Happy Memorial Day! Please note that AoPS Online is closed May 24-26th.

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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.

Are you interested in working towards MATHCOUNTS and don’t know where to start? We have you covered! If you have taken Prealgebra, then you are ready for MATHCOUNTS/AMC 8 Basics. Already aiming for State or National MATHCOUNTS and harder AMC 8 problems? Then our MATHCOUNTS/AMC 8 Advanced course is for you.

Summer camps are starting next month at the Virtual Campus in math and language arts that are 2 - to 4 - weeks in duration. Spaces are still available - don’t miss your chance to have an enriching summer experience. There are middle and high school competition math camps as well as Math Beasts camps that review key topics coupled with fun explorations covering areas such as graph theory (Math Beasts Camp 6), cryptography (Math Beasts Camp 7-8), and topology (Math Beasts Camp 8-9)!

Be sure to mark your calendars for the following upcoming events:
[list][*]May 9th, 4:30pm PT/7:30pm ET, Casework 2: Overwhelming Evidence — A Text Adventure, a game where participants will work together to navigate the map, solve puzzles, and win! All are welcome.
[*]May 19th, 4:30pm PT/7:30pm ET, What's Next After Beast Academy?, designed for students finishing Beast Academy and ready for Prealgebra 1.
[*]May 20th, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 1 Math Jam, Problems 1 to 4, join the Canada/USA Mathcamp staff for this exciting Math Jam, where they discuss solutions to Problems 1 to 4 of the 2025 Mathcamp Qualifying Quiz!
[*]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]
Our full course list for upcoming classes is below:
All classes run 7:30pm-8:45pm ET/4:30pm - 5:45pm PT unless otherwise noted.

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MathWOOT Level 1
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Relativity
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0 replies
jlacosta
May 1, 2025
0 replies
k i Peer-to-Peer Programs Forum
jwelsh   157
N Dec 11, 2023 by cw357
Many of our AoPS Community members share their knowledge with their peers in a variety of ways, ranging from creating mock contests to creating real contests to writing handouts to hosting sessions as part of our partnership with schoolhouse.world.

To facilitate students in these efforts, we have created a new Peer-to-Peer Programs forum. With the creation of this forum, we are starting a new process for those of you who want to advertise your efforts. These advertisements and ensuing discussions have been cluttering up some of the forums that were meant for other purposes, so we’re gathering these topics in one place. This also allows students to find new peer-to-peer learning opportunities without having to poke around all the other forums.

To announce your program, or to invite others to work with you on it, here’s what to do:

1) Post a new topic in the Peer-to-Peer Programs forum. This will be the discussion thread for your program.

2) Post a single brief post in this thread that links the discussion thread of your program in the Peer-to-Peer Programs forum.

Please note that we’ll move or delete any future advertisement posts that are outside the Peer-to-Peer Programs forum, as well as any posts in this topic that are not brief announcements of new opportunities. In particular, this topic should not be used to discuss specific programs; those discussions should occur in topics in the Peer-to-Peer Programs forum.

Your post in this thread should have what you're sharing (class, session, tutoring, handout, math or coding game/other program) and a link to the thread in the Peer-to-Peer Programs forum, which should have more information (like where to find what you're sharing).
157 replies
jwelsh
Mar 15, 2021
cw357
Dec 11, 2023
k i C&P posting recs by mods
v_Enhance   0
Jun 12, 2020
The purpose of this post is to lay out a few suggestions about what kind of posts work well for the C&P forum. Except in a few cases these are mostly meant to be "suggestions based on historical trends" rather than firm hard rules; we may eventually replace this with an actual list of firm rules but that requires admin approval :) That said, if you post something in the "discouraged" category, you should not be totally surprised if it gets locked; they are discouraged exactly because past experience shows they tend to go badly.
-----------------------------
1. Program discussion: Allowed
If you have questions about specific camps or programs (e.g. which classes are good at X camp?), these questions fit well here. Many camps/programs have specific sub-forums too but we understand a lot of them are not active.
-----------------------------
2. Results discussion: Allowed
You can make threads about e.g. how you did on contests (including AMC), though on AMC day when there is a lot of discussion. Moderators and administrators may do a lot of thread-merging / forum-wrangling to keep things in one place.
-----------------------------
3. Reposting solutions or questions to past AMC/AIME/USAMO problems: Allowed
This forum contains a post for nearly every problem from AMC8, AMC10, AMC12, AIME, USAJMO, USAMO (and these links give you an index of all these posts). It is always permitted to post a full solution to any problem in its own thread (linked above), regardless of how old the problem is, and even if this solution is similar to one that has already been posted. We encourage this type of posting because it is helpful for the user to explain their solution in full to an audience, and for future users who want to see multiple approaches to a problem or even just the frequency distribution of common approaches. We do ask for some explanation; if you just post "the answer is (B); ez" then you are not adding anything useful.

You are also encouraged to post questions about a specific problem in the specific thread for that problem, or about previous user's solutions. It's almost always better to use the existing thread than to start a new one, to keep all the discussion in one place easily searchable for future visitors.
-----------------------------
4. Advice posts: Allowed, but read below first
You can use this forum to ask for advice about how to prepare for math competitions in general. But you should be aware that this question has been asked many many times. Before making a post, you are encouraged to look at the following:
[list]
[*] Stop looking for the right training: A generic post about advice that keeps getting stickied :)
[*] There is an enormous list of links on the Wiki of books / problems / etc for all levels.
[/list]
When you do post, we really encourage you to be as specific as possible in your question. Tell us about your background, what you've tried already, etc.

Actually, the absolute best way to get a helpful response is to take a few examples of problems that you tried to solve but couldn't, and explain what you tried on them / why you couldn't solve them. Here is a great example of a specific question.
-----------------------------
5. Publicity: use P2P forum instead
See https://artofproblemsolving.com/community/c5h2489297_peertopeer_programs_forum.
Some exceptions have been allowed in the past, but these require approval from administrators. (I am not totally sure what the criteria is. I am not an administrator.)
-----------------------------
6. Mock contests: use Mock Contests forum instead
Mock contests should be posted in the dedicated forum instead:
https://artofproblemsolving.com/community/c594864_aops_mock_contests
-----------------------------
7. AMC procedural questions: suggest to contact the AMC HQ instead
If you have a question like "how do I submit a change of venue form for the AIME" or "why is my name not on the qualifiers list even though I have a 300 index", you would be better off calling or emailing the AMC program to ask, they are the ones who can help you :)
-----------------------------
8. Discussion of random math problems: suggest to use MSM/HSM/HSO instead
If you are discussing a specific math problem that isn't from the AMC/AIME/USAMO, it's better to post these in Middle School Math, High School Math, High School Olympiads instead.
-----------------------------
9. Politics: suggest to use Round Table instead
There are important conversations to be had about things like gender diversity in math contests, etc., for sure. However, from experience we think that C&P is historically not a good place to have these conversations, as they go off the rails very quickly. We encourage you to use the Round Table instead, where it is much more clear that all posts need to be serious.
-----------------------------
10. MAA complaints: discouraged
We don't want to pretend that the MAA is perfect or that we agree with everything they do. However, we chose to discourage this sort of behavior because in practice most of the comments we see are not useful and some are frankly offensive.
[list] [*] If you just want to blow off steam, do it on your blog instead.
[*] When you have criticism, it should be reasoned, well-thought and constructive. What we mean by this is, for example, when the AOIME was announced, there was great outrage about potential cheating. Well, do you really think that this is something the organizers didn't think about too? Simply posting that "people will cheat and steal my USAMOO qualification, the MAA are idiots!" is not helpful as it is not bringing any new information to the table.
[*] Even if you do have reasoned, well-thought, constructive criticism, we think it is actually better to email it the MAA instead, rather than post it here. Experience shows that even polite, well-meaning suggestions posted in C&P are often derailed by less mature users who insist on complaining about everything.
[/list]
-----------------------------
11. Memes and joke posts: discouraged
It's fine to make jokes or lighthearted posts every so often. But it should be done with discretion. Ideally, jokes should be done within a longer post that has other content. For example, in my response to one user's question about olympiad combinatorics, I used a silly picture of Sogiita Gunha, but it was done within a context of a much longer post where it was meant to actually make a point.

On the other hand, there are many threads which consist largely of posts whose only content is an attached meme with the word "MAA" in it. When done in excess like this, the jokes reflect poorly on the community, so we explicitly discourage them.
-----------------------------
12. Questions that no one can answer: discouraged
Examples of this: "will MIT ask for AOIME scores?", "what will the AIME 2021 cutoffs be (asked in 2020)", etc. Basically, if you ask a question on this forum, it's better if the question is something that a user can plausibly answer :)
-----------------------------
13. Blind speculation: discouraged
Along these lines, if you do see a question that you don't have an answer to, we discourage "blindly guessing" as it leads to spreading of baseless rumors. For example, if you see some user posting "why are there fewer qualifiers than usual this year?", you should not reply "the MAA must have been worried about online cheating so they took fewer people!!". Was sich überhaupt sagen lässt, lässt sich klar sagen; und wovon man nicht reden kann, darüber muss man schweigen.
-----------------------------
14. Discussion of cheating: strongly discouraged
If you have evidence or reasonable suspicion of cheating, please report this to your Competition Manager or to the AMC HQ; these forums cannot help you.
Otherwise, please avoid public discussion of cheating. That is: no discussion of methods of cheating, no speculation about how cheating affects cutoffs, and so on --- it is not helpful to anyone, and it creates a sour atmosphere. A longer explanation is given in Seriously, please stop discussing how to cheat.
-----------------------------
15. Cutoff jokes: never allowed
Whenever the cutoffs for any major contest are released, it is very obvious when they are official. In the past, this has been achieved by the numbers being posted on the official AMC website (here) or through a post from the AMCDirector account.

