On the Skitovich-Darmois theorem for a-adic solenoids

Let $X$ be a compact connected Abelian group. It is well-known that then there exist topological automorphisms $\alpha_j, \beta_j$ of $X$ and independent random variables $\xi_1$ and $\xi_2$ with values in $X$ and distributions $\mu_1, \mu_2$ such that the linear forms $L_1 = \alpha_1\xi_1 + \alpha_2\xi_2$ and $L_2 = \beta_1\xi_1 + \beta_2\xi_2$ are independent, whereas $\mu_1$ and $\mu_2$ are not represented as convolutions of Gaussian and idempotent distributions. This means that the Skitovich--Darmois theorem fails for such groups. We prove that if we consider three linear forms of three independent random variables taking values in $X$, where $X$ is an ${\boldsymbol a}$-adic solenoid, then the independence of the linear forms implies that at least one of the distributions is idempotent. We describe all such solenoids.