Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

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Propositional interval temporal logics: some promising paths Angelo Montanari Dipartimento di Matematica e Informatica Universit` a di Udine, Italy

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

2

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Interval temporal logics There exists a broad and multidisciplinary interest in interval temporal logic: • philosophy • linguistics • artificial intelligence • theoretical computer science

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

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Halpern and Shoham’s HS HS features four basic unary operators: hBi (begins) and hEi (ends), and their transposes hBi (begun by) and hEi (ended by) Given a formula φ and an interval [d0 , d1 ], hBiφ holds over [d0 , d1 ] if φ holds over [d0 , d2 ], for some d0 ≤ d2 < d1 , and hEiφ holds over [d0 , d1 ] if φ holds over [d2 , d1 ], for some d0 < d2 ≤ d1 Many other operators can be derived from the basic ones Halpern and Shoham have shown that the validity/satisfiability problem for HS over various classes of linear orders is (highly) undecidable by a suitable encoding of the halting problem Later, Lodaya showed that some undecidability proofs for HS can actually be tailored to its hBihEi fragment &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

4

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Venema’s CDT Venema’s CDT logic has three binary operators C (chop), D, and T , which correspond to the ternary interval relations occurring when an extra point is added in one of the three possible distinct positions with respect to the two endpoints of the current interval (between, before, and after ), plus a modal constant π which holds over a given interval if and only if it is a point-interval Since HS can be embedded into CDT, undecidability results for the latter follow from those for the former

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

5

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Moszkowski’s PITL Moszkowski PITL features the two modalities ° (next) and C (the specialization of the chop operator for discrete structures) Given two formulas φ, ψ and a (finite) interval d0 , . . . , dn , °φ holds over d0 , . . . , dn if and only if φ holds over d1 , . . . , dn , while φCψ holds over d0 , . . . , dn if and only if there exists i, with 0 ≤ i ≤ n, such that φ holds over d0 , . . . , di and ψ holds over di , . . . , dn

PITL has been proved to be undecidable by a reduction from the problem of testing the emptiness of the intersection of two grammars in Greibach form &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

6

$

Goranko, Montanari, and Sciavicco’s PNL Goranko, Montanari, and Sciavicco’s PNL has two unary modalities for right and left interval neighborhoods hAi and hAi Given a formula φ and an interval [d0 , d1 ], hAiφ holds over [d0 , d1 ] if φ holds over [d1 , d2 ], for some d2 > d1 , and hAiφ holds over [d0 , d1 ] if φ holds over [d2 , d0 ], for some d2 < d0 (strict)

While the undecidability of first-order Neighborhood Logic (NL) can be easily proved by embedding HS in it, the decidability problem for its propositional fragments, which can be embedded into HS, is still open &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

7

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Interval temporal logics are very expressive

Propositional interval temporal logics are very expressive temporal logics, with simple syntax and semantics, which allow one to naturally express statements that refer to time intervals and continuous processes.

It can be shown that interval logics such as HS and CDT are strictly more expressive than every point-based temporal logic on linear orderings

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

8

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Interval temporal logics are (highly) undecidable We have that the validity problem for HS, interpreted over any class of ordered structures with an infinitely ascending sequence, is at least r.e.-hard In the case of Dedekind-complete ordered structures having an infinitely ascending sequence, it becomes Π11 -hard (this means that the validity problem for HS over natural numbers, integers, or reals is not recursively axiomatizable) As a matter of fact, it is possible to show that undecidability occurs even without existence of infinitely ascending sequences The problem of finding fragments which are expressive enough to express meaningful statements about time intervals and decidable has been recognized as a fundamental one by several authors &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

9

$

A simple path to decidability In propositional interval temporal logics undecidability is the rule and decidability the exception Interval logics make it possible to express properties of pairs of time points (think of intervals as constructed out of points), rather than single time points. In most cases, this feature prevents one from the possibility of reducing interval-based temporal logics to point-based ones However, there are a few exceptions where the logic satisfies suitable syntactic and/or semantic restrictions, and such a reduction can be defined, thus allowing one to benefit from the good computational properties of point-based logics &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

