A First Look at Rotation in Inactive Late-Type M Dwarfs

We have examined the relationship between rotation and activity in 14 late-type (M6-M7) M dwarfs, using high resolution spectra taken at the W.M. Keck Observatory and flux-calibrated spectra from the Sloan Digital Sky Survey. Most were selected to be inactive at a spectral type where strong H-alpha emission is quite common. We used the cross-correlation technique to quantify the rotational broadening; six of the stars in our sample have vsini > 3.5 km/s. Our most significant and perplexing result is that three of these stars do not exhibit H-alpha emission, despite rotating at velocities where previous work has observed strong levels of magnetic field and stellar activity. Our results suggest that rotation and activity in late-type M dwarfs may not always be linked, and open several additional possibilities including a rotationally-dependent activity threshold, or a possible dependence on stellar parameters of the Rossby number at which magnetic/activity "saturation" takes place in fully convective stars.Comment: 8 pages, 4 figures, accepted for publication in Ap

Context-free grammars and XML languages

We study the decision properties of XML languages. It was known that given a context-free language included in the Dyck language with sufficiently many pairs of parentheses, it is undecidable whether or not it is an XML language. We improve on this result by showing that the problem remains undecidable when the language is written on a unique pair of parentheses. We also prove that if the given language is deterministic, then the problem is decidable; while establishing whether its surfaces are regular turns out to be undecidable whenever the deterministic language is contained in the Dyck language with two pairs of parentheses. Our results are based on a \u201cpumping property\u201d of what Boasson and Berstel call the surface of a context-free language

Context-free grammars and XML languages

We study the decision properties of XML languages. It was known that given a context-free language included in the Dyck language with sufficiently many pairs of parentheses, it is undecidable whether or not it is an XML language. We improve on this result by showing that the problem remains undecidable when the language is written on a unique pair of parentheses. We also prove that if the given language is deterministic, then the problem is decidable; while establishing whether its surfaces are regular turns out to be undecidable whenever the deterministic language is contained in the Dyck language with two pairs of parentheses. Our results are based on a “pumping property” of what Boasson and Berstel call the surface of a context-free language

Pac-learning unambiguous nts languages

Abstract. Non-terminally separated (NTS) languages are a subclass of deterministic context free languages where there is a stable relationship between the substrings of the language and the non-terminals of the grammar. We show that when the distribution of samples is generated by a PCFG, based on the same grammar as the target language, the class of unambiguous NTS languages is PAC-learnable from positive data alone, with polynomial bounds on data and computation.

Identification in the limit of substitutable context-free languages

This paper formalisms the idea of substitutability introduced by Zellig Harris in the 1950s and makes it the basis for a learning algorithm from positive data only for a subclass of context-free grammars. \ud We show that there is a polynomial characteristic set, and thus prove polynomial identification in the limit of this class. We discuss the relationship of this class of languages to other common classes discussed in grammatical inference. We also discuss modifications to the algorithm \ud that produces a reduction system rather than a context-free grammar, that will be much more compact. We discuss the relationship to Angluin’s notion of reversibility for regular languages

Beyond Regular Model Checking

In recent years, it has been established that regular model checking can be successfully applied to several parameterized verification problems. However, there are many parameterized verification problems that cannot be described by regular languages, and thus cannot be verified using regular model checking. In this study we try to practice symbolic model checking using classes of languages more expressive than the regular languages. We provide three methods for the uniform verification of non-regular parameterized systems