27 research outputs found
The Well Structured Problem for Presburger Counter Machines
International audienceWe introduce the well structured problem as the question of whether a model (here a counter machine) is well structured (here for the usual ordering on integers). We show that it is undecidable for most of the (Presburger-defined) counter machines except for Affine VASS of dimension one. However, the strong well structured problem is decidable for all Presburger counter machines. While Affine VASS of dimension one are not, in general, well structured, we give an algorithm that computes the set of predecessors of a configuration; as a consequence this allows to decide the well structured problem for 1-Affine VASS
A lower bound for the coverability problem in acyclic pushdown VAS
We investigate the coverability problem for a one-dimensional restriction of pushdown vector addition systems with states. We improve the lower complexity bound to PSpace, even in the acyclic case
Reachability in Continuous Pushdown VASS
Pushdown Vector Addition Systems with States (PVASS) consist of finitely many
control states, a pushdown stack, and a set of counters that can be incremented
and decremented, but not tested for zero. Whether the reachability problem is
decidable for PVASS is a long-standing open problem.
We consider continuous PVASS, which are PVASS with a continuous semantics.
This means, the counter values are rational numbers and whenever a vector is
added to the current counter values, this vector is first scaled with an
arbitrarily chosen rational factor between zero and one. We show that
reachability in continuous PVASS is NEXPTIME-complete. Our result is unusually
robust: Reachability can be decided in NEXPTIME even if all numbers are
specified in binary. On the other hand, NEXPTIME-hardness already holds for
coverability, in fixed dimension, for bounded stack, and even if all numbers
are specified in unary
Verification problems for timed and probabilistic extensions of Petri Nets
In the first part of the thesis, we prove the decidability (and PSPACE-completeness) of
the universal safety property on a timed extension of Petri Nets, called Timed Petri Nets.
Every token has a real-valued clock (a.k.a. age), and transition firing is constrained by
the clock values that have integer bounds (using strict and non-strict inequalities). The
newly created tokens can either inherit the age from an input token of the transition or
it can be reset to zero.
In the second part of the thesis, we refer to systems with controlled behaviour that
are probabilistic extensions of VASS and One-Counter Automata. Firstly, we consider
infinite state Markov Decision Processes (MDPs) that are induced by probabilistic
extensions of VASS, called VASS-MDPs. We show that most of the qualitative problems
for general VASS-MDPs are undecidable, and consider a monotone subclass in which
only the controller can change the counter values, called 1-VASS-MDPs. In particular,
we show that limit-sure control state reachability for 1-VASS-MDPs is decidable, i.e.,
checking whether one can reach a set of control states with probability arbitrarily close
to 1. Unlike for finite state MDPs, the control state reachability property may hold limit
surely (i.e. using an infinite family of strategies, each of which achieving the objective
with probability ≥ 1-e, for every e > 0), but not almost surely (i.e. with probability 1).
Secondly, we consider infinite state MDPs that are induced by probabilistic extensions of
One-Counter Automata, called One-Counter Markov Decision Processes (OC-MDPs).
We show that the almost-sure {1;2;3}-Parity problem for OC-MDPs is at least as hard
as the limit-sure selective termination problem for OC-MDPs, in which one would
like to reach a particular set of control states and counter value zero with probability
arbitrarily close to 1
Reachability in Two-Dimensional Vector Addition Systems with States: One Test Is for Free
Vector addition system with states is an ubiquitous model of computation with extensive applications in computer science. The reachability problem for vector addition systems is central since many other problems reduce to that question. The problem is decidable and it was recently proved that the dimension of the vector addition system is an important parameter of the complexity. In fixed dimensions larger than two, the complexity is not known (with huge complexity gaps). In dimension two, the reachability problem was shown to be PSPACE-complete by Blondin et al. in 2015. We consider an extension of this model, called 2-TVASS, where the first counter can be tested for zero. This model naturally extends the classical model of one counter automata (OCA). We show that reachability is still solvable in polynomial space for 2-TVASS. As in the work Blondin et al., our approach relies on the existence of small reachability certificates obtained by concatenating polynomially many cycles
Coverability Is Undecidable in One-Dimensional Pushdown Vector Addition Systems with Resets
We consider the model of pushdown vector addition systems with resets. These
consist of vector addition systems that have access to a pushdown stack and
have instructions to reset counters. For this model, we study the coverability
problem. In the absence of resets, this problem is known to be decidable for
one-dimensional pushdown vector addition systems, but decidability is open for
general pushdown vector addition systems. Moreover, coverability is known to be
decidable for reset vector addition systems without a pushdown stack. We show
in this note that the problem is undecidable for one-dimensional pushdown
vector addition systems with resets.Comment: 8 page