Measure-Theoretic Aspects of Star-Free and Group Languages
A language $L$ is said to be ${\cal C}$-measurable, where ${\cal C}$ is a class of languages, if there is an infinite sequence of languages in ${\cal C}$ that ``converges'' to $L$. We investigate the properties of ${\cal C}$-measurability in the cases where ${\cal C}$ is SF, the class of all star-free languages, and G, the class of all group languages. It is shown that a language $L$ is SF-measurable if and only if $L$ is GD-measurable, where GD is the class of all generalised definite languages (a more restricted subclass of star-free languages). This means that GD and SF have the same ``measuring power'', whereas GD is a very restricted proper subclass of SF. Moreover, we give a purely algebraic characterisation of SF-measurable regular languages, which is a natural extension of Schutzenberger's theorem stating the correspondence between star-free languages and aperiodic monoids. We also show the probabilistic independence of star-free and group languages, which is an important application of the former result. Finally, while the measuring power of star-free and generalised definite languages are equal, we show that the situation is rather opposite for subclasses of group languages as follows. For any two local subvarieties ${\cal C} \subsetneq {\cal D}$ of group languages, we have $\{L \mid L \text{ is } {\cal C}\text{-measurable}\} \subsetneq \{ L \mid L \text{ is } {\cal D}\text{-measurable}\}$.