Power-like functional equation

I would like to know what are the especifications of a functional equation that give us a power function as a solution.

For example, if $f:\Bbb R \to \Bbb R$ is continuous and monotonic, such that

$$f(x)+f(y)=f(z)$$ iif $$f(\lambda x)+f(\lambda y)=f(\lambda z)$$ for all $\lambda>0$, then $f(x)=ax^b$.

Does anyone know another functional equation that gives a power function as a solution?

Answer

For all $\ x\ $ and $\ y\ne 0,\ $ the only continuous solutions of the equation $\ f(x)^2 = f(xy)f(x/y)\ $ is
$\ f(x) = ax^b.$

An alternative equation similar to

Cauchy's functional equation is
$\ f(1)f(x\ y) = f(x)f(y)\ $ for all $x,y$.