Let $\alpha$ be a fixed real number. Find all functions $f:\mathbb R \to \mathbb R$ such that $$f(f(x + y)f(x - y)) = x^2 + \alpha yf(y)$$for all $x,y \in \mathbb R$.

[i]Proposed by Walther Janous[/i]
1. Let $P(x, y)$ be the assertion of the given functional equation:
   \[
   P(x, y): f(f(x + y)f(x - y)) = x^2 + \alpha y f(y)
   \]

2. Consider $P(x, 0)$:
   \[
   P(x, 0): f(f(x)^2) = x^2
   \]
   This implies that $f(f(x)^2) = f(f(-x)^2) = x^2$. Therefore, $f(a) = f(b)$ implies $a^2 = b^2$.

3. From the above, we have $f(f(x)^2) = f(f(-x)^2)$, which implies:
   \[
   f(x)^4 = f(-x)^4
   \]
   Hence, $f(-x) = \pm f(x)$.

4. Consider $P(0, x)$:
   \[
   P(0, x): f(f(x)f(-x)) = \alpha x f(x)
   \]
   Since $f(-x) = \pm f(x)$, we have:
   \[
   f(f(x)f(-x)) = f(\pm f(x)^2) = \pm f(f(x)^2) = \pm x^2
   \]
   Therefore:
   \[
   \alpha x f(x) = \pm x^2 \quad \forall x
   \]

5. This implies $\alpha \neq 0$ (otherwise, there would be no solution). Thus, we have:
   \[
   f(x) = \pm \frac{x}{\alpha} \quad \forall x \neq 0
   \]

6. Plugging $f(x) = \pm \frac{x}{\alpha}$ into $f(f(x)^2) = x^2$, we get:
   \[
   f\left(\left(\pm \frac{x}{\alpha}\right)^2\right) = x^2
   \]
   Simplifying, we have:
   \[
   f\left(\frac{x^2}{\alpha^2}\right) = x^2
   \]
   This implies:
   \[
   \frac{x^2}{\alpha^2} = x^2 \quad \Rightarrow \quad \alpha^2 = 1
   \]

7. Since $\alpha^2 = 1$, we have $\alpha = \pm 1$. 

8. If $\alpha = 1$, then $f(x) = \pm x$. However, we need to check if $f(x) = -x$ is a valid solution:
   \[
   f(f(x + y)f(x - y)) = f((-x - y)(-x + y)) = f((x + y)(x - y)) = f(x^2 - y^2)
   \]
   \[
   = -(x^2 - y^2) = -x^2 + y^2
   \]
   On the other hand:
   \[
   x^2 + \alpha y f(y) = x^2 + y(-y) = x^2 - y^2
   \]
   Therefore, $f(x) = -x$ is indeed a valid solution when $\alpha = 1$.

9. If $\alpha = -1$, then $f(x) = \pm x$. We need to check if $f(x) = x$ is a valid solution:
   \[
   f(f(x + y)f(x - y)) = f((x + y)(x - y)) = f(x^2 - y^2) = x^2 - y^2
   \]
   On the other hand:
   \[
   x^2 + \alpha y f(y) = x^2 - y(y) = x^2 - y^2
   \]
   Therefore, $f(x) = x$ is indeed a valid solution when $\alpha = -1$.

10. Finally, we conclude that the solutions are:
   \[
   \boxed{f(x) = -x \text{ if } \alpha = 1 \text{ and } f(x) = x \text{ if } \alpha = -1}
   \]