You must never post fake cutoffs, even as a joke. You should also refrain from posting cutoffs that you've heard of via email, etc., because it is better to wait for the obvious official announcement. A longer explanation is given in A Treatise on Cutoff Trolling.
-----------------------------
16. Meanness: never allowed
Being mean is worse than being immature and unproductive. If another user does something which you think is inappropriate, use the Report button to bring the post to moderator attention, or if you really must reply, do so in a way that is tactful and constructive rather than inflammatory.
-----------------------------

Finally, we remind you all to sit back and enjoy the problems. :D

-----------------------------
(EDIT 2024-09-13: AoPS has asked to me to add the following item.)

Advertising paid program or service: never allowed

Per the AoPS Terms of Service (rule 5h), general advertisements are not allowed.

While we do allow advertisements of official contests (at the MAA and MATHCOUNTS level) and those run by college students with at least one successful year, any and all advertisements of a paid service or program is not allowed and will be deleted.
0 replies
v_Enhance
Jun 12, 2020
0 replies
k i Stop looking for the "right" training
v_Enhance   50
N Oct 16, 2017 by blawho12
Source: Contest advice
EDIT 2019-02-01: https://blog.evanchen.cc/2019/01/31/math-contest-platitudes-v3/ is the updated version of this.

EDIT 2021-06-09: see also https://web.evanchen.cc/faq-contest.html.

Original 2013 post
50 replies
v_Enhance
Feb 15, 2013
blawho12
Oct 16, 2017
Inspired by 2025 Beijing
sqing   8
N 28 minutes ago by ytChen
Source: Own
Let $ a,b,c,d >0  $ and $ (a^2+b^2+c^2)(b^2+c^2+d^2)=36. $ Prove that
$$ab^2c^2d \leq 8$$$$a^2bcd^2 \leq 16$$$$ ab^3c^3d \leq \frac{2187}{128}$$$$ a^3bcd^3 \leq \frac{2187}{32}$$
8 replies
1 viewing
sqing
Yesterday at 4:56 PM
ytChen
28 minutes ago
four points lie on a circle
pohoatza   78
N 31 minutes ago by ezpotd
Source: IMO Shortlist 2006, Geometry 2, AIMO 2007, TST 1, P2
Let $ ABCD$ be a trapezoid with parallel sides $ AB > CD$. Points $ K$ and $ L$ lie on the line segments $ AB$ and $ CD$, respectively, so that $AK/KB=DL/LC$. Suppose that there are points $ P$ and $ Q$ on the line segment $ KL$ satisfying \[\angle{APB} = \angle{BCD}\qquad\text{and}\qquad \angle{CQD} = \angle{ABC}.\]Prove that the points $ P$, $ Q$, $ B$ and $ C$ are concyclic.

Proposed by Vyacheslev Yasinskiy, Ukraine
78 replies
pohoatza
Jun 28, 2007
ezpotd
31 minutes ago
JBMO TST Bosnia and Herzegovina 2023 P4
FishkoBiH   2
N 36 minutes ago by Stear14
Source: JBMO TST Bosnia and Herzegovina 2023 P4
Let $n$ be a positive integer. A board with a format $n*n$ is divided in $n*n$ equal squares.Determine all integers $n$3 such that the board can be covered in $2*1$ (or $1*2$) pieces so that there is exactly one empty square in each row and each column.
2 replies
FishkoBiH
Today at 1:38 PM
Stear14
36 minutes ago
Does there exist 2011 numbers?
cyshine   8
N 41 minutes ago by TheBaiano
Source: Brazil MO, Problem 4
Do there exist $2011$ positive integers $a_1 < a_2 < \ldots < a_{2011}$ such that $\gcd(a_i,a_j) = a_j - a_i$ for any $i$, $j$ such that $1 \le i < j \le 2011$?
8 replies
cyshine
Oct 20, 2011
TheBaiano
41 minutes ago
4th grader qual JMO
HCM2001   38
N Today at 1:14 PM by blueprimes
i mean.. whattttt??? just found out about this.. is he on aops? (i'm sure he is) where are you orz lol..
https://www.mathschool.com/blog/results/celebrating-success-douglas-zhang-is-rsm-s-youngest-usajmo-qualifier
38 replies
HCM2001
May 22, 2025
blueprimes
Today at 1:14 PM
An FE. Who woulda thunk it?
nikenissan   120
N Today at 12:32 PM by NerdyNashville
Source: 2021 USAJMO Problem 1
Let $\mathbb{N}$ denote the set of positive integers. Find all functions $f : \mathbb{N} \rightarrow \mathbb{N}$ such that for positive integers $a$ and $b,$ \[f(a^2 + b^2) = f(a)f(b) \text{ and } f(a^2) = f(a)^2.\]
120 replies
nikenissan
Apr 15, 2021
NerdyNashville
Today at 12:32 PM
Zsigmondy's theorem
V0305   3
N Today at 9:00 AM by CatCatHead
Is Zsigmondy's theorem allowed on the IMO, and is it allowed on the AMC series of proof competitions (e.g. USAJMO, USA TSTST)?
3 replies
V0305
Yesterday at 6:22 PM
CatCatHead
Today at 9:00 AM
Base 2n of n^k
KevinYang2.71   50
N Today at 1:39 AM by ray66
Source: USAMO 2025/1, USAJMO 2025/2
Let $k$ and $d$ be positive integers. Prove that there exists a positive integer $N$ such that for every odd integer $n>N$, the digits in the base-$2n$ representation of $n^k$ are all greater than $d$.
50 replies
KevinYang2.71
Mar 20, 2025
ray66
Today at 1:39 AM
How Math WOOT Level 2 prepare you for olympiad contest
AMC10JA   0
Yesterday at 11:35 PM
I know how you do on Olympiad is based on your effort and your thinking skill, but I am just curious is WOOT level 2 is generally for practicing the beginner olympiad contest (like USAJMO or lower), or also good to learn for hard olympiad contest (like USAMO and IMO).
Please share your thought and experience. Thank you!
0 replies
AMC10JA
Yesterday at 11:35 PM
0 replies
Equilateral triangle $ABC$, $DEF$ has twice the area
v_Enhance   122
N Yesterday at 10:37 PM by lpieleanu
Source: JMO 2017 Problem 3, Titu, Luis, Cosmin
Let $ABC$ be an equilateral triangle, and point $P$ on its circumcircle. Let $PA$ and $BC$ intersect at $D$, $PB$ and $AC$ intersect at $E$, and $PC$ and $AB$ intersect at $F$. Prove that the area of $\triangle DEF$ is twice the area of $\triangle ABC$.

Proposed by Titu Andreescu, Luis Gonzales, Cosmin Pohoata
122 replies
v_Enhance
Apr 19, 2017
lpieleanu
Yesterday at 10:37 PM
Perfect Square Dice
asp211   67
N Yesterday at 9:27 PM by A7456321
Source: 2019 AIME II #4
A standard six-sided fair die is rolled four times. The probability that the product of all four numbers rolled is a perfect square is $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m+n$.
67 replies
asp211
Mar 22, 2019
A7456321
Yesterday at 9:27 PM
HCSSiM results
SurvivingInEnglish   75
N Yesterday at 7:25 PM by cowstalker
Anyone already got results for HCSSiM? Are there any point in sending additional work if I applied on March 19?
75 replies
SurvivingInEnglish
Apr 5, 2024
cowstalker
Yesterday at 7:25 PM
Perfect squares: 2011 USAJMO #1
v_Enhance   227
N Yesterday at 7:23 PM by ray66
Find, with proof, all positive integers $n$ for which $2^n + 12^n + 2011^n$ is a perfect square.
227 replies
v_Enhance
Apr 28, 2011
ray66
Yesterday at 7:23 PM
Mustang Math Recruitment is Open!
MustangMathTournament   0
Yesterday at 7:02 PM
The Interest Form for joining Mustang Math is open!

Hello all!

We're Mustang Math, and we are currently recruiting for the 2025-2026 year! If you are a high school or college student and are passionate about promoting an interest in competition math to younger students, you should strongly consider filling out the following form: https://link.mustangmath.com/join. Every member in MM truly has the potential to make a huge impact, no matter your experience!

