv 0 \pmod 8, n! \equiv 0 \pmod 3, n! \equiv 0 \pmod 5.$
First, if $b \ge 1$ with $\mod 3$, we see that $2^a+5^c \equiv (-1)^a+(-1)^c \equiv 0 \pmod 3$ which means $a$ and $c$ have different parities.
Next, if we take $\mod 8$, we have two cases.

Case 1: $2^a \equiv 4 \pmod 8, 3^b+5^c \equiv 4 \pmod 8.$
We immediately see that $a = 2$. Next, for $3^b+5^c \equiv 4 \pmod 8$, we must have $3^b \equiv 3 \pmod 8$ and $5^c \equiv 1 \pmod 8$, which means $c \equiv 0 \pmod 2$. Because $b \ge 1$ as $b \equiv 1 \pmod 2$, we see that $a$ and $c$ have the same parity, a contradiction.

Case 2: $3^b+5^c \equiv 0 \pmod 8.$
We must have $3^b \equiv 3 \pmod 8$ and $5^c \equiv 5 \pmod 8$ in this case, which is equivalent to $b \equiv c \equiv 1\pmod 2$. Because $b \ge 1$, we have $a$ and $c$ must be different parities, so $a$ is even. Finally, if we take $\mod 5$, we see that $2^a \equiv 4, 1\pmod 5$, while $3^b \equiv 3, 2 \pmod 5$, a because $2^a+3^b \equiv 0 \pmod 5$, we have a contradiction.

Therefore, the only solutions are the ones stated above when $n \le 4.$
This post has been edited 1 time. Last edited by jeteagle, Jan 1, 2021, 5:12 PM
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pad
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#20
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When $n \le 4$, it can be confirmed that the solutions are $(a, b, c, n) = (2, 0, 0, 3), (1, 1, 0, 3), (4, 1, 1, 4)$. Henceforth assume $n\ge 5$.
  • Case 1: $a\ge 2$. Then mod 4 gives $(-1)^b+1\equiv 0 \pmod4$, so $b$ is odd; let $b=2k+1$ for some $k\ge 0$. If $a$ is even, then $4^{a/2}+3\cdot 9^k+5^c=n!$, so taking mod 5 gives $(-1)^{a/2} + 3(-1)^k \equiv 0 \pmod5$. This is impossible since the LHS is even and absolutely bounded by 4. Hence $a$ is odd; let $a=2\ell+1$ for some $\ell\ge 0$. Now $2\cdot 4^\ell+3\cdot 9^k+5^c=n!$, so taking mod 3 gives $2+(-1)^c \equiv 0 \pmod3$. Hence $c$ is even; let $c=2m$ for some $m\ge 0$. Now the equation becomes $2\cdot 4^\ell+3\cdot 9^k+25^m=n!$. Then mod 8 gives $2\cdot 4^\ell +3+1 \equiv 0 \pmod8$, but this is impossible both when $\ell=0$ and $\ell\ge 1$.
  • Case 2: $a=1$. The equation is $3^b+5^c=n!-2$. Then mod 4 gives $(-1)^b+1\equiv -2\pmod4$, so $b$ is even; let $b=2k$ for some $k\ge 0$. Taking instead mod 3 gives $(-1)^c\equiv -2\pmod3$, so $c$ is even; let $c=2m$ for some $m\ge 0$. The equation is $9^k+25^m=n!-2$. Taking mod 8 gives $2\equiv -2\pmod8$, contradiction.
  • Case 3: $a=0$. The equation is $3^b+5^c=n!-1$. Now mod 4 gives $(-1)^b+1\equiv -1\pmod4$, contradiction.
So there are no solutions for $n\ge 5$, and we conclude.
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AMN300
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#21
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First suppose $n \le 4$. $n=0, 1, 2$ clearly yield no solutions since then the left hand side is less than 3.
-If $n=3$ we have $2^a + 3^b + 5^c = 6$, so $c \le 1$. $c=1$ means $2^a+3^b=1$ which can't hold. So $c=0$. We want to solve $2^a+3^b=5$ and we can check that the solutions are $\boxed{a=1,b=1,c=0,n=3}$ and $\boxed{a=2,b=0,c=0,n=3}$ by inspection.
-If $n=4$ we have $2^a + 3^b + 5^c = 24$, so $c \le 1$.
--If $c=0$ then $2^a+3^b=23$, so $b \le 2$, but we can check there are no solutions by hand.
--If $c=1$ then $2^a+3^b=19$. Thus $b \le 2$. We can check by inspection that the only solution is $\boxed{a=4,b=1,c=1,n=4}$.

