Erik Meineche Schmidt

IO and OI. II

A fixed-point characterization of the inside-out (IO) and outside-in (OI) context-free tree languages is given. This characterization is used to obtain a theory of nondeterministic systems of context-free equations with parameters. Several “Mezei-and-Wright-like” results are obtained which relate the context-free tree languages to recognizable tree languages and to nondeterministic recursive program(scheme)s (called by value and called by name). Closure properties of the context-free tree languages are discussed. Hierarchies of higher level equational subsets of an algebra are considered.

Algorithm Theory — SWAT 2002

We define a quasidictionary to be a data structure that supports the following operations: check-in(v) inserts a data item v and returns a positive integer tag to be used in future references to v; check-out(x) deletes the data item with tag x; access(x) inspects and/or modifies the data item with tag x. A quasidictionary is similar to a dictionary, the difference being that the names identifying data items are chosen by the data structure rather than by its user. We describe a deterministic quasidictionary that executes the operations check-in and access in constant time and check-out in constant amortized time, works in linear space, and uses only tags bounded by the maximum number of data items stored simultaneously in the quasidictionary since it was last empty.

Pascal semantics by a combination of denotational semantics and high-level Petri nets

This paper describes the formal semantics of a subset of PASCAL, by means of a semantic model based on a combination of denotational semantics and high-level Petri nets. It is our intention that the paper can be used as part of the written material for an introductory course in computer science.

BRICS and Quantum Information Processing

BRICS is a research centre and international PhD school in theoretical computer science, based at the University of Aarhus, Denmark. The centre has recently become engaged in quantum information processing in cooperation with the Department of Physics, also University of Aarhus.

A Universal Relational Operator

We present a single relational operator which, in combination with a simple core language for manipulating atoms and tuples, generalizes all standard unary and binary operators in relational databases, while permitting a more intuitive query style. The new operator, factor , is based on a unique factorization of relations. We present an example language and demonstrate how the usual operators appear as simple and intuitive instances of factor . We further show that many new operators and combinations of old ones can be expressed in concise terms using factor. The factor versions will always be evaluated as efficiently as the originals and will sometimes even lead to a speed-up.

Space-Restricted Attribute Grammars

Restricting the size of attribute values, relative to the length of the string under consideration, leads to a model of attribute grammars in which grammars with both inherited and synthesized attributes can be significantly more economical than grammars with synthesized attributes only.