10

$

Case 1: constraining interval modalities This is the case with the hBihBi and hEihEi fragments of HS. Consider the case of hBihBi (the case of hEihEi is similar). The decidability of hBihBi can be obtained by embedding it into the propositional temporal logic of linear time LTL[F,P] with temporal modalities F (sometime in the future) and P (sometime in the past) The formulas of hBihBi are simply translated into formulas of LTL[F,P] by a mapping that replaces hBi by P and hBi by F . LTL[F,P] has the finite model property and is decidable

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

11

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Case 2: constraining temporal structures This is the case with the so-called Split Logics (SLs) SLs are propositional interval logics equipped with operators borrowed from HS and CDT, but interpreted over specific structures, called split structures. The distinctive feature of split structures is that every interval can be ‘chopped’ in at most one way The decidability of various SLs has been proved by embedding them into the first-order fragments of monadic second-order decidable theories of time granularity (which are proper extensions of the well-known monadic second-order theory of one successor S1S) &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

12

$

Case 3: constraining semantic interpretations Another possibility is to constrain the relation between the truth value of a formula over an interval and its truth value over the subintervals of that interval This is the case with the decidable fragment of PITL extended with quantification over propositional variables (QPITL) which has been obtained by imposing a suitable locality constraint which states that a propositional variable is true over an interval if and only if it is true at its starting point (point-interval) By exploiting such a constraint, decidability of QPITL can be proved by embedding it into Quantified LTL (QLTL) &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

13

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Some promising alternative paths In view of the previous analysis, a (maybe the) major challenge in the area of interval temporal logics is to identify expressive enough, yet decidable, fragments and/or logics which are genuinely interval-based A logic is genuinely interval-based if it cannot be directly translated into point-based logics and it does not invoke locality, or other semantic restrictions, reducing the interval-based semantics to the point-based one

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

14

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Path 1: tableau-based decision procedures By combining syntactic restrictions (temporal operators: no past operators) and semantic ones (temporal structure: natural numbers), we recently succeeded in devising a tableau-based decision procedure for the future fragment of PNL, interpreted over natural numbers Unlike the case of the hBihBi and hEihEi fragments, in such a case we cannot abstract way from the left endpoint of intervals: there can be contradictory formulas that hold over intervals that have the same right endpoint, but a different left one

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

15

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Path 1: tableau-based decision procedures (con’t) The proposed tableau method partly resembles the tableau-based decision procedure for LTL However, while the latter takes advantage of the so-called fix-point definition of temporal operators which makes it possible to proceed stepwise by splitting every temporal formula into a (possibly empty) part related to the current state and a part related to the next state, the former must also keep track of universal and (pending) existential requests coming from the past.

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

16

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Path 2: restrict attention to finite models There are contexts where interpretations in which infinitely many statements (events) hold (occur) in a finite space of time are of no interest. Examples can be found in computational linguistics Pratt developed a decidable interval logic of temporal prepositions which is interpreted over finite models (notice that, under such an assumption, any formula with only infinite models turns out to be unsatisfiable) Since the restriction to finite models is neither a necessary nor a sufficient condition for decidability, it would be interesting to investigate the effects of imposing it to other interval logics &

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Interval Temporal Logics

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

17

$

Path 3: connections with other decidable logics Research on interval temporal logics can benefit from interesting connections that there seem to be between them and other decidable logics Examples Montanari and Sciavicco are investigating the relations between full PNL and two variable first-order logic over ordered domains Shapirovsky and Shehtman explored the relations between the logics of subintervals, the so-called D logics, and the logics of Minkowski spacetime &

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Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