If $a=0$ and $n \ge 5$: then $1+3^b+5^c=n!$. The left hand side is odd and the right hand side is even, contradiction.

If $b=0$ and $n \ge 5$: then $2^a+5^c+1=n!$. We have already done the case $a=0$.
-If $a=1$ then we want to solve $5^c+3=n!$, but the left hand side is not $0 \pmod{3}$ and the right hand side is, contradiction.
-If $a \ge 2$ then we have $1^c+1 \equiv 0 \pmod{4}$ which cannot hold.

If $c=0$ and $n \ge 5$: we have already done the cases when $a=0$ or $b=0$, so $a, b \ge 1$. Now $(-1)^a+1 \equiv 0 \pmod{3}$ so $a$ is odd.
-If $a=1$ then we want to solve $3+3^b = n!$. The left hand side is not divisible by $9$ but the right hand side is, contradiction.
-If $a \ge 2$ then since $a$ is odd, $a \ge 3$. Then we have $3^b+1 \equiv 0 \pmod{8}$ which is not possible.

Thus $a,b,c \ge 1$ and $n \ge 5$: then $(-1)^a + (-1)^c \equiv 0 \pmod{3}$, so $a, c$ have opposite parity.
-If $a=1$ then by the above, $c$ is even. Now $2+(-2)^b \equiv 0 \pmod{5}$, so $b \equiv 1 \pmod{4}$, so $b$ is odd. But $2+3^b+(-3)^c \equiv 0 \pmod{8}$. But $b$ is odd and $c$ is even so the left hand side is $2+3+1 = 6 \pmod{8}$, contradiction.
-If $a=2$ then by the above, $c$ is odd. Now $4+(-2)^b \equiv 0 \pmod{5}$ so $b$ is even. Then $4+3^b+(-3)^c \equiv 0 \pmod{8}$. But $b$ is even and $c$ is odd so the left hand side is $4+1+5 = 10 \pmod{8}$, contradiction.
-If $a \ge 3$ then $2^a \equiv 0 \pmod{4}$, so $(-1)^b+1 \equiv 0 \pmod{4}$, so $b$ is odd. Then $2^a+(-2)^b \equiv 0 \pmod{5}$. This means $a \equiv 1 \pmod{4}$, so $a$ is odd. Therefore $c$ is even. Since $a \ge 3$, $2^a \equiv 0 \pmod{8}$ so $3^b+(-3)^c \equiv 0 \pmod{8}$. But $b$ is odd and $c$ is even, so the left hand side is $3+1 = 4 \pmod{8}$, contradiction.
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megarnie
5532 posts
#22
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If $n<4$, we can find that the only solutions are $(a,b,c,n)$ $\boxed{(2,0,0,3)}$, $\boxed{(1,1,0,3)}$.

If $n=4$, we have $c<2, a<5, b<3$. If $c=0$, we find that there are no solutions by testing out values for $b$. If $c=1$, then we find no solutions unless $b=1$. Therefore, $\boxed{(4,1,1,4)}$ is the only solution when $n=4$.

Now it remains to find all solutions when $n>4$. We can take powers of $2,3,5$ $\pmod{120}$.

