Find all The Functions !

**Perelman** · February 5th 2010, 09:24 AM

Hii !

Can You Help Me For This Exercice :

Find all The Functions $f$ : $\mathbb{R}$ $\longrightarrow$ $\mathbb{R}$ Such as :

$f((f(x)+y)=f(x^2-y)+4f(x)y$

**Opalg** · February 5th 2010, 11:01 AM

Originally Posted by Perelman

Hii !

Can You Help Me For This Exercice :

Find all The Functions $f$ : $\mathbb{R}$ $\longrightarrow$ $\mathbb{R}$ Such as :

$f((f(x)+y)=f(x^2-y)+4f(x)y$

Put $y=-f(x)$ :

$(1)\qquad f(0) = f(x^2 + f(x)) - 4(f(x))^2.$

Put $y=x^2$ :

$(2)\qquad f(f(x)+x^2) = f(0) + 4f(x)x^2.$

Combine (1) and (2) to get $f(x)\bigl(x^2-f(x)\bigr) = 0.$

That gives you two solutions: $f(x) = 0$ and $f(x)=x^2$ . I'll leave you to think about whether or not there are also hybrid solutions, where f(x) is 0 for some values of x, and x^2 for other values of x.

**Media_Man** · February 5th 2010, 03:50 PM

A particular solution is $f(x)=x^2$ .

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