A proof of Furstenberg’s conjecture on the intersections of $\times p$- and $\times q$-invariant sets

Abstract

We prove the following conjecture of Furstenberg (1969): if $A,B\subset [0,1]$ are closed and invariant under $\times p \mod 1$ and $\times q \mod 1$, respectively, and if $\log p/\log q\notin \Bbb{Q}$, then for all real numbers $u$ and $v$,
\[
\dim_{\rm H}(uA+v)\cap B\le \max\{0,\dim_{\rm H}A+\dim_{\rm H}B-1\}.
\]
We obtain this result as a consequence of our study on the intersections of incommensurable self-similar sets on $\Bbb{R}$. Our methods also allow us to give upper bounds for dimensions of arbitrary slices of planar self-similar sets satisfying SSC and certain natural irreducible conditions.