To solve the functional equation
f(xy)(f(x)−f(y))=(x−y)f(x)f(y)
for all real numbers x and y, we proceed as follows:
### Step 1: Analyze the Equation
Given the equation:
f(xy)(f(x)−f(y))=(x−y)f(x)f(y)
we can perform a few initial checks. First, consider x=y:
f(x2)(f(x)−f(x))=(x−x)f(x)f(x)
This simplifies to 0=0, which is trivially true, but it does not provide any new information.
### Step 2: Substituting Special Values
- **Substituting y=0:**
f(0)(f(x)−f(0))=xf(x)f(0)
If f(0)=0, then dividing both sides by f(0) gives:
f(x)−f(0)=xf(x)
Rearranging gives:
f(x)(1−x)=f(0)
This implies f(x)=1−xf(0). However, this must hold for all x, which is not possible unless f(x)=0 for all x.
- **Substituting x=0:**
f(0)(f(0)−f(y))=(−y)f(0)f(y)
If f(0)=0, then:
f(y)=0for all y
Hence, f(x)=0 is a possible solution.
### Step 3: Consider Non-zero Solutions
Assume f(x)=0 for some non-zero x. Substituting y=1, we get:
f(x)(f(x)−f(1))=(x−1)f(x)f(1)
Simplifying this gives:
f(x)f(x)−f(x)f(1)=(x−1)f(x)f(1)
If f(x)=0, cancel f(x) from both sides:
f(x)−f(1)=(x−1)f(1)
Thus,
f(x)=f(1)x
This implies that f is a linear function of x, specifically f(x)=cx for some constant c.
### Step 4: Formulate the General Solution
The general solution is:
- If x∈S, where S is some subset of R, then f(x)=f(1)x.
- If x∈/S, then f(x)=0.
Putting it together, we get for some subset S⊆R,
f(x)=f(1)⋅x∣x∈S;f(x)=0∣x∈/S
This solution framework satisfies the original functional equation for any choices of sets S by considering both the zero and non-zero functional cases.