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mtayab1994
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Homework Statement
for every (x,y) in ℝ^2:
f(x^2+f(y))= y-x^2
Homework Equations
Find all functions.
The Attempt at a Solution
I was wondering if rewriting it as x^2+f(y)=f^-1(y-x^2)?
Yes that's what it is. Find all functions satisfying that condition.deluks917 said:I believe the idea is to find all functions satisfying that condition.
I don't get what you're trying to tell me with gx∘gx.micromass said:Note that
[tex]x^2+f(x^2+f(y))=y[/tex]
So if we define
[tex]g_x(y):=x^2+f(y)[/tex]
Then what can you say about [itex]g_x\circ g_x[/itex]?
I can't get this too. Maybe one should apply the derivative ?Then what can you say about [itex]g_x\circ g_x[/itex]?
Dick said:I'd try just differentiating both sides with respect to x.
mtayab1994 said:I don't get what you're trying to tell me with gx∘gx.
I understand that its a category from gx to gx, so u can say: gx(g(x)). right?
Dick said:I'd try just differentiating both sides with respect to x.
Quinzio said:I'm sorry for my intrusion, but after having spent some time on this I would like to add my comment.
From intuition and some trials I can say the [itex]f(x) = -x[/itex] works.
So that:
[itex]f(y) = -y[/itex]
[itex]f(y) + x^2= -y+x^2[/itex]
[itex]f(f(y) + x^2)= y-x^2[/itex]
But is there an analytical or mechanical way to solve these problems ?
I can't get this too. Maybe one should apply the derivative ?
micromass said:Yes, but what if f is not differentiable??
Quinzio said:But it's not a function of 2 variables. There's only one "input".
I don't know that's what I'm trying to reach but i also got this one:Quinzio said:But the problem says "find all the solutions". So there are more than 1 ? What the others look like then ?
mtayab1994 said:I don't know that's what I'm trying to reach but i also got this one:
y=f^-1(f^-1(y-x^2)-x^2)
micromass said:Yes, but you got to watch out. f is not necessarily invertible. So the function [itex]f^{-1}[/itex] might not exist!
Not until you show f to be invertible. What happens if x=0 ?Yes but is it correct?
mtayab1994 said:for x=0 you get fof(y)=y and f(y)=f^-1(y)
x^2=f(-x^2)micromass said:Indeed! So f is its now inverse!
Now, take your original relationship again
[tex]f(x^2+f(y))=y-x^2[/tex]
Now take f of both sides. What happens if y=0?
mtayab1994 said:x^2=f(-x^2)
micromass said:No, that's not what I get. How did you get that?
mtayab1994 said:did u get f(y)=f(-x^2)-x^2?
micromass said:No. Remember that we put y=0!
mtayab1994 said:idk but i keep getting x^2=f(-x^2)
micromass said:Can you post how you get there??
micromass said:In your first line you use f(a+b)=f(a)+f(b). But you don't know whether that property holds! So you can't do that!
mtayab1994 said:What solution did u get for Y=0 just curious to know.
I can't grasp what you mean by if you take the f of both sides?micromass said:Hold on, let me guide you there step by step.
So, you have
[tex]f(x^2+f(y))=y-x^2[/tex]
What happens if you take f of both sides??
mtayab1994 said:I can't grasp what you mean by if you take the f of both sides?
micromass said:For example, if we have a=b. If we take f of both sides, then we have f(a)=f(b).
mtayab1994 said:ooohh alright i got: fof(x^2+f(y))=f(y-x^2)