One central issue in the formal design and analysis of reactive systems is
the notion of refinement that asks whether all behaviors of the implementation
is allowed by the specification. The local interpretation of behavior leads to
the notion of simulation. Alternating transition systems (ATSs) provide a
general model for composite reactive systems, and the simulation relation for
ATSs is known as alternating simulation. The simulation relation for fair
transition systems is called fair simulation.
We consider 2-player games played on a finite state space for infinite
rounds. The games are concurrent: in each round, the two players choose their
moves simultaneously; the current state and the moves determine the successor.
We consider omega-regular winning conditions given as parity objectives. We
consider the qualitative analysis problems: the computation of the almost-sure
and limit-sure winning set of states, where player 1 can ensure to win with
probability 1 and with probability arbitrarily close to 1, respectively.
In two-player finite-state stochastic games of partial observation on graphs,
in every state of the graph, the players simultaneously choose an action, and
their joint actions determine a probability distribution over the successor
states. We consider reachability objectives where player 1 tries to ensure a
target state to be visited almost-surely or positively. On the basis of
information, the game can be one-sided with either (a)player 1 or (b)player 2
having partial observation, or two-sided with both players having partial
observation.
Turn-based stochastic games and its important subclass Markov decision
processes (MDPs) provide models for systems with both probabilistic and
nondeterministic behaviors. We consider turn-based stochastic games with two
classical quantitative objectives: discounted-sum and long-run average
objectives. The game models and the quantitative objectives are widely used in
probabilistic verification, planning, optimal inventory control, network
protocol and performance analysis.
We consider Markov Decision Processes (MDPs) with mean-payoff parity and
energy parity objectives. In system design, the parity objective is used to
encode \omega-regular specifications, and the mean-payoff and energy objectives
can be used to model quantitative resource constraints. The energy condition
requires that the resource level never drops below 0, and the mean-payoff
condition requires that the limit-average value of the resource consumption is
within a threshold.
Weighted automata are nondeterministic automata with numerical weights on
transitions. They can define quantitative languages~$L$ that assign to each
word~$w$ a real number~$L(w)$. In the case of infinite words, the value of a
run is naturally computed as the maximum, limsup, liminf, limit-average, or
discounted-sum of the transition weights. The value of a word $w$ is the
supremum of the values of the runs over $w$. We study expressiveness and
closure questions about these quantitative languages.
Simulation and bisimulation metrics for stochastic systems provide a
quantitative generalization of the classical simulation and bisimulation
relations. These metrics capture the similarity of states with respect to
quantitative specifications written in the quantitative {\mu}-calculus and
related probabilistic logics. We first show that the metrics provide a bound
for the difference in long-run average and discounted average behavior across
states, indicating that the metrics can be used both in system verification,
and in performance evaluation.
We study the automatic synthesis of fair non-repudiation protocols, a class
of fair exchange protocols, used for digital contract signing. First, we show
how to specify the objectives of the participating agents and the trusted third
party (TTP) as path formulas in LTL and prove that the satisfaction of these
objectives imply fairness and abuse-freeness; properties required of fair
exchange protocols. We then show that weak (co-operative) co-synthesis and
classical (strictly competitive) co-synthesis fail, whereas assume-guarantee
synthesis (AGS) succeeds.
Gist is a tool that (a) solves the qualitative analysis problem of turn-based
probabilistic games with {\omega}-regular objectives; and (b) synthesizes
reasonable environment assumptions for synthesis of unrealizable
specifications. Our tool provides the first and efficient implementations of
several reduction-based techniques to solve turn-based probabilistic games, and
uses the analysis of turn-based probabilistic games for synthesizing
environment assumptions for unrealizable specifications.
Often one has a preference order among the different systems that satisfy a
given specification. Under a probabilistic assumption about the possible
inputs, such a preference order is naturally expressed by a weighted automaton,
which assigns to each word a value, such that a system is preferred if it
generates a higher expected value.
Energy parity games are infinite two-player turn-based games played on
weighted graphs. The objective of the game combines a (qualitative) parity
condition with the (quantitative) requirement that the sum of the weights
(i.e., the level of energy in the game) must remain positive. Beside their own
interest in the design and synthesis of resource-constrained omega-regular
specifications, energy parity games provide one of the simplest model of games
with combined qualitative and quantitative objective.
The standard procedure for hardware design consists of describing circuit in
a hardware description language at logic level followed by extensive
verification and logic-synthesis. However, this process consumes significant
time and needs a lot of effort. An alternative is to use formal specification
language as a high-level hardware description language and synthesize hardware
from formal specification. Bloem et.al. gave formal specifications and
synthesize the AMBA AHB Arbiter.
Nondeterministic weighted automata are finite automata with numerical weights
on transitions. They define quantitative languages L that assign to each word w
a real number L(w). The value of an infinite word w is computed as the maximal
value of all runs over w, and the value of a run as the maximum, limsup,
liminf, limit average, or discounted sum of the transition weights. We
introduce probabilistic weighted automata, in which the transitions are chosen
in a randomized (rather than nondeterministic) fashion. Under almost-sure
semantics (resp.
We study observation-based strategies for partially-observable Markov
decision processes (POMDPs) with omega-regular objectives. An observation-based
strategy relies on partial information about the history of a play, namely, on
the past sequence of observations. We consider the qualitative analysis
problem: given a POMDP with an omega-regular objective, whether there is an
observation-based strategy to achieve the objective with probability~1
(almost-sure winning), or with positive probability (positive winning). Our
main results are twofold.