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Algebra Difficulty 8.2 Shortlist Find the answer

Find all functions f:RRf: \mathbb{R} \rightarrow \mathbb{R}, satisfying f(xy)(f(x)f(y))=(xy)f(x)f(y) f(xy)(f(x) - f(y)) = (x-y)f(x)f(y) for all x,yx,y.

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

Solution

To solve the functional equation
f(xy)(f(x)f(y))=(xy)f(x)f(y) f(xy)(f(x) - f(y)) = (x-y)f(x)f(y)
for all real numbers xx and yy, we proceed as follows:

### Step 1: Analyze the Equation

Given the equation:
f(xy)(f(x)f(y))=(xy)f(x)f(y) f(xy)(f(x) - f(y)) = (x-y)f(x)f(y)
we can perform a few initial checks. First, consider x=yx = y:
f(x2)(f(x)f(x))=(xx)f(x)f(x) f(x^2)(f(x) - f(x)) = (x-x)f(x)f(x)
This simplifies to 0=00 = 0, which is trivially true, but it does not provide any new information.

### Step 2: Substituting Special Values

- **Substituting y=0 y = 0 :**
f(0)(f(x)f(0))=xf(x)f(0) f(0)(f(x) - f(0)) = xf(x)f(0)
If f(0)0 f(0) \neq 0 , then dividing both sides by f(0) f(0) gives:
f(x)f(0)=xf(x) f(x) - f(0) = xf(x)
Rearranging gives:
f(x)(1x)=f(0) f(x)(1-x) = f(0)
This implies f(x)=f(0)1xf(x) = \frac{f(0)}{1-x}. However, this must hold for all xx, which is not possible unless f(x)=0f(x) = 0 for all xx.

- **Substituting x=0 x = 0 :**
f(0)(f(0)f(y))=(y)f(0)f(y) f(0)(f(0) - f(y)) = (-y)f(0)f(y)
If f(0)0f(0) \neq 0, then:
f(y)=0for all y f(y) = 0 \quad \text{for all } y
Hence, f(x)=0f(x) = 0 is a possible solution.

### Step 3: Consider Non-zero Solutions

Assume f(x)0f(x) \neq 0 for some non-zero xx. Substituting y=1y = 1, we get:
f(x)(f(x)f(1))=(x1)f(x)f(1) f(x)(f(x) - f(1)) = (x-1)f(x)f(1)
Simplifying this gives:
f(x)f(x)f(x)f(1)=(x1)f(x)f(1) f(x)f(x) - f(x)f(1) = (x-1)f(x)f(1)
If f(x)0 f(x) \neq 0 , cancel f(x) f(x) from both sides:
f(x)f(1)=(x1)f(1) f(x) - f(1) = (x-1)f(1)
Thus,
f(x)=f(1)x f(x) = f(1)x
This implies that ff is a linear function of xx, specifically f(x)=cx f(x) = cx for some constant cc.

### Step 4: Formulate the General Solution

The general solution is:
- If xSx \in S, where SS is some subset of R\mathbb{R}, then f(x)=f(1)xf(x) = f(1)x.
- If xSx \notin S, then f(x)=0f(x) = 0.

Putting it together, we get for some subset SRS \subseteq \mathbb{R},
f(x)=f(1)xxS;f(x)=0xS \boxed{f(x) = f(1)\cdot x \mid x \in S; \quad f(x) = 0 \mid x \notin S}

This solution framework satisfies the original functional equation for any choices of sets SS by considering both the zero and non-zero functional cases.

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