89 research outputs found
Forward Analysis for WSTS, Part III: Karp-Miller Trees
This paper is a sequel of "Forward Analysis for WSTS, Part I: Completions"
[STACS 2009, LZI Intl. Proc. in Informatics 3, 433-444] and "Forward Analysis
for WSTS, Part II: Complete WSTS" [Logical Methods in Computer Science 8(3),
2012]. In these two papers, we provided a framework to conduct forward
reachability analyses of WSTS, using finite representations of downward-closed
sets. We further develop this framework to obtain a generic Karp-Miller
algorithm for the new class of very-WSTS. This allows us to show that
coverability sets of very-WSTS can be computed as their finite ideal
decompositions. Under natural effectiveness assumptions, we also show that LTL
model checking for very-WSTS is decidable. The termination of our procedure
rests on a new notion of acceleration levels, which we study. We characterize
those domains that allow for only finitely many accelerations, based on ordinal
ranks
Decidability Issues for Petri Nets
This is a survey of some decidability results for Petri nets, covering the last three decades. The presentation is structured around decidability of specific properties, various behavioural equivalences and finally the model checking problem for temporal logics
Dense-Timed Petri Nets: Checking Zenoness, Token liveness and Boundedness
We consider Dense-Timed Petri Nets (TPN), an extension of Petri nets in which
each token is equipped with a real-valued clock and where the semantics is lazy
(i.e., enabled transitions need not fire; time can pass and disable
transitions). We consider the following verification problems for TPNs. (i)
Zenoness: whether there exists a zeno-computation from a given marking, i.e.,
an infinite computation which takes only a finite amount of time. We show
decidability of zenoness for TPNs, thus solving an open problem from [Escrig et
al.]. Furthermore, the related question if there exist arbitrarily fast
computations from a given marking is also decidable. On the other hand,
universal zenoness, i.e., the question if all infinite computations from a
given marking are zeno, is undecidable. (ii) Token liveness: whether a token is
alive in a marking, i.e., whether there is a computation from the marking which
eventually consumes the token. We show decidability of the problem by reducing
it to the coverability problem, which is decidable for TPNs. (iii) Boundedness:
whether the size of the reachable markings is bounded. We consider two versions
of the problem; namely semantic boundedness where only live tokens are taken
into consideration in the markings, and syntactic boundedness where also dead
tokens are considered. We show undecidability of semantic boundedness, while we
prove that syntactic boundedness is decidable through an extension of the
Karp-Miller algorithm.Comment: 61 pages, 18 figure
Computing Optimal Coverability Costs in Priced Timed Petri Nets
We consider timed Petri nets, i.e., unbounded Petri nets where each token
carries a real-valued clock. Transition arcs are labeled with time intervals,
which specify constraints on the ages of tokens. Our cost model assigns token
storage costs per time unit to places, and firing costs to transitions. We
study the cost to reach a given control-state. In general, a cost-optimal run
may not exist. However, we show that the infimum of the costs is computable.Comment: 26 pages. Contribution to LICS 201
The Semilinear Home-Space Problem Is Ackermann-Complete for Petri Nets
A set of configurations H is a home-space for a set of configurations X of a Petri net if every configuration reachable from (any configuration in) X can reach (some configuration in) H. The semilinear home-space problem for Petri nets asks, given a Petri net and semilinear sets of configurations X, H, if H is a home-space for X. In 1989, David de Frutos Escrig and Colette Johnen proved that the problem is decidable when X is a singleton and H is a finite union of linear sets with the same periods. In this paper, we show that the general (semilinear) problem is decidable. This result is obtained by proving a duality between the reachability problem and the non-home-space problem. In particular, we prove that for any Petri net and any linear set of configurations L we can effectively compute a semilinear set C of configurations, called a non-reachability core for L, such that for every set X the set L is not a home-space for X if, and only if, C is reachable from X. We show that the established relation to the reachability problem yields the Ackermann-completeness of the (semilinear) home-space problem. For this we also show that, given a Petri net with an initial marking, the set of minimal reachable markings can be constructed in Ackermannian time
On the Home-Space Problem for Petri Nets and its Ackermannian Complexity
A set of configurations H is a home-space for a set of configurations X of a
Petri net if every configuration reachable from (any configuration in) X can
reach (some configuration in) H. The semilinear home-space problem for Petri
nets asks, given a Petri net and semilinear sets of configurations X, H, if H
is a home-space for X. In 1989, David de Frutos Escrig and Colette Johnen
proved that the problem is decidable when X is a singleton and H is a finite
union of linear sets with the same periods. In this paper, we show that the
general (semilinear) problem is decidable. This result is obtained by proving a
duality between the reachability problem and the non-home-space problem. In
particular, we prove that for any Petri net and any semilinear set of
configurations H we can effectively compute a semilinear set C of
configurations, called a non-reachability core for H, such that for every set X
the set H is not a home-space for X if, and only if, C is reachable from X. We
show that the established relation to the reachability problem yields the
Ackermann-completeness of the (semilinear) home-space problem. For this we also
show that, given a Petri net with an initial marking, the set of minimal
reachable markings can be constructed in Ackermannian time
Coverability Synthesis in Parametric Petri Nets
We study Parametric Petri Nets (PPNs), i.e., Petri nets for which some arc weights can be parameters. In that setting, we address a problem of parameter synthesis, which consists in computing the exact set of values for the parameters such that a given marking is coverable in the instantiated net.
Since the emptiness of that solution set is already undecidable for general PPNs, we address a special case where parameters are used only as input weights (preT-PPNs), and consequently for which the solution set is
downward-closed. To this end, we invoke a result for the representation of
upward closed set from Valk and Jantzen.
To use this procedure, we show we need to decide universal coverability,
that is decide if some marking is coverable for every possible values of the parameters.
We therefore provide a proof of its EXPSPACE-completeness,
thus settling the previously open problem of its decidability.
We also propose an adaptation of this reasoning to the case of
parameters used only as output weights (postT-PPNs).
In this case, the condition to use this procedure can be reduced to the decidability of the existential coverability,
that is decide if there exists values of the parameters making a given marking coverable.
This problem is known decidable but we provide here a cleaner proof, providing its EXPSPACE-completeness, by reduction to Omega Petri Nets
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