The multifractal spectra of planar waiting sets in beta expansions

Let $β>1$ be a real number. In this paper, the Hausdorff dimension of sets consisting of pairs of numbers with prescribed quantitative waiting time indicators in $β$-expansions are determined. More precisely, let $I$ be the unit interval $[0,1)$ and write $\underline{R}^β(x,y)$ and $\overline{R}^β(x,y)$ as the lower and upper quantitative waiting time indicators of $y$ by $x$ in $β$-expansions, respectively. Define the waiting set on the plane by \[E_β(a,b)=\left\{(x,y)\in I^2\colon\underline{R}^β(x,y)=a,\overline{R}^β(x,y)=b\right\}.\] where $0\leq a\leq b\leq\infty$, then the set $E_β(a,b)$ is always of Hausdorff dimension two for any pair of numbers $a$ and $b$. In addition, some generalizations for this result are also given in the last section.