Small Furstenberg sets

For $\alpha$ in $(0,1]$, a subset $E$ of \RR is called Furstenberg set of type $\alpha$ or $F_\alpha$-set if for each direction $e$ in the unit circle there is a line segment $\ell_e$ in the direction of $e$ such that the Hausdorff dimension of the set $E\cap\ell_e$ is greater or equal than $\alpha$. In this paper we show that if $\alpha > 0$, there exists a set $E\in F_\alpha$ such that \HH{g}(E)=0 for $g(x)=x^{1/2+3/2\alpha}\log^{-\theta}(\frac{1}{x})$, $\theta>\frac{1+3\alpha}{2}$, which improves on the the previously known bound, that $H^{\beta}(E) = 0$ for $\beta>1/2+3/2\alpha$. Further, by refining the argument in a subtle way, we are able to obtain a sharp dimension estimate for a whole class of zero-dimensional Furstenberg type sets. Namely, for \h_\gamma(x)=\log^{-\gamma}(\frac{1}{x}), $\gamma>0$, we construct a set E_\gamma\in F_{\h_\gamma} of Hausdorff dimension not greater than 1/2. Since in a previous work we showed that 1/2 is a lower bound for the Hausdorff dimension of any E\in F_{\h_\gamma}, with the present construction, the value 1/2 is sharp for the whole class of Furstenberg sets associated to the zero dimensional functions \h_\gamma.Comment: Final versio