$2^a$ $\{1,2,4,8,16,32,64\}$
$3^b$ $\{1,3,9,27,81\}$
$5^c$ $\{1,5,25\}$.
No three add up to $0\pmod{120}$ so we are done.
This post has been edited 1 time. Last edited by megarnie, Mar 6, 2022, 1:16 AM
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HamstPan38825
8853 posts
#23 • 1 Y
Y by megarnie
Assume $n \geq 5$. Mod 5, we we have $$2^a + 3^b \equiv 0 \pmod 5,$$which implies that $a, b$ must be the same parity. Mod 3, we have $$2^a+5^c \equiv 0 \pmod 3,$$implying that $a$ and $c$ must be opposite parity. Now assume $a \geq 2$ -- then mod 4, $$3^b + 5^c \equiv 0 \pmod 4,$$implies that $b$ and $c$ are the same parity mod 5. Combined with the previous two facts, this is an obvious contradiction.

Now we get rid of edge cases. If $a=1$, then we have $$3^b + 5^c + 2 = n!.$$Modulo 120, $3^b+5^c \equiv 118 \pmod {120}$. But since $3^b \equiv 3, 9, 27, 81 \pmod {120}$ and $5^c \equiv 5, 25 \pmod {120}$, no combination of the two can attain 118, contradiction. Thus we only need to consider $n \leq 4$.

For $n=4$, $3^b+5^c=22$ has no solutions. For $n=3$, $3^b+5^c=4$ solves to get $(b, c) = (1, 0)$; for any smaller $n$, there are also no solutions. Thus we have exhausted this case.

Now, we consider $$2^a+3^b+5^c = 24.$$If $c=1, 2^a+3^b=19$, and if $b=1$ we obtain $a=4$ works. If $c=0$, then $2^a+3^b=23$, which has no solutions. For $n=3$, we also have the solution $(a, b, c)=(2, 0, 0)$. Thus the answers are $(4, 1, 1, 4)$, $(1, 1, 0, 3)$ and $(2, 0, 0, 3)$.
This post has been edited 1 time. Last edited by HamstPan38825, Sep 5, 2021, 2:40 AM
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OlyMan
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#25
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The casework solutions are quite good, but mod 120 also works very well. The only solutions are $(a,b,c,n)=(1,1,0,3),(2,0,0,3),(4,1,1,4).$ These clearly work. Now we analyse $2^{a},3^{b},5^{c}\pmod{120}$. We get $2^{a}\equiv \{1,2,4,8,16,32,64\}\pmod {120}$, $3^{b}\equiv \{1,3,9,27,81\}\pmod {120}$ and $5^{c}\equiv \{1,5,25\}\pmod {120}$. Now, clearly, we have $2^{a}+3^{b}+5^{c}$ is not congruent to $0$ mod 120, so there exist no non-negative integer $a,b,c$ which satisfy $2^{a}+3^{b}+5^{c} =5!$. But we have $(n-1)!$ divides $n!$ for all $n.$ Hence no interger $n > 5$ satisfies the equation. We are done. $\boxed{}$
This post has been edited 3 times. Last edited by OlyMan, Oct 14, 2021, 12:01 PM
Reason: Typo
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OlyMan
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#26
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sunfishho wrote:
v_Enhance wrote:
A tricky way to do this is to take modulo $120$, since \begin{align*} 	2^a \pmod{120} &\in \{ 1, 2, 4, 8, 16, 32, 64 \} \\ 	3^b \pmod{120} &\in \{ 1, 3, 9, 27, 81 \} \\ 	5^c \pmod{120} &\in \{ 1, 5, 25 \} \end{align*}and by inspection one notes that no three elements have vanishing sum modulo $120$.
Just wondering, what is the motivation for picking 120?

Isn't that "intuitive", what I did was that I found the possible sols and then tried to find sols for n=5 , but ofc could not succeed, and I though about the fact (n-1)! divides n! .
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VulcanForge
624 posts
#28
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Pure. Spammage.