1

$

Propositional interval temporal logics: some promising paths Angelo Montanari Dipartimento di Matematica e Informatica Universit` a di Udine, Italy

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

2

$

Interval temporal logics There exists a broad and multidisciplinary interest in interval temporal logic: • philosophy • linguistics • artificial intelligence • theoretical computer science

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

3

$

Halpern and Shoham’s HS HS features four basic unary operators: hBi (begins) and hEi (ends), and their transposes hBi (begun by) and hEi (ended by) Given a formula φ and an interval [d0 , d1 ], hBiφ holds over [d0 , d1 ] if φ holds over [d0 , d2 ], for some d0 ≤ d2 < d1 , and hEiφ holds over [d0 , d1 ] if φ holds over [d2 , d1 ], for some d0 < d2 ≤ d1 Many other operators can be derived from the basic ones Halpern and Shoham have shown that the validity/satisfiability problem for HS over various classes of linear orders is (highly) undecidable by a suitable encoding of the halting problem Later, Lodaya showed that some undecidability proofs for HS can actually be tailored to its hBihEi fragment &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

4

$

Venema’s CDT Venema’s CDT logic has three binary operators C (chop), D, and T , which correspond to the ternary interval relations occurring when an extra point is added in one of the three possible distinct positions with respect to the two endpoints of the current interval (between, before, and after ), plus a modal constant π which holds over a given interval if and only if it is a point-interval Since HS can be embedded into CDT, undecidability results for the latter follow from those for the former

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

5

$

Moszkowski’s PITL Moszkowski PITL features the two modalities ° (next) and C (the specialization of the chop operator for discrete structures) Given two formulas φ, ψ and a (finite) interval d0 , . . . , dn , °φ holds over d0 , . . . , dn if and only if φ holds over d1 , . . . , dn , while φCψ holds over d0 , . . . , dn if and only if there exists i, with 0 ≤ i ≤ n, such that φ holds over d0 , . . . , di and ψ holds over di , . . . , dn

PITL has been proved to be undecidable by a reduction from the problem of testing the emptiness of the intersection of two grammars in Greibach form &

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Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

6

$

Goranko, Montanari, and Sciavicco’s PNL Goranko, Montanari, and Sciavicco’s PNL has two unary modalities for right and left interval neighborhoods hAi and hAi Given a formula φ and an interval [d0 , d1 ], hAiφ holds over [d0 , d1 ] if φ holds over [d1 , d2 ], for some d2 > d1 , and hAiφ holds over [d0 , d1 ] if φ holds over [d2 , d0 ], for some d2 < d0 (strict)

While the undecidability of first-order Neighborhood Logic (NL) can be easily proved by embedding HS in it, the decidability problem for its propositional fragments, which can be embedded into HS, is still open &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

7

$

Interval temporal logics are very expressive

Propositional interval temporal logics are very expressive temporal logics, with simple syntax and semantics, which allow one to naturally express statements that refer to time intervals and continuous processes.

It can be shown that interval logics such as HS and CDT are strictly more expressive than every point-based temporal logic on linear orderings

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

8

$

Interval temporal logics are (highly) undecidable We have that the validity problem for HS, interpreted over any class of ordered structures with an infinitely ascending sequence, is at least r.e.-hard In the case of Dedekind-complete ordered structures having an infinitely ascending sequence, it becomes Π11 -hard (this means that the validity problem for HS over natural numbers, integers, or reals is not recursively axiomatizable) As a matter of fact, it is possible to show that undecidability occurs even without existence of infinitely ascending sequences The problem of finding fragments which are expressive enough to express meaningful statements about time intervals and decidable has been recognized as a fundamental one by several authors &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

9

$

A simple path to decidability In propositional interval temporal logics undecidability is the rule and decidability the exception Interval logics make it possible to express properties of pairs of time points (think of intervals as constructed out of points), rather than single time points. In most cases, this feature prevents one from the possibility of reducing interval-based temporal logics to point-based ones However, there are a few exceptions where the logic satisfies suitable syntactic and/or semantic restrictions, and such a reduction can be defined, thus allowing one to benefit from the good computational properties of point-based logics &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

