Upper and lower fast Khintchine spectra in continued fractions

For an irrational number $x\in [0,1)$, let $x=[a\_1(x), a\_2(x),\cdots]$ be its continued fraction expansion. Let $ψ: \mathbb{N} \rightarrow \mathbb{N}$ be a function with $ψ(n)/n\to \infty$ as $n\to\infty$. The (upper, lower) fast Khintchine spectrum for $ψ$ is defined as the Hausdorff dimension of the set of numbers $x\in (0,1)$ for which the (upper, lower) limit of $\frac{1}{ψ(n)}\sum\_{j=1}^n\log a\_j(x)$ is equal to $1$. The fast Khintchine spectrum was determined by Fan, Liao, Wang, and Wu. We calculate the upper and lower fast Khintchine spectra. These three spectra can be different.

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