By inspection we see for $n\le 4$ the solutions are $(a,b,c,n)=(1,1,0,3),(2,0,0,3),(4,1,1,4)$. We undergo pain in order to show there are no solutions when $n \ge 5$.

If $a \ge 3$ then taking$\pmod{4}$ gives $b$ odd, and then taking$\pmod{8}$ gives $c$ odd, so we're solving $2^a+3\cdot 9^{b'}+5\cdot 25^{c'}=n!$. Taking$\pmod{3}$ gives $a$ even, and then$\pmod{5}$ gives $(-1)^{a'}+3\cdot (-1)^{b'} \equiv 0 \pmod{5}$: this is impossible.

If $a=2$ then$\pmod{4}$ gives $b$ odd, and then (noting that $c>0$, otherwise$\pmod{3}$ gives a contradiction)$\pmod{5}$ gives $4+3 \cdot (-1)^{b'} \equiv 0 \pmod{5}$: this is impossible.

If $a=1$ then$\pmod{4}$ gives $b$ even, and then (noting that $c>0$, otherwise$\pmod{8}$ gives a contradiction)$\pmod{5}$ gives $2+(-1)^b \equiv 0 \pmod{5}$: this is impossible.

If $a=0$ then $2^a+3^b+5^c$ is odd.
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sanyalarnab
920 posts
#29
Y by
Cute one!
For $n\le 4$, just case bash to get some (a,b,c,n). Please search up for those. Real fun is for showing no solutions for $n\ge 5$.
Assume FTSOC $n\ge 5$.
Then $n! \equiv 0 \pmod{5}$
$\implies 2^a+3^b+5^c \equiv 0 \pmod{5}$
$\implies 2^a \equiv -3^b \pmod{5}$ (*)
Now $n! = 2^a+3^b+5^c \equiv 2^a+(-1)^b+1 \equiv 0 \pmod{4}$
If b is even,
$2^a \equiv 2 \pmod{4}$
This implies $a=1$.
Thus putting in (*),
$3^b \equiv 3 \pmod{5}$
$\implies 9^m \equiv 3 \pmod{5}$, where $b=2m$ for some natural $m$.
This is not possible for any $m$ and hence a contradiction to the assumption that $b$ is even.
Hence $\boxed{\text{b Is odd!}}$.
This gives in (*) $\implies 2^a \equiv (-3)^b \pmod{5}$
$\implies 2^a \equiv 2^b \pmod{5}$
$\implies 2^{a-b} \equiv 1 \pmod{5}$
If $a-b$ is odd,
$2^{a-b} \equiv \{2,3\} \pmod{5}$, which is not possible.
Hence, $a-b$ is even, or $\boxed{\text{a is odd!}}$.
Now $n!=2^a+3^b+5^c \equiv (-1)^a+(-1)^c \equiv -1+(-1)^c \equiv 0 \pmod{3}$
$\implies (-1)^c \equiv 1 \pmod{3}$
Hence $\boxed{\text{c is even!}}$.
Final move: checking (mod 8).
$n! \equiv 2^a+3^b+5^c \equiv 2^a+3+1 \equiv 0 \pmod{8}$
$\implies 2^a \equiv 4 \pmod{8}$
This immediately implies $a=2$ which is not odd. Contradiction on the initial assumption that $n\ge 5$. So done!
Remark: this problem was good but is it good enough for a TST#4?? It's just spamming modulo 3,4,5,8! That's all!
This post has been edited 4 times. Last edited by sanyalarnab, Oct 27, 2021, 4:58 PM
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Mogmog8
1080 posts
#30 • 1 Y
Y by centslordm
Assume FTSOC $n\ge 5,$ noting $2^a+3^b+5^c\equiv n!\equiv 0\pmod{120}.$ We see $2^a\equiv 1,2,4,8,16,32,64\pmod{120},$ $3^b\equiv 1,3,27,81\pmod{120},$ and $5^c\equiv 1,5,25\pmod{120}.$ Since $64+27+25<120,$ we know $2^b\equiv 81\pmod{120}$ so $2^a+5^c\equiv 39\pmod{120},$ which is absurd. Hence, $n=1,2,3,4.$

Case 1: $n=1.$ Note $2^a+3^b+5^c\ge 1+1+1=3$ so we have no solutions.

