We explore the borderline between decidability and undecidability of the following question: "Let C be a class of codes. Given a machine M of type X, is it decidable whether the language L(M) lies in C or not?" for codes in general, !-codes, codes of finite and bounded deciphering delay, prefix, suffix and bi(pre)fix codes, and for finite automata equipped with different versions of push-down stores and counters.

Abstract. This paper presents some features of the Vaucanson platform. We describe some original algorithms on weighted automata and transducers (computation of the quotient, conversion of a regular expression into a weighted automaton, and composition). We explain how complex declarations due to the generic programming are masked from the user and finally we present a proposal for an XML format that allows implicit descriptions for simple types of automata. 1

Abstract: In this paper, we sharpen the results of Gartner on the universality of partition-limited ET0L systems by showing that such deterministic systems characterize the recursively enumerable sets, and, furthermore, the propagating deterministic partition-limited ET0L systems characterize the programmed languages with appearance checking disallowing erasing productions. The main results of this paper have been announced in [10].

We study the problem of reducing the size of a language model while preserving recognition performance (accuracy and speed). A successful approach has been to represent language models by weighted finite-state automata (WFAs). Analogues of classical automata determinization and minimization algorithms then provide a general method to produce smaller but equivalent WFAs. We extend this approach by introducing the notion of approximate determinization. We provide an algorithm that, when applied to language models for the North American Business task, achieves 25--35% size reduction compared to previous techniques, with negligible effects on recognition time and accuracy. 1. INTRODUCTION An important goal of language model engineering is to produce small language models that guarantee fast and accurate automatic speech recognition (ASR). In practice we see tradeoffs: e.g., in size vs. accuracy and in accuracy vs. speed. There has been recent progress, however, on automatic methods for r...

. In this note, we solve questions regarding closure properties of ordered languages. Especially, ordered languages form a full abstract family of recursive languages which is neither intersection- nor complementationclosed. 1 Introduction and Definitions Ordered languages (or equivalently, forbidden random context languages) were introduced by Fris in 1968 [4]. Therefore, they belong to the classic topics of formal language theory. Surprisingly, several points on closure properties of the corresponding language families were marked as open in the monograph [3]. Most of these questions are solved in this note. We presuppose some knowledge of formal language theory on side of the reader. Especially, the Chomsky hierarchy L(CF) ( L(CS) ( L(REC) ( L(RE) should be known. An ordered grammar is a quintuple G = (VN ; V T ; P; S; OE), where VN , V T , P , and S 2 VN are the nonterminal alphabet, terminal alphabet, set of context-free productions, and axiom, respectively. OE is a partial ord...

We introduce a new variant of PC grammar systems, called PC grammar systems with terminal transmission, PCGSTT for short. We show that right-linear centralized PCGSTT have nice formal language theoretic properties: they are closed under gsm mappings (in particular, under intersection with regular sets and under and homomorphisms) and union; a slight variant is, in addition, closed under concatenation and star; their power lies between that of n-parallel grammars introduced by Wood and that of matrix languages of index n, and their relation to equal matrix grammars of degree n is discussed. We show that membership for these language classes is complete for NL. In a second part of the paper, we discuss questions concerning grammatical inference of these systems. More precisely, we show that PCGSTT whose component grammars are terminal distinguishable right-linear, a notion introduced by Radhakrishnan and Nagaraja in [29, 30], are identi able in the limit if certain data comm...

Flip-pushdown automata are pushdown automata with the additional power to ip or reverse its pushdown, and were recently introduced by Sarkar. We solve most of Sarkar's open problems. In particular, we show that k+1 pushdown reversals are better than k for both deterministic and nondeterministic ip-pushdown automata, i.e., there are languages which can be recognized by a deterministic ippushdown automaton with k + 1 pushdown reversals but which cannot be recognized by a k-ip-pushdown (deterministic or nondeterministic). Furthermore, we investigate closure and non-closure properties as well as computational complexity problems such as xed and general membership.

We give an algorithm for solving equations and inequations with rational constraints in virtually free groups. Our algorithm is based on Rips classification of measured band complexes. Using canonical representatives, we deduce an algorithm for solving equations and inequations in hyperbolic groups (maybe with torsion). Additionnally, we can deal with quasi-isometrically embeddable rational constraints. 1

We show that so-called deterministic even linear simple matrix grammars can be inferred in polynomial time using the query-based learner-teacher model MAT proposed by Angluin for learning deterministic regular languages in [4]. In this way, we extend the class of efficiently learnable languages beyond both the even linear languages and the even equal matrix languages proposed in [41, 48, 52, 53, 54, 57, 56].

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