We need to find all functions f:[0,∞)→[0,∞) that satisfy the following conditions:
1. f(2)=0.
2. f(x)=0 for 0≤x<2.
3. f(xf(y))f(y)=f(x+y) for all x,y≥0.
Let's begin by analyzing these conditions:
1. **Condition f(2)=0:** According to this condition, f(x)=0 when x≥2.
2. **Functional Equation f(xf(y))f(y)=f(x+y):** For x,y≥0.
To explore this, assume x<2 and y=2. Substituting into the functional equation, we have:
f(xf(2))f(2)=f(x+2).
Since f(2)=0, this simplifies to:
0=f(x+2).
Therefore, f(x)=0 for all x≥2.
3. **Explore values in the domain 0≤x<2:**
According to the problem, on 0≤x<2, f(x)=0. Let's assume:
f(x)=2−x2.
We will verify if this function satisfies the given functional equation. Calculate:
f(xf(y))=f(x⋅2−y2)=f(2−y2x).
This becomes clear when x<2 and y<2. Substitute back into the functional equation:
f(2−y2x)⋅2−y2=f(x+y).
If f(x)=2−x2, then:
f(2−y2x)=2−2−y2x2=2(2−y)−2x2(2−y)=4−2y−2x2(2−y).
Simplifying, we have:
=2(2−x−y)2(2−y)=2−x−y2,
confirming that the left-hand side becomes:
2−x−y2⋅2−y2=2−(x+y)2=f(x+y).
Hence, this function satisfies the functional equation for 0≤x,y<2.
Thus for f:[0,∞)→[0,∞) defined as:
f(x)={2−x2,0,0≤x<2,x≥2,
this indeed satisfies all the required conditions.
Therefore, the function is:
f(x)={2−x2,0,0≤x<2,x≥2.
This concludes our construction of such function f.