Case 2: $n=2.$ Note $2^a+3^b+5^c\ge 3>2!$ so we have no solutions.

Case 3: $n=3.$ We proceed by casework on $a.$ If $a=0,$ we have no solutions. If $a=1,$ $(1,1,0,3)$ works. If $a=2,$ $(2,0,0,3)$ works.

Case 4: $n=4.$ Again, we use caswork on $a.$ If $a=0,1,2,3$ we have no solutions. If $a=4,$ $b=c=1$ or $(4,1,1,4)$ works.

Our solutions are $(a,b,c,n)=\boxed{(1,1,0,3),(2,0,0,3),(4,1,1,4)}.$ $\square$
This post has been edited 1 time. Last edited by Mogmog8, Mar 6, 2022, 1:14 AM
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jasperE3
11098 posts
#31
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Exhaustive casework.

Case 1: $a\ge3$ and $n\ge5$
Taking$\pmod8$, we have:
$$3^b+(-3)^c\equiv0\pmod8.$$Let $b=2y+e_1$ and $c=2z+e_2$ with $\{e_1,e_2\}\subseteq\{0,1\}$, then $3^{e_1}+(-3)^{e_2}\equiv0\pmod8$, so $e_1=e_2=0$. Then:
$$2^a+9^y+25^z=n!.$$Taking$\pmod3$ gives:
$$(-1)^a\equiv-1\pmod3$$so $a$ is odd. Let $a=2x+3$ for some $x\ge0$, then:
$$8\cdot4^x+9^y+25^z=n!.$$Finally,$\pmod5$ gives:
$$3\cdot(-1)^x+(-1)^y\equiv0\pmod5,$$a contradiction (since this is impossible for any $x$ and $y$).

Case 2: $a\le2$ and $n\ge5$
Case 2.1: $a=0$ and $n\ge5$
We have:
$$1+3^b+5^c=n!.$$By$\pmod4$, we have $2+(-1)^b\equiv0\pmod4$, so no solutions exist in this case.

Case 2.2: $a=1$ and $n\ge5$
We have:
$$2+3^b+5^c=n!.$$By$\pmod4$, $3+(-1)^b\equiv0\pmod4$ and so $b$ is even. Let $b=2y$.
By$\pmod3$, $2+(-1)^c\equiv0\pmod3$ and so $c$ is even. Let $c=2z$.
The equation transforms into:
$$2+9^y+25^z=n!.$$By$\pmod8$, $4\equiv0\pmod8$ which is absurd. Hence no solutions.

Case 2.3: $a=2$ and $n\ge5$
We have:
$$4+3^b+5^c=n!.$$By$\pmod4$, $(-1)^b+1\equiv0\pmod4$ and so $b$ is odd. Let $b=2y+1$.
By$\pmod5$, $-1+3\cdot(-1)^y\equiv0\pmod5$ which is impossible.

