Baltic Way 2014

by utkarshgupta, Nov 4, 2015, 1:23 PM

I hate to post this as it is too easy for a new entry after such a long time but I hate to see my blog unupdated for such a long time.

Problem (Baltic Way 2014):
Find all functions $f$ defined on all real numbers and taking real values such that $f(f(y)) + f(x - y) = f(xf(y) - x),$ for all real numbers $x, y.$

Solution :
Let $P(x,y)$ be the assertion $f(f(y)) + f(x - y) = f(xf(y) - x)$

$f \equiv 1$ isn't obviously a solution.
Thus there exists some $a \in \mathbb R$ such that $f(a) \neq 1$

$P(\frac{f(a)}{f(a)-1},a) \implies$
$f(\frac{f(a)}{f(a)-1}-a)=0$
$implies$ there exists a $k$ such that $f(k)=0$

$P(x,k) \implies f(0)+f(x-k)=f(-x)$
$P(0.k) \implies f(-k)=0$
$P(k,k) \implies f(0)=0$

$\implies f(0)=0$

$P(0,y) \implies f(f(y)) + f(-y) = 0$
$P(\frac{-y}{f(y)-2},y) \implies f(y)=0$ for all $f(y) \neq 2$

Now let there exist some $t \in R$ such that $f(t)=2$
$f(f(t))= - f(t) = -2$
But we know that that for all $f(x) \neq 2$ , we have $f(x)=0$

A contradiction.

$\implies \boxed{f \equiv 0}$ which indeed is a solution.

functional equation

easy

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Here goes first post of 2025! Great blog.
by math_holmes15, Jan 14, 2025, 8:53 AM
First post of 2024
by Yiyj1, Feb 8, 2024, 5:40 AM
First post of 2023
by HoRI_DA_GRe8, Jul 22, 2023, 7:45 AM
Nice blog ! Your isogonality lemma is really powerful !
by 554183, Oct 14, 2021, 8:55 AM
Post plss....
by samrocksnature, Apr 11, 2021, 10:12 PM
alas,this is ded
by Hamroldt, Mar 18, 2021, 4:13 PM
Thanks for the nice blog.
by Feridimo, Mar 6, 2020, 4:17 PM
I think this might be silly but ... when should we expect to have another post ?? I am very keen to see it
by gamerrk1004, Nov 4, 2019, 4:54 PM
Let's all echo what's written in the blog description - Stay Insane / 'Cause it's your labor, will and pain/ That takes you to the top of soda fountain
by Kayak, Oct 2, 2017, 7:18 PM
hey utkarsh jee is over now ... continue your elementary blog pleaseeeeeee!
by kk108, Jun 17, 2017, 11:19 AM
Congrats on becoming a contest moderator!
by Ankoganit, Mar 9, 2017, 5:22 AM
INTERSTING BLOG
by kk108, Feb 19, 2017, 2:04 PM
I have no plans for this blog right now....
No time here people !
But lets see....
I may try some combinatorics
by utkarshgupta, Feb 15, 2017, 12:47 PM
Thanks for the nice blog!
by Orkhan-Ashraf_2002, Feb 13, 2017, 6:34 PM
Revive it!!!
Best blog out there, for sure!
by rmtf1111, Jan 12, 2017, 6:02 PM
Oh you certainly will..
Bu I don't think I will
by achilles04, Feb 11, 2015, 6:24 PM
Thanks
Hope you are there too ...

And hope I get selected
by utkarshgupta, Feb 11, 2015, 11:14 AM
Nice blog ..
keep it up and you might one day become a mathematician lIke me
( just joking )
Btw don't forget us after going to imotc.
by achilles04, Feb 10, 2015, 4:49 PM
nice blog!!!!!
by pradhit, Feb 7, 2015, 11:51 AM
No ideas about the cut off but should be less than 60 according to me...
So you never know
by utkarshgupta, Feb 6, 2015, 4:06 PM
Hi utkarsh,
what do u expect the cutoff to be this year??

by kgdpsdurg, Feb 6, 2015, 12:41 PM
Hi utkarsh
How many did you clear in INMO 2015?
by moldevort, Feb 1, 2015, 3:59 PM
@ Anonymous Bunny
If we both (that should have included me) are lucky enough !
by utkarshgupta, Sep 17, 2014, 2:44 AM
Nice blog. Great job!

If I am lucky enough, hopefully we shall meet at IMOTC.
by AnonymousBunny, Sep 16, 2014, 8:08 PM
Thanx all !!

If anyone wish to contribute anything just pm !

I will add u to the contributors' list !
by utkarshgupta, Sep 16, 2014, 6:35 AM
doing good progress!! btw nice blog.
by rachitgoel, Sep 15, 2014, 4:21 PM
Nice blog.
by Kezer, Aug 5, 2014, 4:04 PM
Hi Utkarsh! Wonderful blog. Keep it up.
by YESMAths, Jul 20, 2014, 1:36 PM
I need some shouts and characters

by utkarshgupta, Jul 20, 2014, 4:22 AM

48 shouts

Elementary

Baltic Way 2014

by utkarshgupta, Nov 4, 2015, 1:23 PM

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