10

$

Case 1: constraining interval modalities This is the case with the hBihBi and hEihEi fragments of HS. Consider the case of hBihBi (the case of hEihEi is similar). The decidability of hBihBi can be obtained by embedding it into the propositional temporal logic of linear time LTL[F,P] with temporal modalities F (sometime in the future) and P (sometime in the past) The formulas of hBihBi are simply translated into formulas of LTL[F,P] by a mapping that replaces hBi by P and hBi by F . LTL[F,P] has the finite model property and is decidable

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

11

$

Case 2: constraining temporal structures This is the case with the so-called Split Logics (SLs) SLs are propositional interval logics equipped with operators borrowed from HS and CDT, but interpreted over specific structures, called split structures. The distinctive feature of split structures is that every interval can be ‘chopped’ in at most one way The decidability of various SLs has been proved by embedding them into the first-order fragments of monadic second-order decidable theories of time granularity (which are proper extensions of the well-known monadic second-order theory of one successor S1S) &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

12

$

Case 3: constraining semantic interpretations Another possibility is to constrain the relation between the truth value of a formula over an interval and its truth value over the subintervals of that interval This is the case with the decidable fragment of PITL extended with quantification over propositional variables (QPITL) which has been obtained by imposing a suitable locality constraint which states that a propositional variable is true over an interval if and only if it is true at its starting point (point-interval) By exploiting such a constraint, decidability of QPITL can be proved by embedding it into Quantified LTL (QLTL) &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

13

$

Some promising alternative paths In view of the previous analysis, a (maybe the) major challenge in the area of interval temporal logics is to identify expressive enough, yet decidable, fragments and/or logics which are genuinely interval-based A logic is genuinely interval-based if it cannot be directly translated into point-based logics and it does not invoke locality, or other semantic restrictions, reducing the interval-based semantics to the point-based one

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

14

$

Path 1: tableau-based decision procedures By combining syntactic restrictions (temporal operators: no past operators) and semantic ones (temporal structure: natural numbers), we recently succeeded in devising a tableau-based decision procedure for the future fragment of PNL, interpreted over natural numbers Unlike the case of the hBihBi and hEihEi fragments, in such a case we cannot abstract way from the left endpoint of intervals: there can be contradictory formulas that hold over intervals that have the same right endpoint, but a different left one

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

15

$

Path 1: tableau-based decision procedures (con’t) The proposed tableau method partly resembles the tableau-based decision procedure for LTL However, while the latter takes advantage of the so-called fix-point definition of temporal operators which makes it possible to proceed stepwise by splitting every temporal formula into a (possibly empty) part related to the current state and a part related to the next state, the former must also keep track of universal and (pending) existential requests coming from the past.

&

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

16

$

Path 2: restrict attention to finite models There are contexts where interpretations in which infinitely many statements (events) hold (occur) in a finite space of time are of no interest. Examples can be found in computational linguistics Pratt developed a decidable interval logic of temporal prepositions which is interpreted over finite models (notice that, under such an assumption, any formula with only infinite models turns out to be unsatisfiable) Since the restriction to finite models is neither a necessary nor a sufficient condition for decidability, it would be interesting to investigate the effects of imposing it to other interval logics &

%

Interval Temporal Logics

'

Burlington, Vermont, USA - TIME 2005 - June 23/25, 2005

17

$

Path 3: connections with other decidable logics Research on interval temporal logics can benefit from interesting connections that there seem to be between them and other decidable logics Examples Montanari and Sciavicco are investigating the relations between full PNL and two variable first-order logic over ordered domains Shapirovsky and Shehtman explored the relations between the logics of subintervals, the so-called D logics, and the logics of Minkowski spacetime &

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