Case 3: $n\le4$
Case 3.1: $n\in\{0,1,2\}$
This is also impossible as:
$$n!=2^a+3^b+5^c\ge1+1+1=3.$$
Case 3.2: $n=3$
Then $2^a+3^b+5^c=6$, so $5^c\le6$. This means that $c\le1$. If $c=1$ then $2^a+3^b=1$ which has no solutions. If $c=0$ then $2^a+3^b=5$, so $b\le1$. If $b=1$ then $a=1$, whereas if $b=0$ then $a=2$, which gives that $(a,b,c,n)=\boxed{(2,0,0,3)}$ and $\boxed{(1,1,0,3)}$ are solutions

Case 3.3: $n=4$
Then $2^a+3^b+5^c=24$, so $c\le1$.
If $c=0$ then $2^a+3^b=23$, so $b\le2$. For each of these cases there is no corresponding value of $a$ that works.
If $c=1$ then $2^a+3^b=19$, so $b\le2$. If $b=0$ or $b=2$ no solutions exist, but if $b=1$ we have that $(a,b,c,n)=\boxed{(4,1,1,4)}$ is a solution.
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kamatadu
466 posts
#32 • 1 Y
Y by HoripodoKrishno
O maa gu turu lob! I never knew $\pmod{120}$ had such nice modulos for powers for $2$, $3$ and $5$.

For $n\ge 5$, take $\pmod{120}$ to get the following powers:
\begin{align*}
    2^a&\equiv\left\{1,2,4,8,16,32,64\right\}\pmod{120}\\
    3^b&\equiv\left\{1,3,9,27,81\right\}\pmod{120}\\
    5^c&\equiv\left\{1,5,25\right\}\pmod{120}
.\end{align*}
Now some case bashing shows none of these actually work (in other words left for the reader to prove :rotfl: ). Now finally some more case bashing for $4!$ and $3!$ (the ones below these don't work due to size reasons) show that $\boxed{(3,2,0,0)}$, $\boxed{(3,1,1,0)}$ and $\boxed{(4,4,1,1)}$ are the only ones that actually work for the tuples of $(n,a,b,c)$ respectively (which is again left for the reader to prove :rotfl: ) and we are done.
This post has been edited 1 time. Last edited by kamatadu, Apr 15, 2023, 6:15 PM
Reason: joi maa kali private limited (*terms and conditions applied)
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F10tothepowerof34
195 posts
#33
Y by
Claim: The only solutions are $\boxed{(a,b,c,n)=(1,1,0,3), (2,0,0,3), (4,1,1,4)}$
Proof:
After a manual check, the only solutions for $n\le4$, are the ones stated in the claim, so from now on, $FTSOC$ assume $n\ge5$

Part 1: Taking$\pmod 4$
$2^a+3^b+5^c\equiv 3+1 \pmod 4 \Longleftrightarrow 3^b\equiv 3 \pmod 4$ however this in only possible when $b\equiv 1 \pmod 2$, thus $b$ is odd.
Part 2: Taking$\pmod 5$
$2^a+3^b+5^c\equiv 0 \pmod 5\Longleftrightarrow 2^a\equiv -3^b$ furthermore, since $b$ is odd, we can rewrite the expression as $2^a\equiv \left(-3\right)^b \pmod 5$ thus since $b$ is odd, $\left(-3\right)^b\equiv 2,3 \pmod 5$, which forces $2^a\equiv 2,3 \pmod 5$ which implies that $a\equiv 1\pmod 2$, or $a$ is odd.
Part 3: Taking $\pmod 3$
$2^a+3^b+5^c\equiv (-1)^a+0+(-1)^c\pmod 3 \Longrightarrow n!\equiv 0\equiv (-1)^a+(-1)^c\pmod 3$ thus $a$ and $c$ are pairwise distinct, however this forces $c\equiv 0\pmod 2$ or $c$ to be even, from the fact that $a$ is odd.
Part 4 (final part): Taking $\pmod 8$
$n!=2^a+3^b+5^c\equiv 4+3+1\equiv0\pmod 8$, thus$2^a\equiv 4\pmod8$ however this is only possible when $a=2$, which contradicts the previously statement that $a$ is odd. Thus we have reached a contradiction, so there exist no solution for $n\ge5$ $\blacksquare$
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huashiliao2020
1292 posts
#34
Y by
lol just throw random things at the board until it sticks

Note that $(n,a,b,c,)=(3,2,0,0),(3,1,1,0),(4,4,1,1)$ are the only solutions manually caseworked for $n<5$, so assume henceforth that n is at least 5. We see $$2^a\equiv 1,2,4,8,16,32,64\pmod{120},3^b\equiv 1,3,27,81\pmod{120},5^c\equiv 1,5,25\pmod{120}.$$$$64+27+25<120\implies 3^b\equiv 81\pmod{120}\implies 2^a+5^c\equiv 39\pmod{120},$$which can be checked to see that it doesn't work, so our only solutions are indeed the ones listed above.

