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Algebra Difficulty 6.7 National olympiad Find the answer

Let R\mathbb{R} denote the set of the reals. Find all f:RRf : \mathbb{R} \to \mathbb{R} such that
f(x)f(y)=xf(f(yx))+xf(2x)+f(x2) f(x)f(y) = xf(f(y-x)) + xf(2x) + f(x^2)
for all real x,yx, y.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the functional equation for f:RR f : \mathbb{R} \to \mathbb{R} ,

f(x)f(y)=xf(f(yx))+xf(2x)+f(x2), f(x)f(y) = xf(f(y-x)) + xf(2x) + f(x^2),

for all real x,y x, y , we proceed as follows:

1. **Substitute y=0 y = 0 :**
Considering y=0 y = 0 , the equation becomes:
f(x)f(0)=xf(f(x))+xf(2x)+f(x2). f(x)f(0) = xf(f(-x)) + xf(2x) + f(x^2).
Notice that if f f is a constant zero function, f(x)=0 f(x) = 0 for all x x , then the equation holds trivially. Hence, f(x)=0 f(x) = 0 is a solution.

2. Check for non-zero solutions:
Assume there exists x0 x_0 such that f(x0)0 f(x_0) \neq 0 . Then f(y) f(y) cannot be zero for all y y . With this assumption, let's explore further.

3. **Substitute x=0 x = 0 :**
f(0)f(y)=0for all y. f(0)f(y) = 0 \quad \text{for all } y.
This implies f(0)=0 f(0) = 0 or f(y)=0 f(y) = 0 for all y y . Since we're considering non-zero solutions, we assume f(0)=0 f(0) = 0 .

4. Substitute specific values to find a pattern:

Let's use simple substitutions to analyze behavior at specific points.

**Substitute x=1 x = 1 :**
f(1)f(y)=f(f(y1))+f(2)+f(1). f(1)f(y) = f(f(y-1)) + f(2) + f(1).

This implies that if f(1) f(1) is known, we could potentially express f(y) f(y) in simpler terms.

5. Try a linear solution:
Assume f(x)=cx f(x) = cx for some constant c c . Substitute into the original equation:
(cx)(cy)=x(c(c(yx)))+x(c(2x))+c(x2). (cx)(cy) = x(c(c(y-x))) + x(c(2x)) + c(x^2).
Simplifying,
c2xy=x(c2(yx))+2cx2+cx2. c^2xy = x(c^2(y-x)) + 2cx^2 + cx^2.
c2xy=c2xyc2x2+3cx2. c^2xy = c^2xy - c^2x^2 + 3cx^2.
Equating both sides, we get:
0=c2x2+3cx2. 0 = -c^2x^2 + 3cx^2.
x2(c23c)=0. x^2(c^2 - 3c) = 0.
Which implies:
c23c=0c(c3)=0. c^2 - 3c = 0 \quad \Rightarrow \quad c(c-3) = 0.
Thus, c=0 c = 0 or c=3 c = 3 , leading to the solutions:
f(x)=0orf(x)=3x. f(x) = 0 \quad \text{or} \quad f(x) = 3x.

Thus, the functions that satisfy the given functional equation are f(x)=0 f(x) = 0 and f(x)=3x f(x) = 3x for all xR x \in \mathbb{R} .

f(x)=0andf(x)=3xfor all xR \boxed{f(x) = 0 \quad \text{and} \quad f(x) = 3x \quad \text{for all } x \in \mathbb{R}}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.