About Mustang Math

Mustang Math is a nonprofit organization of high school and college volunteers that is dedicated to providing middle schoolers access to challenging, interesting, fun, and collaborative math competitions and resources. Having reached over 4000 U.S. competitors and 1150 international competitors in our first six years, we are excited to expand our team to offer our events to even more mathematically inclined students.

PROJECTS
We have worked on various math-related projects. Our annual team math competition, Mustang Math Tournament (MMT) recently ran. We hosted 8 in-person competitions based in Washington, NorCal, SoCal, Illinois, Georgia, Massachusetts, Nevada and New Jersey, as well as an online competition run nationally. In total, we had almost 900 competitors, and the students had glowing reviews of the event. MMT International will once again be running later in August, and with it, we anticipate our contest to reach over a thousand students.

In our classes, we teach students math in fun and engaging math lessons and help them discover the beauty of mathematics. Our aspiring tech team is working on a variety of unique projects like our website and custom test platform. We also have a newsletter, which, combined with our social media presence, helps to keep the mathematics community engaged with cool puzzles, tidbits, and information about the math world! Our design team ensures all our merch and material is aesthetically pleasing.

Some highlights of this past year include 1000+ students in our classes, AMC10 mock with 150+ participants, our monthly newsletter to a subscriber base of 6000+, creating 8 designs for 800 pieces of physical merchandise, as well as improving our custom website (mustangmath.com, 20k visits) and test-taking platform (comp.mt, 6500+ users).

Why Join Mustang Math?

As a non-profit organization on the rise, there are numerous opportunities for volunteers to share ideas and suggest projects that they are interested in. Through our organizational structure, members who are committed have the opportunity to become a part of the leadership team. Overall, working in the Mustang Math team is both a fun and fulfilling experience where volunteers are able to pursue their passion all while learning how to take initiative and work with peers. We welcome everyone interested in joining!

More Information

To learn more, visit https://link.mustangmath.com/RecruitmentInfo. If you have any questions or concerns, please email us at contact@mustangmath.com.

https://link.mustangmath.com/join
0 replies
MustangMathTournament
Yesterday at 7:02 PM
0 replies
Functional Inequality Implies Uniform Sign
peace09   35
N May 20, 2025 by EpicBird08
Source: 2023 ISL A2
Let $\mathbb{R}$ be the set of real numbers. Let $f:\mathbb{R}\rightarrow\mathbb{R}$ be a function such that \[f(x+y)f(x-y)\geqslant f(x)^2-f(y)^2\]for every $x,y\in\mathbb{R}$. Assume that the inequality is strict for some $x_0,y_0\in\mathbb{R}$.

Prove that either $f(x)\geqslant 0$ for every $x\in\mathbb{R}$ or $f(x)\leqslant 0$ for every $x\in\mathbb{R}$.
35 replies
peace09
Jul 17, 2024
EpicBird08
May 20, 2025
Functional Inequality Implies Uniform Sign
G H J
G H BBookmark kLocked kLocked NReply
Source: 2023 ISL A2
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peace09
5419 posts
#1 • 5 Y
Y by OronSH, MarkBcc168, Rounak_iitr, Sedro, anyuhang
Let $\mathbb{R}$ be the set of real numbers. Let $f:\mathbb{R}\rightarrow\mathbb{R}$ be a function such that \[f(x+y)f(x-y)\geqslant f(x)^2-f(y)^2\]for every $x,y\in\mathbb{R}$. Assume that the inequality is strict for some $x_0,y_0\in\mathbb{R}$.

Prove that either $f(x)\geqslant 0$ for every $x\in\mathbb{R}$ or $f(x)\leqslant 0$ for every $x\in\mathbb{R}$.
This post has been edited 2 times. Last edited by peace09, Jul 17, 2024, 12:27 PM
Z K Y
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peace09
5419 posts
#2 • 3 Y
Y by OronSH, Sedro, TensorGuy666
First, letting $y=x$ gives $f(0)f(2x)\ge0$, which implies the result unless $f(0)=0$; henceforth suppose so.

Next, letting $y=-x$ produces $0\ge f(x)^2-f(-x)^2$ or $f(x)^2\le f(-x)^2$; but swapping $x\mapsto-x$ analogously yields $f(-x)^2\le f(x)^2$, and so $f(x)^2=f(-x)^2~(\ast)$.

Then, letting $(x,y)=(x,y),(y,x)$ gives
\begin{align*}
        f(x+y)f(x-y)&\ge f(x)^2-f(y)^2\\
        f(y+x)f(y-x)&\ge f(y)^2-f(x)^2,
    \end{align*}and chaining the $1^\text{st}$ inequality with the negation of the $2^\text{nd}$ produces
\begin{align*}
        f(x+y)f(x-y)\ge-f(x+y)f(y-x)\\
        \iff f(w)f(z)\ge-f(w)f(-z),~(\dagger)
    \end{align*}where in the last step we only redenote $(x+y,x-y)\mapsto(w,z)$ for simplicity.

Now, it is given that $(\dagger)$ is strict for some $(\tilde{w},\tilde{z})$: \[f(\tilde{w})f(\tilde{z})>-f(\tilde{w})f(-\tilde{z}).\]But $(\ast)$ implies that the magnitudes of both sides are equal, and so both $f(\tilde{w})f(\tilde{z})$ and $f(\tilde{w})f(-\tilde{z})$ are strictly positive. In particular, $f(\tilde{z})=f(-\tilde{z})\neq0$.

Finally, letting $z=\tilde{z}$ in $(\dagger)$ yields $f(w)f(\tilde{z})\ge-f(w)f(-\tilde{z})=-f(w)f(\tilde{z})$, i.e., each $f(w)$ has the same sign as $f(\tilde{z})$. $\square$
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OronSH
1748 posts
#3 • 1 Y
Y by peace09
Call the assertion $P(x,y).$ If $P(a,b)$ is strict, then $P(a,b)+P(b,a)$ gives $f(a+b)(f(a-b)+f(b-a))>0,$ so $f(a-b)+f(b-a)\ne 0.$

Now $P\left(\frac{x+a-b}2,\frac{x-a+b}2\right)+P\left(\frac{x-a+b}2,\frac{x+a-b}2\right)$ gives $f(x)(f(a-b)+f(b-a))\ge 0.$ If $f(a-b)+f(b-a)>0$ then $f(x)\ge 0$ for all $x,$and if $f(a-b)+f(b-a)<0$ then $f(x)\le 0$ for all $x.$
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Anzoteh
126 posts
#4 • 20 Y
Y by OronSH, peace09, ehuseyinyigit, kamatadu, navi_09220114, pingupignu, Seicchi28, avisioner, Supertinito, BlazingMuddy, Sedro, CaptainLevi16, SatisfiedMagma, Aryan-23, EpicBird08, alexanderhamilton124, NicoN9, CyclicISLscelesTrapezoid, aidan0626, MS_asdfgzxcvb
I am the first proposer of this problem :) Co-authored with Ivan Chan (who proposed IMO 2023 P3 which imho a lot better than this), and Tristan Chaang (who was the first to supplya clean and neat solution; the original solution by Ivan and me made it looked like it's an A5).

I originally thought of the problem with sign flipped, i.e. $f(x + y) f(x - y) \le f(x)^2 - f(y)^2$, and then realized that leads to equality (i.e. no room on what to do). Then I decided to try the other direction and took me a while to notice the fact that became the problem statement!

Unfortunately I can't further characterize such functions. (What happens if we have $f(x + y) f(x - y) = f(x)^2 - f(y)^2$ and $f(x_0), f(y_0)$ different signs? How about the case where $f(x + y) f(x - y) > f(x)^2 - f(y)^2$ for all $x, y$? Those characterization seems elusive. )
This post has been edited 1 time. Last edited by Anzoteh, Jul 17, 2024, 12:12 PM
Reason: Typo
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kamatadu
480 posts
#5 • 1 Y
Y by SilverBlaze_SY
Thanks to Sammy27 for pointing out the point-wise trap in my previous (fake-)solution :ninja: . Here is the previous solution if anyone wants. incorrect solution

Here is the corrected solution.


Let $P(x,y)$ denote the assertion.

$P(x,x)\implies f(2x)f(0) \ge 0$.

Now if $f(0) \neq 0 $, then we can just divide both sides by $f(0)$ and switch $x \mapsto \frac{x}{2}$ to get that $f(x) \ge 0$ or $f(x) \le 0$ for all $x$.

Otherwise, assume $f(0) = 0$.

$P(x,-x) \implies 0 \ge f(x)^2 - f(-x)^2 \implies f(-x)^2 \ge f(x)^2$.

Now switching $x \mapsto -x$ in the above inequality, we get that $f(x)^2 \ge f(-x)^2$. Adding these two up, we get $f(-x)^2 + f(x)^2 \ge f(x)^2 + f(-x)^2$ which forces that equality holds in both the cases, i.e., $f(x)^2 = f(-x)^2$ for all $x$.

\[
P(x,y) \implies f(x+y)f(x-y) \ge f(x)^2 - f(y)^2.
\]\[
P(-y,x) \implies f(-y + x)f(-y-x) \ge f(-y)^2 - f(x)^2 = f(y)^2 - f(x)^2.
\]Adding these two up, we get that,
\[
f(x-y)(f(x+y) + f(-(x+y))) \ge 0
.\]
Now we substitute $x \mapsto \frac{p+q}{2}$ and $y \mapsto \frac{q-p}{2}$ to get,
\[
f(p)(f(q) + f(-q)) \ge 0
.\]
Now if $f(q)+f(-q) \neq 0$ for some $q$, then we are basically done. FTSOC assume that $f(q) = -f(-q)$ for all $q$, i.e., $f$ is odd.