I also had a decently long casework bash but its just elementary techniques and since I'm not going to learn anything from it (usually imo diophantine you don't really need any skill its like fe but usually easier) i wont type it up
This post has been edited 1 time. Last edited by huashiliao2020, Aug 9, 2023, 6:59 PM
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Captain_Baran
36 posts
#35
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$\textcolor{red}{Claim:}$ $n<5.$
$\textcolor{red}{Proof:}$ Assume $n\geq5.$ Then if $a\geq1$ we have $3^{b}+5^{c}\equiv0\pmod 4$ and $b\equiv c\equiv 1\pmod 2.$
Say $b=2b_1+1$ and $c=2c_1+1.$ Then if we look at the expression in modulo 3 we see $2^{a}+5\equiv0\pmod3$ and, $a\equiv 0\pmod 2.$ Say $a=2a_1.$ Then in modulo 5 we have $2^{2a_1}+3^{2b_1+1}\equiv 0\pmod 5.$ Since $2^{2a_1}\equiv\{4,1\}$ and $3^{2b_1+1}\equiv\{3,2\}$ we have a contradiction and $a$ must be $0$ and $b=4b_2+1$.
Now let's solve $2+3^{4b_2+1}+5^{2c_1+1}$ Looking at the expression in modulo 9 gives us $5^{2c_1+1}\equiv7\pmod9$ and $c=6k+2$ but since $c=2c_1+1=6k+2$ means $c\equiv1\equiv0\pmod 2$ we have a contradiction. b must be equal to 0. Now we have $5+5^{2c_1+1}=n!$ but for $n\geq5$ this means $10\equiv 0\pmod 3.$ Contradiction. Our claim is right.

Now let's solve for $n\leq4$:
For $n=4,$ we have $3^{b}+5^{c}=\{14,8,6,10,4,2\}$. In this case only $\{a,b,c,n\}=\{4,1,1,4\}$ is possible.
For $n=3$ we have $2^{a}+3^{b}={5,4,3,2}$. Only $2^{a}+3^{b}=5$ meaning either $\{a,b,c,n\}=\{2,0,0,3\}$ or $\{a,b,c,n\}=\{1,1,0,3\}$ is possible in this case.
For $n\leq2$ we have $n!\leq 2$ but $2^{a}+3^{b}+5^{c}\geq 3$ a contradiction. The only possible solutions are $\{a,b,c,n\}=\boxed{\{4,1,1,4\}},\boxed{\{2,0,0,3\}},\boxed{\{1,1,0,3\}}.$
This post has been edited 3 times. Last edited by Captain_Baran, Sep 8, 2023, 11:00 AM
Reason: Typo
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dolphinday
1310 posts
#36
Y by
We claim that the only solutions are $(a, b, c, n) = (1, 1, 0, 3), (2, 0, 0, 3), (4, 1, 1, 4)$.
There are clearly no other solutions for $a, b, c, n \leq 4$. (Feeling kind of lazy)

Taking $\pmod{120}$ for $n > 4$, then

\[2^a + 3^b + 5^c \equiv 0\pmod{12}\]
\[2^a \equiv 2, 4, 8, 16, 32, 64\pmod{120}\]
\[3^b \equiv 3, 9, 27, 81\pmod{120}\]
\[5^c \equiv 5, 25\pmod{120}\].

