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arXiv · 2107.10605

A combinatorial proof of a sumset conjecture of Furstenberg

Daniel Glasscock·Joel Moreira·Florian K. Richter
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Abstract

We give a new proof of a sumset conjecture of Furstenberg that was first proved by Hochman and Shmerkin in 2012: if $\log r / \log s$ is irrational and $X$ and $Y$ are $\times r$- and $\times s$-invariant subsets of $[0,1]$, respectively, then $\dim_\text{H} (X+Y) = \min ( 1, \dim_\text{H} X + \dim_\text{H} Y)$. Our main result yields information on the size of the sumset $\lambda X + \eta Y$ uniformly across a compact set of parameters at fixed scales. The proof is combinatorial and avoids the machinery of local entropy averages and CP-processes, relying instead on a quantitative, discrete Marstrand projection theorem and a subtree regularity theorem that may be of independent interest.

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Daniel Glasscock, Joel Moreira, Florian K. Richter. 2021-07-22. A combinatorial proof of a sumset conjecture of Furstenberg. https://doi.org/10.1007/s00493-023-00008-9

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