Then,
\[
P(x,y) \implies f(x+y)f(x-y) \ge f(x)^2 - f(y)^2
.\]
\[
P(y,x) \implies f(y+x)f(y-x) \ge f(y)^2 - f(x)^2 \implies -f(x+y)f(x-y) \ge f(y)^2 - f(x)^2
.\]
Adding these two, we get that $0\ge 0$ which forces the equality of both the equations, that is
\[
f(x+y)f(x-y) = f(x)^2 - f(y)^2
\]for all $x$, $y$. But this is a contradiction because for $(x_0,y_0)$ the equality fails and we are done.



Can someone clarify what the $\gg$ means?

@below, thanks :thumbup:
This post has been edited 7 times. Last edited by kamatadu, Jul 18, 2024, 2:15 PM
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peace09
5419 posts
#6
Y by
kamatadu wrote:
Can someone clarify what the $\gg$ means? I have a solution that seems to be pretty weird :maybe: . Will be posting soon.
Sorry, I meant $\geqslant$, non-strict inequality.
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OptimalFEian
10 posts
#7
Y by
Let $P(x, y)$ denote the given assertion. First, $P(x/2, x/2)$ gives $f(x)f(0) \geq 0$ for all $x$. Thus, we are done if $f(0) \neq 0$. Now, assume $f(0) = 0$. Then, $P(x, -x)$ and $P(-x, x)$ give
\[f(-x)^2 \geq f(x)^2 \geq f(-x)^2 \implies f(x)^2 = f(-x)^2 \enspace \forall x \in \mathbb{R}.\]We have
\begin{align*}
		P(\frac{x+y}{2}, \frac{x-y}{2}): f(x)f(y) \geq f\left(\frac{x+y}{2} \right)^2 - f\left(\frac{x-y}{2} \right)^2
	\end{align*}for all $x, y \in \mathbb{R}$, and denote this by $Q(x, y)$. Suppose $f(-a) = -f(a)$ for some $a$. Then $Q(x, a)$ and $Q(x, -a)$ yield
\begin{align*}
		f(x)f(-a) =f\left(\frac{x-a}{2} \right)^2 - f\left(\frac{x+a}{2} \right)^2 \enspace \forall x \in \mathbb{R}.
	\end{align*}Again, replacing $x$ by $-x$ gives $f(-x) f(-a) = -f(x)f(-a)$. It follows that $f(-x) = -f(x)$ for all real $x$, assuming that $f(a) \neq 0$. Then, for any $x, y \in \mathbb{R}$,
\begin{align*}
		f(y+x)f(y-x)\geq f(y)^2 - f(x)^2 \geq -f(x+y)f(x-y)=f(x+y)f(y-x)
	\end{align*}which forces $P(x, y)$ to be the equality, contradicting the hypothesis.
Hence, we now have $f(-x) = f(x)$ for all $x$. It follows that
\begin{align*}
		f(x)f(-y) \geq f\left(\frac{x-y}{2} \right)^2 - f\left(\frac{x+y}{2} \right)^2 \geq -f(x)f(y).
	\end{align*}That is, $f(x)f(y) \geq 0$ for all real $x, y$. Now, the conclusion follows.
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Marinchoo
407 posts
#8 • 1 Y
Y by isomoBela
Denote by $P(x,y)$ the assertion of $(x,y)$ into the functional equation. Combining the strict $P(x_0, y_0)$ and $P(y_0, x_0)$, we get:
\begin{align*}
    f(x_0+y_0)f(x_0-y_0)&>f(x_0)^2-f(y_0)^2 \\
    f(x_0+y_0)f(y_0-x_0)&\geq f(y_0)^2-f(x_0)^2 \\
    \Longrightarrow f(x_0+y_0)\cdot (f(x_0-y_0)+f(y_0-x_0)) &> 0.
\end{align*}Note that $f$ works iff $-f$ does, so WLOG $f(s)+f(-s)>0$ where $s = x_0-y_0$. Summing $P(x+s,x)$ and $P(x,x+s)$ now shows $f$ is non-negative, as desired.
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ihatemath123
3449 posts
#9 • 1 Y
Y by TensorGuy666
Let $(a,b)$ be a pair that makes the inequality strict. Plugging in the pairs $(a,b)$ and $(b,a)$ and summing gives us
\[ f(a+b) [ f(a-b) + f(b-a) ] > 0 \implies f(a-b)+f(b-a) \neq 0.\]Let $k$ be any constant; plugging in the pairs $(a+k, b+k)$ and $(b+k, a+k)$ and summing gives us
\[ f(a+b+2k) [ f(a-b) + f(b-a) ] \geq 0 \implies \text{sgn} (f(a+b+2k)) = \text{sgn} (f(a-b)+f(b-a))\]Since $k$ varies, $a+b+2k$ can be any real number, so we're done.
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sami1618
913 posts
#10
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We will prove the converse that $f$ achieving both signs implies that the inequality is an equality.

Claim: $f(a)$ and $f(-a)$ are of different signs (if they are not both zero)
Let $b$ be such that $f(a)$ and $f(b)$ have different signs. Then choose $x$ and $y$ such that $x+y=b$ and $x-y=a$. Then $P(x,y)$ gives that $0>f(x)^2-f(y)^2$ so $P(y,x)$ gives that $f(b)f(-a)$ must be positive, as desired.
Claim: $f(0)=0$
If $f(0)\neq 0$ then $P(x,x)$ would contradict our assumption.
Claim: $f(a)=-f(-a)$
Compare $P(a,-a)$ and $P(-a,a)$.

To finish comparing $P(x,y)$ and $P(y,x)$ shows that the inequality must be an equality.
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dkedu
180 posts
#11
Y by
Let $P(x,y)$ denote the assertion.

Rewrite the condition as
\[-f(x+y)f(y-x) \le f(x)^2 - f(y)^2 \le f(x+y)f(x-y)\]\[-f(x+y)f(y-x) \le f(x+y)f(x-y)\]
Case 1: If there is $r$ such that $f(r) = f(-r) \neq 0$, then we have that by $P(\frac{n+r}{2},\frac{n-r}{2})$, which gives that $-f(n)f(-r) \le f(n)f(r)$ which implies that $f(n)$ has the same sign as $f(r)$ which implies $f(x) \le 0$ or $f(x) \ge 0$.

Case 2: If $f(x) = -f(-x)$ for all $x \in \mathbb R$, then we get that $f(x)^2 - f(y)^2 = f(x+y)f(x-y)$ so the inequality is never strict.

Having exhausted all cases, we are done.
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Sammy27
83 posts
#12 • 1 Y
Y by Eka01
Let $P(x, y)$ denote the assertion
$$f(x+y)f(x-y)\geq f(x)^2-f(y)^2.$$
  • $P(x_0,y_0)$ and $P(y_0, x_0)$ give
    $$f(x_0+y_0)f(x_0-y_0)> f(x_0)^2-f(y_0)^2$$and
    $$f(x_0+y_0)f(y_0-x_0)\geq f(y_0)^2-f(x_0)^2,$$respectively. We add them to get
    $$f(x_0+y_0)(f(x_0-y_0)+f(y_0-x_0))>0\implies f(x_0-y_0)+f(y_0-x_0)\neq 0.$$
  • $P\left(\frac{x+x_0-y_0}{2}, \frac{x+y_0-x_0}{2}\right)$ and $P\left(\frac{x+y_0-x_0}{2}, \frac{x+x_0-y_0}{2}\right)$ give
    $$f(x)f(x_0-y_0)\geq f\left(\frac{x+x_0-y_0}{2}\right)^2-f\left(\frac{x+y_0-x_0}{2}\right)^2$$and
    $$f(x)f(y_0-x_0)\geq f\left(\frac{x+y_0-x_0}{2}\right)^2-f\left(\frac{x+x_0-y_0}{2}\right)^2,$$respectively. We add them to get
    $$f(x)(f(x_0-y_0)+f(y_0-x_0))\geq 0,$$so
    $$f(x_0-y_0)+f(y_0-x_0)>0 \implies f(x)\geq 0$$or
    $$f(x_0-y_0)+f(y_0-x_0)<0 \implies f(x)\leq 0$$for all $x\in\mathbb{R}$ as desired, and we are done. $\blacksquare$
This post has been edited 2 times. Last edited by Sammy27, Jul 18, 2024, 5:01 PM
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crocodilepradita
145 posts
#13
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Let $P(x,y)$ be the assertion of $f(x+y)f(x-y)\ge f(x)^2-f(y)^2$.