By inspection, none of these add up to $0\pmod{120}$, so it is impossible for $n > 4$.

Hence, the only solutions are $(a, b, c, n) = (1, 1, 0, 3), (2, 0, 0, 3), (4, 1, 1, 4)$.
This post has been edited 2 times. Last edited by dolphinday, Oct 20, 2023, 5:54 PM
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joshualiu315
2513 posts
#37
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The solutions are

\begin{align*}
(a,b,c, n) &= (2,0,0,3) \\
&= (1,1,0,3) \\
&= (4, 1, 1, 4)
\end{align*}
Notice that if $n \le 4$, we can manually check each case and find each of the above solutions. We will prove that there are no more solutions for $n>4$. For each $n>4$, note that

\[120 \mid n! = 2^a+3^b+5^c.\]
However, analyzing each of the residues of each individual power, we see that none of them cancel modulo $120$. Thus, there are no solutions when $n>4$.
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mannshah1211
651 posts
#38
Y by
Ah yes, the famous 5M TSTST problem.
Assume that $n \ge 5$.
We'll analyze modulo $120$. $2^a$ modulo $60$ is $\{1, 2, 4, 8, 16, 32, 64\}$, $3^b$ modulo $120$ is $\{1, 3, 9, 27, 81\}$, and $5^c$ modulo $120$ is $\{ 1, 5, 25 \}$. Note that the max sum of residues is $170,$ which means that our residue sum must be exactly $120$. Note that if the second residue is $\le 27,$ then the max sum of residues is $116 < 120,$ so our second residue must be $81$. Then, the sum of the remaining residues must be $39$. Note that this isn't possible (just check that none of $39 - 1, 39 - 5, 39 - 25$ appear in the first set), so there's no solutions for $n \ge 5$. It remains to find the solutions for $n \le 4$. For $n \in \{0, 1, 2 \},$ $2 \ge n! = 2^a + 3^b + 5^c \ge 3,$ clearly a contradiction. So, only cases remaining are $n = 3,$ and $n = 4$. For $n = 3,$ we have $2^a + 3^b + 5^c = 6,$ which gives solution $(a, b, c) = (1, 1, 0), (2, 0, 0).$ For $n = 4,$ we have $2^a + 3^b + 5^c = 24,$ whic gives solution $(a, b, c) = (4, 1, 1).$ So, the only solutions are $(a, b, c, n) = (2, 0, 0, 3), (1, 1, 0, 3), (4, 1, 1, 4).$
This post has been edited 1 time. Last edited by mannshah1211, Jan 17, 2024, 6:29 PM
Reason: Forgot the (2, 0, 0, 3) trivial solution :( I need sleep
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Cusofay
85 posts
#39
Y by
First we can see that $(a,b,c,n)\in\{(1,1,0,3),(2,0,0,3),(4,1,1,4) \}$ are the only solutions for $n\leq 4$. Now if $n\geq 5$. Mod $4$ implies $b$ is odd and mod $3$ gives us $a,c$ have different parities. If $c$ is even and $a,b$ are odd, taking the expression mod $8$ ends the case. If $a$ is even and $b,c$ are odd then mod $5$ ends the problem.

$$\mathbb{Q.E.D.}$$
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megahertz13
3168 posts
#40
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The only solutions are $(2,0,0)$, $(1,1,0)$, and $(4,1,1)$, which clearly work. One can manually check that these are the only solutions for $n\le 4$. Now we prove that there are no other solutions for $n\ge 5$.

The idea is to take $\mod {120}$. Note that $$2^a\pmod {120}\in \{1,2,4,8,16,32,64\}$$$$3^b\pmod {120}\in \{1,3,9,27,81\}$$$$5^c\pmod {120}\in \{1,3,9,27,81\}.$$Note that if you choose one element from each of these sets, they cannot sum to $0\pmod {120}$. However, $n\ge 5$ implies $$n!\equiv 0\pmod {120},$$done.
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