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Algebra Difficulty 8.3 Shortlist Find the answer

Find all functions ff defined on the non-negative reals and taking non-negative real values such that: f(2)=0,f(x)0f(2)=0,f(x)\ne0 for 0x<20\le x<2, and f(xf(y))f(y)=f(x+y)f(xf(y))f(y)=f(x+y) for all x,yx,y.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We need to find all functions f:[0,)[0,) f: [0, \infty) \to [0, \infty) that satisfy the following conditions:
1. f(2)=0 f(2) = 0 .
2. f(x)0 f(x) \neq 0 for 0x<2 0 \leq x < 2 .
3. f(xf(y))f(y)=f(x+y) f(xf(y))f(y) = f(x+y) for all x,y0 x, y \geq 0 .

Let's begin by analyzing these conditions:

1. **Condition f(2)=0 f(2) = 0 :** According to this condition, f(x)=0 f(x) = 0 when x2 x \geq 2 .

2. **Functional Equation f(xf(y))f(y)=f(x+y) f(xf(y))f(y) = f(x+y) :** For x,y0 x, y \geq 0 .

To explore this, assume x<2 x < 2 and y=2 y = 2 . Substituting into the functional equation, we have:
f(xf(2))f(2)=f(x+2). f(xf(2))f(2) = f(x + 2).
Since f(2)=0 f(2) = 0 , this simplifies to:
0=f(x+2). 0 = f(x + 2).
Therefore, f(x)=0 f(x) = 0 for all x2 x \geq 2 .

3. **Explore values in the domain 0x<2 0 \leq x < 2 :**

According to the problem, on 0x<2 0 \leq x < 2 , f(x)0 f(x) \neq 0 . Let's assume:
f(x)=22x. f(x) = \frac{2}{2-x}.
We will verify if this function satisfies the given functional equation. Calculate:
f(xf(y))=f(x22y)=f(2x2y). f(xf(y)) = f\left(x \cdot \frac{2}{2-y}\right) = f\left(\frac{2x}{2-y}\right).
This becomes clear when x<2 x < 2 and y<2 y < 2 . Substitute back into the functional equation:
f(2x2y)22y=f(x+y). f\left(\frac{2x}{2-y}\right) \cdot \frac{2}{2-y} = f(x+y).
If f(x)=22x f(x) = \frac{2}{2-x} , then:
f(2x2y)=222x2y=2(2y)2(2y)2x=2(2y)42y2x. f\left(\frac{2x}{2-y}\right) = \frac{2}{2 - \frac{2x}{2-y}} = \frac{2(2-y)}{2(2-y) - 2x} = \frac{2(2-y)}{4 - 2y - 2x}.
Simplifying, we have:
=2(2y)2(2xy)=22xy, = \frac{2(2-y)}{2(2-x-y)} = \frac{2}{2-x-y},
confirming that the left-hand side becomes:
22xy22y=22(x+y)=f(x+y). \frac{2}{2-x-y} \cdot \frac{2}{2-y} = \frac{2}{2-(x+y)} = f(x+y).
Hence, this function satisfies the functional equation for 0x,y<2 0 \leq x, y < 2 .

Thus for f:[0,)[0,) f: [0, \infty) \to [0, \infty) defined as:
f(x)={22x,0x<2,0,x2, f(x) = \begin{cases} \frac{2}{2-x}, & 0 \leq x < 2, \\ 0, & x \geq 2, \end{cases}
this indeed satisfies all the required conditions.

Therefore, the function is:
f(x)={22x,0x<2,0,x2. \boxed{ f(x) = \begin{cases} \frac{2}{2-x}, & 0 \leq x < 2, \\ 0, & x \geq 2. \end{cases} }

This concludes our construction of such function f f .

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