89 research outputs found
Reachability analysis of first-order definable pushdown systems
We study pushdown systems where control states, stack alphabet, and
transition relation, instead of being finite, are first-order definable in a
fixed countably-infinite structure. We show that the reachability analysis can
be addressed with the well-known saturation technique for the wide class of
oligomorphic structures. Moreover, for the more restrictive homogeneous
structures, we are able to give concrete complexity upper bounds. We show ample
applicability of our technique by presenting several concrete examples of
homogeneous structures, subsuming, with optimal complexity, known results from
the literature. We show that infinitely many such examples of homogeneous
structures can be obtained with the classical wreath product construction.Comment: to appear in CSL'1
Timed pushdown automata revisited
This paper contains two results on timed extensions of pushdown automata
(PDA). As our first result we prove that the model of dense-timed PDA of
Abdulla et al. collapses: it is expressively equivalent to dense-timed PDA with
timeless stack. Motivated by this result, we advocate the framework of
first-order definable PDA, a specialization of PDA in sets with atoms, as the
right setting to define and investigate timed extensions of PDA. The general
model obtained in this way is Turing complete. As our second result we prove
NEXPTIME upper complexity bound for the non-emptiness problem for an expressive
subclass. As a byproduct, we obtain a tight EXPTIME complexity bound for a more
restrictive subclass of PDA with timeless stack, thus subsuming the complexity
bound known for dense-timed PDA.Comment: full technical report of LICS'15 pape
Automata theory in nominal sets
We study languages over infinite alphabets equipped with some structure that
can be tested by recognizing automata. We develop a framework for studying such
alphabets and the ensuing automata theory, where the key role is played by an
automorphism group of the alphabet. In the process, we generalize nominal sets
due to Gabbay and Pitts
Definable isomorphism problem
We investigate the isomorphism problem in the setting of definable sets
(equivalent to sets with atoms): given two definable relational structures, are
they related by a definable isomorphism? Under mild assumptions on the
underlying structure of atoms, we prove decidability of the problem. The core
result is parameter-elimination: existence of an isomorphism definable with
parameters implies existence of an isomorphism definable without parameters
Orbit-finite linear programming
An infinite set is orbit-finite if, up to permutations of the underlying
structure of atoms, it has only finitely many elements. We study a
generalisation of linear programming where constraints are expressed by an
orbit-finite system of linear inequalities. As our principal contribution we
provide a decision procedure for checking if such a system has a real solution,
and for computing the minimal/maximal value of a linear objective function over
the solution set. We also show undecidability of these problems in case when
only integer solutions are considered. Therefore orbit-finite linear
programming is decidable, while orbit-finite integer linear programming is not.Comment: Full version of LICS 2023 pape
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