$P(x_0,y_0) \implies f(x_0+y_0)f(x_0-y_0)>f(x_0)^2-f(y_0)^2$
$P(y_0,x_0) \implies f(x_0+y_0)f(y_0-x_0)\ge f(y_0)^2-f(x_0)^2$

Adding those $2$ inequality yields :
$f(x_0+y_0)[f(x_0-y_0)+f(y_0-x_0)]>0$ ...$(1)$

Claim 1. If $f(0)=0$ then $f(x)^2=f(-x)^2$ for all $x\in \mathbb{R}$
Proof.
Using $P(x,x)$ we get $f(2x)f(0)\ge 0$.
1) If $f(0)>0$ thus $f(2x)\ge 0 \implies f(x)\ge 0$ for all $x\in \mathbb{R}$.
2) If $f(0)<0 $ thus $f(2x)\le 0 \implies f(x) \le 0$ for all $x \in \mathbb{R}$.
3) If $f(0)=0$ then
$P(x,-x) \implies f(x)^2\le f(-x)^2$
$P(-x,x) \implies f(x)^2\ge f(-x)^2$
Therefore, $f(x)^2=f(-x)^2 ,\forall x \in \mathbb{R}$. $\square$

By substituting $x \to x_0-y_0$ we have
$f(x_0-y_0)^2=f(y_0-x_0)^2$
Therefore, $f(x_0-y_0)=f(y_0-x_0)$ or $f(x_0-y_0)=-f(y_0-x_0)$.

Now, consider inequality $(1)$.

If $f(x_0-y_0)=-f(y_0-x_0) \implies 0>0$, contradiction. Therefore, $f(x_0-y_0)=f(y_0-x_0)$.

Claim 2. If there exist $a\in \mathbb{R}$ such that $f(a)=f(-a)\neq 0$, then $f(x)\ge 0$ or $f(x)\le 0$ for all $x \in \mathbb{R}$
Proof.
$P(\frac{a+b}{2},\frac{a-b}{2}) \implies f(a)f(b)\ge f(\frac{a+b}{2})^2-f(\frac{a-b}{2})^2$
$P(\frac{-a+b}{2},\frac{-a-b}{2}) \implies f(-a)f(b) \ge f(\frac{b-a}{2})^2-f(\frac{-a-b}{2})^2=f(\frac{a-b}{2})^2-f(\frac{a+b}{2})^2$
Adding those $2$ inequality yields :
$f(b)[f(a)+f(-a)]\ge 0$

Now, if $f(a)=f(-a)>0$ then $f(b)\ge 0, \forall b \in \mathbb{R} \implies f(x)\ge 0, \forall x \in \mathbb{R} $. If $f(a)=f(-a)<0$ then $f(b)\le 0, \forall b \in \mathbb{R} \implies f(x)\le 0, \forall x \in \mathbb{R}$. $\square$

Now, consider the information $f(x_0-y_0)=f(y_0-x_0)$. We claim that $f(x_0-y_0)=f(y_0-x_0)\neq 0$.

Assume $f(x_0-y_0)=f(y_0-x_0)=0$.

$P(x_0,y_0) \implies f(x_0)^2 < f(y_0)^2$
$P(y_0,x_0) \implies f(x_0) \ge f(y_0)^2$
Contradiction. Therefore $f(x_0-y_0)=f(y_0-x_0)\neq 0$.

Therefore, there exist $a=x_0-y_0$ such that $f(a)=f(-a)\neq 0$. Using Claim $2$, we are finished. Done. $\blacksquare$
This post has been edited 5 times. Last edited by crocodilepradita, Jul 18, 2024, 12:59 AM
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YaoAOPS
1541 posts
#14
Y by
Guess who didn't know what strict meant.


Note that if $f$ is a solution then $c \cdot f$ is a solution. As such, WLOG scale such that $f(1) = 1$.
Denote the assertion with $P(x, y)$. By $P(x, x)$ we get that $f(2x)f(0) \ge 0$ which finishes if $f(0) \ne 0$, so assume $f(0) = 0$.
Now, by $P(x, -x)$ we get that \[ 0 \ge f(x)^2 - f(-x)^2 \]Comparing with $P(-x, x)$, we get that $f(x)^2 = f(-x)^2$.

Claim: $f$ is either even or odd.
Proof. Suppose that $f(a) = f(-a) \ne 0, f(b) = -f(-b) \ne 0$. Then set $x + y = a, x - y = b$. Now $P(x, y), P(-x, -y)$ have LHS with opposite signs so $f(x)^2 - f(y)^2 \le 0$. By comparing $P(y, x), P(-y, -x)$ we get $f(y)^2 - f(x)^2 \le 0$ as well.
As such, $f(x)^2 = f(y)^2$ and thus $f(a)f(b) \ge 0, f(-a)f(-b) \ge 0$, contradiction. $\blacksquare$
First suppose $f$ is odd. Note that by $P(y, x)$ we have that $f(x + y)f(y - x) \ge f(y)^2 - f(x)^2$. Since $f(x + y)f(y - x) = -f(x + y)f(y - x) \le -(f(x)^2 - f(y)^2)$ it follows that equality must hold for all $x, y$, contradiction.
Thus, $f$ is even. Take $x_0, y_0$ such that $f(x_0 + y_0)f(x_0 - y_0) > f(x_0)^2 - f(y_0)^2$.
Now, suppose $f(a)$ has a negative sign. Then take $x + y = a, x - y = 1$.
We then get that \[ f(a) \ge f(x)^2 - f(y)^2 \]By $P(y, x)$, we also get \[ f(a) \le f(x)^2 - f(y)^2 \]This gives a contradiction.
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ericxyzhu
49 posts
#15 • 2 Y
Y by BlazingMuddy, Anzoteh
Anzoteh wrote:
Unfortunately I can't further characterize such functions. (What happens if we have $f(x + y) f(x - y) = f(x)^2 - f(y)^2$ and $f(x_0), f(y_0)$ different signs? How about the case where $f(x + y) f(x - y) > f(x)^2 - f(y)^2$ for all $x, y$? Those characterization seems elusive. )

The problem with the sign being equal was actually already an existing problem from IMOR 2018, proposed by Nuno Arala and Miguel Moreira from Portugal. https://artofproblemsolving.com/community/c6h1673292p10651933
And yes there are unexpected extraneous solutions.
This post has been edited 2 times. Last edited by ericxyzhu, Jul 18, 2024, 5:07 AM
Reason: Typo
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megarnie
5610 posts
#17
Y by
Let $P(x,y)$ denote the given assertion. Suppose that the result was not true and choose reals $a,b$ such that $f(a) < 0$ and $f(b) > 0$. Additionally, choose $c,d$ such that $f(c+d) f(c-d) > f(c)^2 - f(d)^2$.

Claim: $f(0) = 0$
Proof: Suppose otherwise.

$P(x,x): f(2x) f(0) \ge 0$. However, choosing $x = \frac{a}{2}$ implies that $f(a) f(0) \ge 0$, so $f(0) < 0$, but $x = \frac b2$ implies that $f(b) f(0) \ge 0$, so $f(0) > 0$, absurd. Thus, $f(0)$ must equal $0$. $\square$


$P(x,-x): 0 \ge f(x)^2 - f(-x)^2$. But $P(-x,x)$ gives $0\ge f(-x)^2 - f(x)^2$, so this means $f(x)^2 = f(-x)^2$, so either $f(x) = f(-x)$ or $f(x) = -f(-x)$.

Claim: $f(c-d) \ne -f(d-c)$
Proof: Suppose that $f(c-d) = -f(d-c)$. Notice that $f(c+d) f(c-d) > f(c)^2 - f(d)^2$ and $P(d, c)$ gives that $f(c+d)f(d-c) \ge f(d)^2 - f(c)^2$. Now, taking the negative of both sides and flipping the inequality gives that $f(c+d) f(c-d) \le f(c)^2  - f(d)^2$, absurd. $\square$

This implies that $f(c-d) = f(d-c)$.

$P \left( \frac{x + c - d}{2}, \frac{x - (c - d) }{2} \right): f(x) f(c-d) \ge f \left( \frac{x + c - d}{2} \right)^2 - f\left( \frac{x - (c-d)}{2} \right)^2 $

$P \left( \frac{x - (c - d)}{2} , \frac{x + c + d}{2} \right): f(x) f(d-c) = f(x) f(c-d)  \ge f\left( \frac{x - (c-d)}{2} \right)^2  -  f \left( \frac{x + c - d}{2} \right)^2$.

This implies if $k = f \left( \frac{x + c - d}{2} \right)^2 - f\left( \frac{x - (c-d)}{2} \right)^2$, then $f(x) f(c-d) \ge k$ and $f(x) f(c-d) \ge -k$, so $f(x) f(c-d) \ge 0$. However, if $f(c-d) = 0$, then $f(d-c) = 0$, contradiction to our claim that $f(c-d) \ne -f(d-c)$. Therefore, $f(c-d) \ne 0$, choosing $x\in \{a,b\}$ such that $f(x)$ and $f(c-d)$ have opposite signs gives that $f(x) f(c-d) < 0$, contradiction.

