1.2 Structure and Summary of the Book Content 11
We begin the chapter by sketching a generic tableaux construction for testing satisability,
illustrated with the basic modal logic BML. This construction builds on, and optimises
further, a blend of ideas going back to the incremental tableaux method introduced for
the propositional dynamic logic PDL in Pratt (1979, 1980) and further developed for LTL
by Wolper (1983, 1985) and for CTL by Emerson and Halpern (1985). That version of the
tableaux method differs essentially from the more traditional approach to tableaux-based
calculi presented in Fitting (1983) and Goré (1999), because of the need for special treat-
ment of the xpoint denable temporal operators. Following the generic construction we
develop and illustrate with running examples the tableau constructions for the logics LTL
and CTL and present in detail proofs of their termination, soundness and completeness.
The most distinctive features of the tableaux building methodology presented in Chapter 13
are uniformity, conceptual simplicity and exibility. That methodology, however, does not
extend readily to the more expressive logics CTL
∗
and the L
μ
, for which alternative deci-
sion methods, using automata and games, have been developed. Such alternative methods
are developed, also for LTL and CTL, respectively in the two subsequent chapters.
In Chapter 14, ‘The Automata-Based Approach’, we present in detail that approach for
a variety of temporal logics and classes of automata and compare the advantages and the
drawbacks of the different constructions. The automata-based approach consists in reduc-
ing logical decision problems to decision problems about automata and their languages and
takes advantage of known results and decision procedures from automata theory. Here tem-
poral logical formulae are represented by automata and their models are considered to be
of uniform nature, viz. innite words (representing innite computations, for linear-time
logics) or trees (representing tree-unfoldings of transition systems, for the branching-time
logics). Thus, the question of truth of a temporal formula in a model is equivalently replaced
by the question of acceptance of that model by the automaton representing the formula.
Respectively, the satisability problem for the formula is reduced to the language nonempti-
ness problem of its representing automaton. Based on this generic transformation, in this
chapter we develop the automata-based approach for solving logical decision problems for
several logics, mainly LTL and its variants (ETL, PSL) as well as the branching-time tem-
poral logic CTL (and its fragment BML) for which we present several reductions from
formulae to automata. To do so, we make use of notions from previous chapters but also we
provide some original reductions, e.g. using alternating automata, thus also illustrating the
diversity of automata-based methods. Moreover, we present several methods for LTL-like
logics, as well as analogous methods for CTL, as a generalisation of some of the methods for
LTL. The chapter does not deal with CTL
∗
and with modal μ-calculus. Instead, we provide
decision methods based on verication games for those two logics in the next Chapter 15.
Chapter 15, ‘The Game-Theoretic Framework’, develops the approach to the main
decision problems for temporal logics based on special logical games. We present model-
checking games and satisability-checking games and use them to characterise the respec-
tive decision problems for temporal logics. These games are dened by means of an arena