Turku Centre for Computer Science

About: Turku Centre for Computer Science is a facility organization based out in Turku, Finland. It is known for research contribution in the topics: Decidability & Word (group theory). The organization has 382 authors who have published 1027 publications receiving 19560 citations.

Contexts on Trajectories

Abstract: We introduce and investigate a new way of generating mildly context-sensitive languages. The main idea is that the contexts are adjoined by shuffling them on certain trajectories. In this way we obtain also a very general class of contextual grammars such that most of the fundamental classes of contextual grammars, for instance, internal contextual grammars, external contextual grammars, n-contextual grammars, are particular cases of contextual grammars with contexts shuffled on trajectories. The approach is very flexible, able to model various aspects from linguistics.

On the frequency of letters in pure binary morphic sequences

Abstract: It is well-known that the frequency of letters in primitive morphic sequences exists We show that the frequency of letters exists in pure binary morphic sequences generated by non-primitive morphisms Therefore, the letter frequency exists in every pure binary morphic sequence We also show that this is somewhat optimal, in the sense that the result does not hold in the class of general binary morphic sequences Finally, we give an explicit formula for the frequency of letters

Theorem Prover Support for Precondition and Correctness Calculation

Abstract: Tools for automatically extracting the conditions for which a program is correct with respect to a precondition and postcondition can make proving program correctness easier. We build a HOL-based tool that uses weakest preconditions and semantically derived rules to prove correctness theorems with the verification conditions as assumptions. The rules include two new rules for calculating loop preconditions and recursion correctness while taking specification variables into consideration. The programming language has (recursive) procedures, and both demonic and angelic nondeterminism, which can be used to model interaction. Program variables can be of arbitrary types. Programs with procedures are handled modularly, and proved facts about individual procedures are stored in a database available to all programs.

Finite sets of words and computing

Abstract: We discuss about two recent undecidability results in formal language theory. The corresponding problems are very simply formulated questions on finite sets of words. In particular, these results underline how finite sets of words can be used to perform powerful computations.

Linear Grammars with One-Sided Contexts and Their Automaton Representation

Abstract: The paper considers a family of formal grammars that extends linear context-free grammars with an operator for referring to the left context of a substring being defined, as well as with a conjunction operation (as in linear conjunctive grammars). These grammars are proved to be computationally equivalent to an extension of one-way real-time cellular automata with an extra data channel. The main result is the undecidability of the emptiness problem for grammars restricted to a one-symbol alphabet, which is proved by simulating a Turing machine by a cellular automaton with feedback. The same construction proves the \(\Sigma^0_2\)-completeness of the finiteness problem for these grammars.