Hence we must have $f(x) \ge 0$ always or $f(x) \le 0$ always.
This post has been edited 2 times. Last edited by megarnie, Jul 19, 2024, 10:14 PM
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VicKmath7
1391 posts
#18
Y by
Solution
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SomeonesPenguin
129 posts
#19 • 1 Y
Y by zzSpartan
Interesting FE. :roll:

Solution
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SatisfiedMagma
461 posts
#20 • 2 Y
Y by Funcshun840, H_Taken
Okay retirement days are kinda interesting. I thought I forgot to flip the sign of the inequality somewhere and fakesolved. Turns out I was right all along! Here's a solution which should be right I think.

Solution: Denote $P(x,y)$ as the assertion to the inequality. With the usual swapping trick, consider $P(x,y)$ and $P(y,x)$. This will yield the following two inequalities.
\begin{align*}
f(x+y)f(x-y) &\ge f(x)^2 - f(y)^2 \\
f(x+y)f(y-x) &\ge f(y)^2 - f(x)^2
\end{align*}Add them up and write $y = x + a$ for some arbitrary real $a$.
\[f(2x+a) \left(f(-a) + f(a) \right) \ge 0.\]If $f$ is not an odd function, then we can find some $a_0 \in \mathbb{R}$ such that $f(a_0) + f(-a_0) \ne 0$. Then dividing the above inequality by $\left(f(a_0) + f(-a_0)\right)$ with an appropriate change in sign of inequality, we would be done. Henceforth assume $f$ is an odd function.

Since $f$ is odd, $f(x-y) = -f(y-x)$. From the above aligned equations, we can now say that
\[f(x+y)f(x-y) = f(x)^2 - f(y)^2 \]for all $x,y \in \mathbb{R}$ which is the desired contradiction since there exists some $x_0,y_0 \in \mathbb{R}$ such that there is strict inequality in $P(x,y)$. This completes the solution. $\blacksquare$
This post has been edited 1 time. Last edited by SatisfiedMagma, Aug 21, 2024, 2:28 AM
Reason: simple sign mistake, fixed!
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sansgankrsngupta
147 posts
#21
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OG, Let $P(x,y)$ denote the given assertion.
$P(x,x) \implies$ $f(2x)f(0) \geq 0.$ If $f(0)$ is not 0, then the problem statement is directly implied, Hence, suppose $f(0)=0$
$P(x,-x) \implies$ $(f(x))^2 \geq (f(-x))^2$, replacing $x$ by $-x$ $\implies$ $(f(x))^2 \leq (f(-x))^2$. Thus, $(f(x))^2 =(f(-x))^2$ $\implies$ $f(x)= f(-x) or -f(-x)$. Now, Note that the problem condition can be translated to $f(x)f(y) \geq (f(\frac{x+y}{2}))^2-(f(\frac{x-y}{2}))^2$ for every $x,y\in\mathbb{R}$
Claim: $f(x)=f(-x)$ for all real $x$
Assume a real number $z /neq 0$(as $f(0)=0$) exists such that $f(z)=-f(-z)$.
Choose $x$ and $y$ such that and $x-y=z$
$P(x,y) \implies$
This post has been edited 1 time. Last edited by sansgankrsngupta, Sep 13, 2024, 11:15 AM
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N3bula
288 posts
#22
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Let $P(x, y)$ denote the assertion.
\[P(x_0, y_0)+P(y_0, x_0)\]\[f(x_0+y_0)(f(x_0-y_0)+f(y_0-x_0))>0\]Let $s=x_0-y_0$.
\[P(x, x+s)+P(x+s, x)\]\[f(2x+s)(f(s)+f(-s))\geq 0\]Thus for all $x$ we have the desired result as $(f(s)+f(-s))>0$.
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bobthesmartypants
4337 posts
#23
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Clearly $f$ is not identically 0; since $f$ satisfies the FE iff $-f$ satisfies the FE, WLOG let $f(1)>0$, and denote the FE as $P(x, y)$.

$P(x/2, x/2)\implies f(x)f(0) \ge 0$, so either $f(x)\ge 0$ in which we're done, or $f(0)=0$.

$P(x, -x)\implies 0 \ge f(x)^2-f(-x)^2\implies f(x)^2\le f(-x)^2$. But $f(-x)^2\le f(x)^2$ as well so $|f(x)|=|f(-x)|\quad (*)$.

$P(x, y) + P(y, x)\implies f(x+y)(f(x-y) + f(y-x))\ge 0$, and if we choose such that $x+y=1$ and $x-y=z \in \mathbb{R}$, then $f(z)+f(-z)\ge 0$ for all $z\in \mathbb{R}$, which by $(*)$ implies $f(z)\ge 0$ or $-f(z)=f(-z)\ne 0$. Let the set of all $z$ that satisfy the latter be $N$; note that if $|N|=0$ we're done.

Note that if $x-y$ satisfies $-f(x-y)=f(y-x)$, then $f(x+y)f(x-y) = -f(x+y)f(y-x)\ge f(x)^2-f(y)^2\implies f(x+y)f(y-x)\le f(y)^2-f(x)^2$. But $P(y, x) \implies f(x+y)f(y-x)\ge f(y)^2-f(x)^2$ already so $f(x+y)f(y-x)=f(y)^2-f(x)^2$. But substituting $(x, y) \to (-x, -y)$, we similarly get $f(-x-y)f(x-y) = f(y)^2-f(x)^2\implies f(-x-y)f(y-x) = f(x)^2-f(y)^2\quad (**)$. If we choose to further specify that $x-y\in N$, adding with the previous equality, gives $$(f(x+y)+f(-x-y))f(y-x) = 0\implies -f(x+y)=f(-(x+y))$$as $f(y-x) \ne 0$. Thus, if $|N|>0$, then $f(x)=-f(x)$ for all $x\in \mathbb{R}$.

This finally gives a contradiction as this would imply, by $(**)$, that $f(-x-y)f(y-x)=f(x)^2-f(y)^2$ for all $x, y\in \mathbb{R}$; yet the problem statement says there should exist $x_0, y_0$ for which this is not true. Thus, $|N|=0$ and we conclude $f(z)\ge 0$ for all $z\in \mathbb{R}$. $\blacksquare$
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poirasss
1 post
#26 • 1 Y
Y by bin_sherlo
Putting $y=x$ gives us $f(2x)f(0) \geq 0 $. This gives $f(0)=0$, if not, then for all $x$, the sign of $f(x)$ doesn't change.
Then setting $x=-y$ gives $0\geq f(-y)^2-f(y)^2$, changing $y$ with $ -y$ gives us $f(y)^2\geq f(-y)^2\geq f(y)^2$, which implies $|f(x)|=|f(-x)|$ and $f(x)^2=f(-x)^2$.
Then swapping $-x$ as $x$ gives $f(y-x)f(-x-y)\geq f(x)^2-f(y)^2$ .
Swapping $x$ and $y$ gives $f(x-y)f(-x-y)\geq f(y)^2-f(x)^2$ which means $f(x)^2-f(y)^2\geq -f(x-y)f(-x-y)$.

Assume $f(a)>0>f(b)$. Choose $x$ and $y$ as $x-y=a$, $x+y=c$, where c is any real number.
$f(x+y)f(x-y)\geq f(x)^2-f(y)^2\geq -f(x-y)f(-x-y)$
$\Rightarrow f(c)f(a) \geq -f(a)f(-c)$
$\Rightarrow f(c) \geq -f(-c)$ Putting $-c$ as $c$ gives $f(c) \geq -f(-c) \geq f(c)$ which implies $f(c)=-f(-c)$.
It is given that $f(x_0+y_0)f(x_0-y_0)>f(x_0)^2-f(y_0)^2$ , $f(y_0+x_0)f(y_0-x_0)\geq f(y_0)^2-f(x_0)^2$.
So, $f(x_0+y_0)f(x_0-y_0)>f(x_0)^2-f(y_0)^2 \geq f(x_0+y_0)f(x_0+y_0)$, which gives us contradiction. $\blacksquare$
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InterLoop
279 posts
#27
Y by
kawaii
solution
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HamstPan38825
8868 posts
#28
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By setting $x=y$, $f(2x)f(0) \geq 0$. Thus, if $f(0) \neq 0$, we may assume without loss of generality that $f(0) > 0$, from where $f(2x) \geq 0$ for all real numbers $x$, so the desired result is true.

