Suppose $\lambda$ is the Lebesgue measure on the Borel $\sigma$-algebra.
Does there exist an example of sets $A,B\subset\mathbb{R}$, where:
- $A\cup B=\mathbb{R}$
- $A\cap B=\emptyset$
- for all non-empty open intervals $I:=(a,b)\subset\mathbb{R}$, such that $c\neq 1$ is a non-zero constant:
$$\lambda(A\cap I)=c\cdot\lambda(B \cap I)?$$
This answer might offer a hint, despite $c$ being non-constant, where $c$ is positive (and):
$$\lim_{t\to\infty}\frac{\lambda(A\cap[-t,t])}{2t}=\frac{2}{3}$$
$$\lim_{t\to\infty}\frac{\lambda(B\cap[-t,t])}{2t}=\frac{1}{3}$$