Ashutosh Trivedi

Reachability-time games on timed automata

In a reachability-time game, players Min and Max choose moves so that the time to reach a final state in a timed automaton is minimised or maximised, respectively. Asarin and Maler showed decidability of reachability-time games on strongly non-Zeno timed automata using a value iteration algorithm. This paper complements their work by providing a strategy improvement algorithm for the problem. It also generalizes their decidability result because the proposed strategy improvement algorithm solves reachability-time games on all timed automata. The exact computational complexity of solving reachability-time games is also established: the problem is EXPTIME-complete for timed automata with at least two clocks.

Recursive timed automata

We study recursive timed automata that extend timed automata with recursion. Timed automata, as introduced by Alur and Dill, are finite automata accompanied by a finite set of real-valued variables called clocks. Recursive timed automata are finite collections of timed automata extended with special states that correspond to (potentially recursive) invocations of other timed automata from their collection. During an invocation of a timed automaton, our model permits passing the values of clocks using both pass-byvalue and pass-by-reference mechanisms. We study the natural reachability and termination (reachability with empty invocation stack) problems for recursive timed automata. We show that these problems are decidable (in many cases with the same complexity as the reachability problem on timed automata) for recursive timed automata satisfying the following condition: during each invocation either all clocks are passed by reference or none is passed by reference. Furthermore, we show that for recursive timed automata that violate this condition reachability/termination problems are undecidable for automata with as few as three clocks. We also establish similar results for two-player game extension of our model against reachability/termination objective.

Concavely-Priced Timed Automata

Concavely-priced timed automata, a generalization of linearly-priced timed automata, are introduced. Computing the minimum value of a number of cost functions--including reachability price, discounted price, average time, average price, price-per-time average, and price-per-reward average--is considered in a uniform fashion for concavely-priced timed automata. All the corresponding decision problems are shown to be PSPACE-complete. This paper generalises the recent work of Bouyer et al. on deciding the minimum reachability price and the minimum ratio-price for linearly-priced timed automata. A new type of a region graph--the boundary region graph--is defined, which generalizes the corner-point abstraction of Bouyer et al. A broad class of cost functions--concave-regular cost functions--is introduced, and the boundary region graph is shown to be a correct abstraction for deciding the minimum value of concave-regular cost functions for concavely-priced timed automata.

Regular Transformations of Infinite Strings

The theory of regular transformations of finite strings is quite mature with appealing properties. This class can be equivalently defined using both logic (Monadic second-order logic) and finite-state machines (two-way transducers, and more recently, streaming string transducers); is closed under operations such as sequential composition and regular choice; and problems such as functional equivalence and type checking, are decidable for this class. In this paper, we initiate a study of transformations of infinite strings. The MSO-based definition for regular string transformations generalizes naturally to infinite strings. We define an equivalent generalization of the machine model of streaming string transducers to infinite strings. A streaming string transducer is a deterministic machine that makes a single pass over the input string, and computes the output fragments using a finite set of string variables that are updated in a copyless manner at each step. We show how Muller acceptance condition for automata over infinite strings can be generalized to associate an infinite output string with an infinite execution. The proof that our model captures all MSO-definable transformations uses two-way transducers. Unlike the case of finite strings, MSO-equivalent definition of two-way transducers over infinite strings needs to make decisions based on omega-regular look-ahead. Simulating this look-ahead using multiple variables with copyless updates, is the main technical challenge in our constructions. Finally, we show that type checking and functional equivalence are decidable for MSO-definable transformations of infinite strings.

Concavely-Priced Probabilistic Timed Automata

Concavely-priced probabilistic timed automata, an extension of probabilistic timed automata, are introduced. In this paper we consider expected reachability, discounted, and average price problems for concavely-priced probabilistic timed automata for arbitrary initial states. We prove that these problems are EXPTIME-complete for probabilistic timed automata with two or more clocks and PTIME-complete for automata with one clock. Previous work on expected price problems for probabilistic timed automata was restricted to expected reachability for linearly-priced automata and integer valued initial states. This work uses the boundary region graph introduced by Jurdzinski and Trivedi to analyse properties of concavely-priced (non-probabilistic) timed automata.