If $f(0) = 0$, then \[f(x)^2-f(-x)^2 \leq f(0)f(2x) = 0.\]In particular, swapping $x$ and $-x$, it follows that $f(x)^2 = f(-x)^2$. Now, summing the equations for $(x, y)$ and $(y, x)$,
\[f(x+y)(f(x-y) + f(y-x)) \geq f(x)^2-f(y)^2+f(y)^2-f(x)^2 = 0.\]In particular, there is $(x_0, y_0)$ such that this inequality is strict; i.e. for this pair $(x_0, y_0)$, it follows that $f(c) = f(-c) \neq 0$ where $c = x_0-y_0$. However, for all pairs $(x, x+c)$, it follows that
\[f(2x+c)(f(c)+f(c))) \geq 0\]by the same argument, hence if $f(c) > 0$, $f(2x+c) \geq 0$ for all $x$ and vice versa.
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AshAuktober
1009 posts
#29
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\subsection{Sol sketch}
We prove the contrapositive, i. e. given $a, b$ with $f(a) > 0, f(b) < 0$, equality must always hold.
The main claims are:
\begin{enumerate}
\item $f(0) = 0$
\item $f(x)^2 = f(-x)^2$.
\end{enumerate}
From here, swap $x, y$ to get $$f(x+y)(f(x-y) + f(y-x)) \ge 0.$$If $f(x-y) = -f(y-x)$, we're done as equality holds and thus must hold in the original equation.
Else if $f(x-y) = f(y-x)$, choosing $x, y$ such that $x-y$ comes out to the value required and $x+ y = a$, we have $f(x-y) \ge 0$. But doing the same such that $x+y = b$, $f(x-y) \le 0$.Thus $f(x-y) = 0$, which would mean (yet again) that equality holds. Thus we're done,
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Mr.Sharkman
501 posts
#30
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Claim: $f(0) = 0$ or the problem is true.

Proof: Applying $P(x,x),$ we get
$$f(2x)f(0) \ge f(x)^{2}-f(x)^{2} =0,$$so if $f(0) \neq 0,$ then all of $f(2x)$ have the same sign, and we are done.

Because of this, we can assume that $f(0)= 0. $

Claim: For all $x,$ $f(x)^{2} = f(-x)^{2}.$

Proof: Notice that, by $P(-x, x),$ we get $$f(0)f(-2x) \ge f(-x)^{2}-f(x)^{2},$$so $f(x)^{2} \ge f(-x)^{2}.$ Also, $f(-x)^{2} \ge f(x)^{2},$ so $f(x)^{2} = f(-x)^{2},$ as desired.

Now, by applying $P(x,y), P(y,x),$ and adding these together, we get
$$f(x-y)f(x+y)+f(y-x)f(x+y)\ge 0 \implies (f(x-y)+f(y-x))f(x+y) \ge 0.$$and we will call this assertion $Q(x,y).$

Claim: If $f(a) = f(-a)$ for some $a,$ then we are done.

Proof: Notice that taking $Q\left(\frac{a+b}{2},\frac{b-a}{2}  \right)$ gives
$$(f(a)+f(-a))f(b)\ge 0 \implies f(a)f(b) \ge 0.$$Since $b$ was arbitrary, we are done. $\blacksquare$

So, now, we need to find such an $a.$

Claim: $a = x_{0}-y_{0}$ satisfies $f(a) = f(-a).$

Proof: Assume FTSOC that $f(a)+f(-a)=0.$ Then, $Q$ would be at an equality case. However, since $Q$ is the sum of two inequalities, one of which is $P(x_{0}, y_{0}),$ which is not an equality case, we are done. $\blacksquare$
Thus, we have completed the proof. $\blacksquare$
This post has been edited 1 time. Last edited by Mr.Sharkman, Jan 29, 2025, 2:41 AM
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dubabuba
1 post
#31
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Call the assertion $P(x, y)$.

Summing up $P(x, y)$ and $P(y, x)$ gives us $f(x + y) (f(x - y) + f(y - x)) \ge 0$.
For any $a, b \in \mathbb{R}$, there exists $x, y \in \mathbb{R}$ such that $a = x + y$ and $b = x - y$.
Therefore, $f(a) (f(b) + f(-b)) \ge 0$ for any $ a, b \in \mathbb{R} ~(\star) $.

Denote $a_0 = x_0 + y_0, b_0 = x_0 - y_0$.
Since the given inequality is strict for $(x_0, y_0)$, $~(\star)$ is also strict for $(a_0, b_0)$.
In otherwords, there exists $a, b \in \mathbb{R}$ such that $f(a)(f(b) + f(-b)) > 0$.

Suppose that $f(a_0) > 0$. This also implies $f(b_0) + f(-b_0) > 0$.
Plugging $(x, b_0)$ in $~(\star)$ gives $f(x)(f(b_0) + f(-b_0)) \ge 0 \Longrightarrow \forall x \in \mathbb{R}: f(x) \ge 0$.
The case $f(a_0) < 0$ can be done in a similar way.
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pie854
243 posts
#32
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Putting $x=y$ we get that $f(2x)f(0)\geq 0$ and so we are done if $f(0)\neq 0$. Suppose $f(0)=0$. Putting $x=-y$ we get $f(y)^2\geq f(-y)^2$ and so, since this holds for all $y$, $f(y)^2=f(-y)^2$.

Suppose there exists some $t$ with $f(t)\neq -f(-t)$ (in particular, note that $f(t)\neq 0$). Putting $x=\frac{a+t}2$, $y=\frac{a-t}2$ we get $$f(a)f(t)\geq f\left (\frac{a+t}2\right)^2-f\left (\frac{a-t}2\right)^2$$and putting $x=\frac{a-t}2$, $y=\frac{a+t}2$ we get $$f(a)f(t)\geq f\left (\frac{a-t}2\right)^2-f\left (\frac{a+t}2\right)^2.$$Thus, $f(a)f(t)\geq 0$ for all $a$ and we're done.

Now suppose $f(x)=-f(-x)$ for all real $x$. Putting $(x,y) \mapsto (y,x)$ we get $$-f(x+y)f(x-y)=f(y+x)f(y-x)\geq f(y)^2-f(x)^2 \implies f(x+y)f(x-y)\leq f(x)^2-f(y)^2,$$thus $f(x+y)f(x-y)=f(x)^2-f(y)^2$ for all $x,y$. But this is a contradiction with the assumption that the inequality is strict for some $x_0,y_0$.
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Ilikeminecraft
658 posts
#33
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what

Rewrite as $f(a)f(b)\geq f\left(\frac{a + b}2\right)^2 + f\left(\frac{a - b}2\right)^2.$ Plug in $b\mapsto-b,$ and add to get $f(a)(f(b) + f(-b))\geq0.$ Since there exist $a_0, b_0$ such that the inequality is strict, we have $f(a_0) + f(-a_0)>0$ or $f(a_0) + f(-a_0) < 0.$ The two cases are the same, so we assume it is positive. Plug in $b = a_0$ and we get $f(a)\geq0.$
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Nari_Tom
117 posts
#34
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We have that: $f(x+y)f(x-y) \geq f(x)^2-f(y)^2$.

By swapping (x,y) we will get that: $f(x+y)f(y-x) \geq f(y)^2-f(x)^2$.

Combining these we get: $f(x+y)[f(x-y)+f(y-x)] \geq 0$. If we take $(x=\frac{a+b}{2}, y=\frac{a-b}{2})$, then last inequality implies $f(a)[f(b)+f(-b)] \geq 0$, for all $a,b \in \mathbb{R}$. If there exist $c \in \mathbb {R}$ such that $f(c)+f(-c) \neq 0$, then $f(a)$ have same sign with $f(c)+f(-c)$.

But there exist such $c$, since $f(x_0+y_0)[f(x_0-y_0)+f(y_0-x_0)]>0$.
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Andyexists
8 posts
#35
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For $x = y$, we get $f(2x)f(0) \geq f(x)^2 - f(x)^2 = 0$. If $f(0) \neq 0$, this implies $f(x)$ and $f(0)$ have the same sign, which finishes the problem.
Otherwise, $f(0) = 0$. Taking $y = -x$, we get $f(0)f(2x) \geq f(x)^2 - f(-x)^2$, from which $f(-x)^2 \geq f(x)^2$. Replacing $x$ with $-x$, we get $f(x)^2 \geq f(-x)^2$, and from the two inequalities we get $f(-x)^2 = f(x)^2, \forall x \in \mathbb{R}$.

This implies $f(-x) = \pm f(x)$. Replacing $x$ and $y$ in the statement, we get $f(y+x)f(y-x) \geq f(y)^2 - f(x)^2$, and by summing with $f(x+y)f(x-y) \geq f(x)^2 - f(y)^2$, we have \[f(x+y)(f(x-y) + f(y-x)) \geq.\]We know there exist $x_0, y_0$ for which the inequality in the statement is strict, which means there exist $x_0, y_0$ for which $f(x_0+y_0)(f(x_0-y_0) + f(y_0-x_0)) > 0$, which implies $f(x_0 - y_0) = f(y_0 - x_0)$, otherwise their sum is null. Let $d = x_0 - y_0$. Then $f(x + y)f(d) \geq 0$, for any $x+y$ with $|x - y| = d$. Let $x = d + y$, then $f(2y+d)f(d) \geq 0$, so $f(d + 2y)$ and $f(d)$ have the same sign, which solves the problem, as $2y + d$ takes any real value.
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lelouchvigeo
183 posts
#36
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Let $P(x,y) $ denote assertion
$P(x,0) : f(2x)f(0)  \geq 0$
Observe if $f(0) \neq 0$ we are done, assume $f(0) = 0 $
$f(x,-x) : f(2x) f(0) \geq 0 \geq  f(x)^2 - f(-x)^2 $
$f(-x,x) : 0 \geq   f(-x)^2  - f(x)^2$
$\implies f(x) = \pm f(-x) $
Let$ x+y = c = x_0 + y_0 $
$P(x,y) : f(x_0 + y_0 ) f(x-y) \geq f(x)^2 -f(y)^2 , f(x)^2 \geq f(y)^2.$ From this we can see that $f(x-y) $ has same sign as $f(x_0 + y_0 )$ or is equal to 0
If $f(x_y) \neq 0 ,$ then $P(y,x) : f(x_0 + y_0 ) f(y-x) \geq f(y)^2 -f(x)^2, \implies f(x-y) = f(y-x) \implies f(x) = f(-x) $
We are done
This post has been edited 1 time. Last edited by lelouchvigeo, May 9, 2025, 8:02 AM
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ezpotd
1293 posts
#37
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Assume that $f$ is not nonnegative or nonpositive. $f$ is obviously not universally zero.

