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Algebra Difficulty 6.0 AIME, harder Prove it Ukraine

Determine all strictly increasing functions f:RRf: \mathbb{R} \to \mathbb{R} that satisfies the conditions:
f(x3+y2+f(y))=x2f(x)+yf(y)+y; \bullet \quad f(x^3 + y^2 + f(y)) = x^2 f(x) + y f(y) + y;
Equality y2+f(y)=t has a solution for every tR. \bullet \quad \text{Equality } y^2 + f(y) = t \text{ has a solution for every } t \in \mathbb{R}.

Solution

Answer. Solution does not exist.

Let us substitute x=y=0x = y = 0 at the first condition f(f(0))=0\Rightarrow f(f(0)) = 0.
Then when x=0x=0 and when x=f(0)x=f(0) we obtain such equations:
f(y2+f(y))=yf(y)+ytaf(f3(0)+y2+f(y))=yf(y)+y. f(y^2 + f(y)) = y f(y) + y \quad \text{ta} \quad f(f^3(0) + y^2 + f(y)) = y f(y) + y.
Since ff is strictly increasing:
f(y2+f(y))=f(f3(0)+y2+f(y))f(0)=0. f(y^2 + f(y)) = f(f^3(0) + y^2 + f(y)) \Rightarrow f(0) = 0.
If we substitute x=0x=0 and y=0y=0, we obtain
f(y2+f(y))=yf(y)+ytaf(x3)=x2f(x)f(x3+y2+f(y))=f(x3)+f(y2+f(y)). \begin{aligned} f(y^2 + f(y)) &= y f(y) + y \quad \text{ta} \quad f(x^3) = x^2 f(x) \\ f(x^3 + y^2 + f(y)) &= f(x^3) + f(y^2 + f(y)). \end{aligned}
Since y2+f(y)y^2 + f(y) can equal any positive value (from the second condition), the last equation can be rewritten in such a way: f(x+t)=f(x)+f(t)f(x+t) = f(x) + f(t), where xR,t0x \in \mathbb{R}, t \ge 0.

Therefore, ff is increasing and satisfies Cauchy's functional equation, thus f(x)=kxf(x) = kx for some real kk. After checking the first condition it is clear that only case k=1k=1 is correct, but such a function does not satisfy the second condition.

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