Optimal scheduling for constant-rate multi-mode systems

Constant-rate multi-mode systems are hybrid systems that can switch freely among a finite set of modes, and whose dynamics is specified by a finite number of real-valued variables with mode-dependent constant rates. The schedulability problem for such systems is to design a mode-switching policy that maintains the state within a specified safety set. The main result of the paper is that schedulability can be decided in polynomial time. We also generalize our result to optimal schedulability problems with average cost and reachability cost objectives. Polynomial-time scheduling algorithms make this class an appealing formal model for design of energy-optimal policies. The key to tractability is that the only constraints on when a scheduler can switch the mode are specified by global objectives. Adding local constraints by associating either invariants with modes, or guards with mode switches, lead to undecidability, and requiring the scheduler to make decisions only at multiples of a given sampling rate, leads to a PSPACE-complete schedulability problem.

Average-Time Games

An average-time game is played on the infinite graph of configurations of a finite timed automaton. The two players, Min and Max, construct an infinite run of the automaton by taking turns to perform a timed transition. Player Min wants to minimise the average time per transition and player Max wants to maximise it. A solution of average-time games is presented using a reduction to average-price game on a finite graph. A direct consequence is an elementary proof of determinacy for average-time games. This complements our results for reachability-time games and partially solves a problem posed by Bouyer et al., to design an algorithm for solving average-price games on priced timed automata. The paper also establishes the exact computational complexity of solving average-time games: the problem is EXPTIME-complete for timed automata with at least two clocks.

Safe schedulability of bounded-rate multi-mode systems

Bounded-rate multi-mode systems (BMS) are hybrid systems that can switch freely among a finite set of modes, and whose dynamics is specified by a finite number of real-valued variables with mode-dependent rates that can vary within given bounded sets. The schedulability problem for BMS is defined as an infinite-round game between two players---the scheduler and the environment---where in each round the scheduler proposes a time and a mode while the environment chooses an allowable rate for that mode, and the state of the system changes linearly in the direction of the rate vector. The goal of the scheduler is to keep the state of the system within a pre-specified safe set using a non-Zeno schedule, while the goal of the environment is the opposite. Green scheduling under uncertainty is a paradigmatic example of BMS where a winning strategy of the scheduler corresponds to a robust energy-optimal policy. We present an algorithm to decide whether the scheduler has a winning strategy from an arbitrary starting state, and give an algorithm to compute such a winning strategy, if it exists. We show that the schedulability problem for BMS is co-NP complete in general, but for two variables it is in PTIME. We also study the discrete schedulability problem where the environment has only finitely many choices of rate vectors in each mode and the scheduler can make decisions only at multiples of a given clock period, and show it to be EXPTIME-complete.

Playing stochastic games precisely

We study stochastic two-player games where the goal of one player is to achieve precisely a given expected value of the objective function, while the goal of the opponent is the opposite. Potential applications for such games include controller synthesis problems where the optimisation objective is to maximise or minimise a given payoff function while respecting a strict upper or lower bound, respectively. We consider a number of objective functions including reachability, ω-regular, discounted reward, and total reward. We show that precise value games are not determined, and compare the memory requirements for winning strategies. For stopping games we establish necessary and sufficient conditions for the existence of a winning strategy of the controller for a large class of functions, as well as provide the constructions of compact strategies for the studied objectives.

Expected reachability-time games

In an expected reachability-time game (ERTG) two players, Min and Max, move a token along the transitions of a probabilistic timed automaton, so as to minimise and maximise, respectively, the expected time to reach a target. These games are concurrent since at each step of the game both players choose a timed move (a time delay and action under their control), and the transition of the game is determined by the timed move of the player who proposes the shorter delay. A game is turn-based if at any step of the game, all available actions are under the control of precisely one player. We show that while concurrent ERTGs are not always determined, turn-based ERTGs are positionally determined. Using the boundary region graph abstraction, and a generalisation of Asarin and Malers simple function, we show that the decision problems related to computing the upper/lower values of concurrent ERTGs, and computing the value of turn-based ERTGs are decidable and their complexity is in NEXPTIME ∩ co-NEXPTIME.