Claim: $f(0) = 0$.
Proof: Take $x =y$, then $f(2x)f(0) \ge 0$, if $f(0) \neq 0$ the expression on the left can have either sign, so $f(0) = 0$.

Claim: $|f(x)| = |f(-x)|$
Proof: Take $x = -y$, then $0 \ge f(x)^2 - f(-x)^2, 0 \ge f(-x)^2 - f(x)^2$, so $f(x)^2 = f(-x)^2$.

Claim: $f$ is odd.
Proof: Take $a, b$ with $f(a + b) > 0 > f(a - b)$, which is possible by our assumption. Then $f(a)^2 < f(b)^2$, so $f(a + b)f(b - a) > 0$, so $f(b - a)$ is positive. Now take any $k = f(x - y)$, if $k > 0$, set $m,n$ with $m - n = x - y$, $m + n = a-b$, thus $0 > f(m + n)f(m- n) > f(m)^2 - f(n)^2$. Then $f(m + n)f(n - m) > f(n)^2 - f(m)^2 > 0$, so $f(n - m)$ is negative. Combining this with the above claim gives $f(x  -y) = -f(y - x)$ for $f(x - y) > 0$. If $k < 0$, set $m -n = x - y, m + n = b - a$, then $ 0>f(m + n)f(m - n) > f(m)^2 - f(n)^2$, then $f(m + n)f(n - m) > f(n)^2 - f(m)^2 > 0$, so $f(n - m)$ is positive, combining this with the above claim gives $f(x - y) = -f(y - x)$ for all values of $f(x - y)$, thus it is always true.

To finish, taking $x = -y, y = -x$ gives $f(-x-y)f(x-y) \ge -(f(x)^2 - f(y)^2)$, forcing $f(x+ y) f(x - y) \le f(x)^2 - f(y)^2$, implying that the inequality is never strict, giving the desired contradiction.
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MathIQ.
46 posts
#38
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Let $P(x,y)$ be the assertion $f(x+y)f(x-y) \ge f(x)^2 - f(y)^2$.

$P(x,x) \implies f(2x)f(0) \ge f(x)^2 - f(x)^2 = 0$.
So, for any $z \in \mathbb{R}$ (by letting $x=z/2$), we have $f(z)f(0) \ge 0$.

If $f(x)$ was identically zero, i.e., $f(x)=0$ for all $x \in \mathbb{R}$, then $0 \cdot 0 \ge 0^2 - 0^2$, which is $0 \ge 0$. This holds with equality for all $x,y$. However, the problem states that the inequality is strict for some $x_0, y_0 \in \mathbb{R}$. This means $f(x)$ cannot be identically zero.

Now, we establish that $f(0) \neq 0$.
Assume, for contradiction, that $f(0)=0$.
The condition $P(x,x)$ then becomes $f(2x) \cdot 0 \ge 0$, which is $0 \ge 0$. This provides no information on its own.
The condition $P(x,0)$ gives $f(x)^2 \ge f(x)^2 - f(0)^2 \implies f(x)^2 \ge f(x)^2$, also equality if $f(0)=0$.
The condition $P(0,y)$ gives $f(y)f(-y) \ge f(0)^2 - f(y)^2 \implies f(y)f(-y) \ge -f(y)^2$.
It is known that if $f(0)=0$ and $f$ satisfies $f(x+y)f(x-y) = f(x)^2 - f(y)^2$ (i.e., equality everywhere), then typical solutions (often requiring continuity, though this problem doesn't state it) are $f(x)=ax$, $f(x)=a\sin(kx)$, or $f(x)=a\sinh(kx)$. For these functions, the equality $f(x+y)f(x-y) = f(x)^2 - f(y)^2$ holds for all $x,y$.
If $f(0)=0$ and the given inequality $f(x+y)f(x-y) \ge f(x)^2 - f(y)^2$ were to hold with equality for all $x,y$, this would contradict the given condition that the inequality is strict for some $x_0, y_0$.
A stronger argument for $f(0) \neq 0$: if $f(0)=0$, then the original inequality for $(x_0,0)$ would be $f(x_0)^2 \ge f(x_0)^2 - 0^2$, i.e., $f(x_0)^2 \ge f(x_0)^2$. This can't be the strict inequality. So $y_0 \ne 0$. Similarly for $(0,y_0)$, $f(y_0)f(-y_0) \ge -f(y_0)^2$. If this were the strict case $f(y_0)f(-y_0)>-f(y_0)^2$, it doesn't force $f(0) \ne 0$.
However, if the pair $(x_0,x_0)$ (for $x_0 \ne 0$) gives the strict inequality, $f(2x_0)f(0) > f(x_0)^2-f(x_0)^2=0$. This would directly imply $f(0) \ne 0$. The problem states "for some $x_0, y_0$". If this pair can be $y_0=x_0$, then $f(0) \ne 0$. If it cannot be $y_0=x_0$ (because $f(2x)f(0)=0$ for all $x$), then $f(0)=0$ (as $f$ is not identically zero). This implies $f(x+y)f(x-y) = f(x)^2-f(y)^2$ must hold with equality for all $x,y$ to satisfy $f(2x)f(0)=0$ when $f(0)=0$, which contradicts strict inequality.
Thus, the strict inequality condition must imply $f(0) \neq 0$.

Since $f(0) \neq 0$:
From $f(z)f(0) \ge 0$ for all $z \in \mathbb{R}$:
If $f(0) > 0$, then $f(z) \ge 0$ for all $z \in \mathbb{R}$
If $f(0) < 0$, then $f(z) \le 0$ for all $z \in \mathbb{R}$
This completes the proof.
This post has been edited 2 times. Last edited by MathIQ., May 18, 2025, 9:37 PM
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EpicBird08
1755 posts
#39
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Let $P(x,y)$ denote the given inequality.

First, $P(x/2,x/2)$ gives $f(x) f(0) \ge 0.$ If $f(0) \ne 0,$ then we are automatically done. Henceforth assume $f(0) = 0.$

Claim 1: $f(x)^2 = f(-x)^2$ for all $x.$
Proof: Use $P(x,-x)$ to get $0 \ge f(x)^2 - f(-x)^2,$ and use $P(-x,x)$ to get $0 \ge f(-x)^2 - f(x)^2.$ Comparing the two gives $f(x)^2 = f(-x)^2,$ as claimed.

Claim 2: There exists a $b$ such that $f(b) = f(-b) \ne 0.$
Proof: Use $P(x_0, y_0)$ and $P(y_0, x_0)$ along with the given condition to get
\begin{align*}
f(x_0 + y_0) f(x_0 - y_0) &> f(x_0)^2 - f(y_0)^2, \\
f(x_0 + y_0) f(y_0 - x_0) &\ge f(y_0)^2 - f(x_0)^2.
\end{align*}
Adding the two gives us $$f(x_0 + y_0) ((f(x_0 - y_0) + f(y_0 - x_0)) > 0.$$If $f(x_0 - y_0) = -f(y_0 - x_0),$ then that gives $0 > 0,$ a contradiction. Hence we must have $f(x_0 - y_0) = f(y_0 - x_0)$ and $f(x_0 + y_0) f(x_0 - y_0) > 0,$ so using $b = x_0 - y_0$ suffices.

Now, $P\left(\frac{x+b}{2}, \frac{x-b}{2}\right)$ and $P\left(\frac{x-b}{2}, \frac{x+b}{2}\right)$ give
\begin{align*}
f(x) f(b) &\ge f\left(\frac{x+b}{2}\right)^2 - f\left(\frac{x-b}{2}\right)^2, \\
f(x) f(-b) &\ge f\left(\frac{x-b}{2}\right)^2 - f\left(\frac{x+b}{2}\right)^2.
\end{align*}Adding the two gives $f(x) ((f(b) + f(-b)) = 2 f(x) f(b) \ge 0.$ If $f(b) < 0,$ then we get $f(x) \le 0$ for all $x.$ If $f(b) > 0,$ then we get $f(x) \ge 0$ for all $x.$ Therefore, we are done.
This post has been edited 1 time. Last edited by EpicBird08, May 20, 2025, 7:48 PM
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