Absorbedores dinámicos de vibraciones

winning iff, for all pairs(RG_{, G}G_{)∈ Ω} GP,

η_{|= ♦R}G _{→ ♦G}G,

i.e., if there are infinitely many vertices in the play that visit RG then there have to be infinitely many vertices that visitGG_.

Infinite plays that have an infinite observation, i.e. that visit vertices inVP

ctrandVenvP in alter-

nation, are only winning ifσ is accepted byP, i.e., σ ∈ L(P), which is ensured by ΩPand

if the observation isFair[_{A]-schedulable, which is ensured by Ω}fair. The infinite plays that

have finite observations, i.e., that reach a vertexv∈ VP

termand stay there forever due to the

self-loop edge, are winning iffv /∈ Rterm_{, as otherwiseR}term _{is visited infinitely often, but}

Gterm_{, being empty, is never visited. This is the case for all fail(s) vertices of the underlying}

game, as these always represent failure to ensure admissibility. This is likewise the case for those vertices where the corresponding state inP disallows divergence or termination, respectively.

Lemma 7.3. Let η = v0 ,→ v1 ,→ . . . ,→ vt ,→ vt ,→ . . . be an infinite play in GP

reaching a vertexvt= (v, p)∈ VtermP . Then,η is winning iff the three following conditions

are satisfied:

(1) v _{6= fail(s) for some state s,}

(2) v = div(s) for some state s implies p∈ Accdiv

(3) v = stop(s) for some state s implies p_{∈ Acc}stop

Remark 7.4. The Streett pair(Rterm_{, G}term_{) is the only pair in Ω}

GP that contains vertices

fromVP

term and likewise does not contain vertices not in VtermP . Plays that visits a vertex

inVP

term then stays infinitely in that vertex. The pair (Rterm, Gterm) is thus the only pair

that determines whether these plays are accepted or not and furthermore does not affect the acceptance of plays that never visitVP

term. It is therefore possible to avoid adding an

extra pair(Rterm_{, G}term_{) by combining it with one or more of the other Streett pairs, without}

changing the set of accepted plays in the game.

7.3

Constructing most general controllers for Streett objectives

We will now show how to construct a most general controller for the objective given byP from the solution of theω-regular game_GP, i.e., the set of winning vertices for player 1

and a deterministic finite memory winning strategy in the game_GP from all the winning

vertices. It is well known that two-player, perfect information turn-basedω-regular games are determined [GTW02], i.e., that each vertex in the game is winning for exactly one of the players, that there are deterministic finite-memory winning strategies for player 1 from each of his winning vertices and that both the winning vertices and winning strategies can be computed.

A deterministic winning strategy in the complete-information Streett game correspond to an admissible decision function (cf. Lemma 5.10). Given the set of winning vertices and a family of winning strategies inGP, i.e., one decision function from each winning vertex, we can construct a most general strategy as follows. The strategy operates in two phases. In the

7.3. Constructing most general controllers for Streett objectives Chapter 7. Most general controllers for ω-regular objectives

first phase, the strategy allows all legal choices that allow staying in the region of winning vertices. The fairness condition of the strategy is used to detect violations of the Streett winning condition in the long run and forces the strategy into its second phase. There, it plays according to the decision function (deterministic winning strategy in GP) for the game vertex where the switch to the second phase occurred.

As we are interested in constructing controllers, we rely on a family of finite-memory win- ning strategies for each vertex. In practice, such finite-memory winning strategies share a common set of modes that correspond to the vertices of the game plus some additional memory. If that is not the case, i.e., each finite-memory winning strategy has its own, sep- arate set of modes, it is easy to construct a “global” winning strategy by taking the disjoint union of the different modes. We can thus consider the winning strategies from the different vertices as a simplified controller, without annotations or fairness condition and having a de- cision function template that offers exactly a single choice. In addition, we need a function mapping the winning vertices to the corresponding mode.

Definition 7.5. LetWGP be the set of vertices inGP that are winning for player 1. A finite-

memory game strategyw = (Mw, ∆w, µw, initw) for player 1 consists of

• Mw, a finite set of modes,

• initw : WGP ∩ V P

ctr → Mw, a function determining the initial mode for a given

winning vertex,

• ∆w: Mw→ 2Obs, a function providing the choice of the successor vertex, and

• µw : Mw× Obs → Mw, a function updating the mode for the observable chosen by

player 2.

Letη = v0 ,→ v00 ,→ v1,→ v01,→ . . . be a play in GP, starting from a vertexv0 ∈ W(GP)∩

VP

ctrand visiting only vertices fromVctrP ∪ VenvP . Letσ = β1β2β3. . . be the observation of

η and let m0, m1, m2, . . . be the corresponding sequence of modes, with m0 = initw(v0)

andmi+1 = µw(mi, βi). We say the play η is played according to w iff allVenvP vertices

v_i0 =hSi, Oii in η satisfy Oi = ∆w(mi). A play that eventually visits onlyVtermP -vertices

is played according tow iff its maximal prefix consisting entirely of_VctrP _{∪ Venv}P -vertices is played according tow.

A finite-memory game strategyw is winning if all maximal plays played according to w and starting in a vertex inWGP are winning inGP. In particular, all vertices visited in plays

played according tow are members of_WGP.

We now construct a most general controller for objectiveP from a solution of the complete- information game_GP, i.e., a set of verticesWGP ⊆ V

P_{where player 1 can ensure winning}

in_GP and a winning finite-memory game strategyw = (Mw, ∆w, µw, initw) in case that

the initial vertex([Q0]∗, p0)∈ WGP is winning for player 1.

The controller consists of two phases. In the first phase, it ensures that the winning region WGP will not be left. The fairness condition ensures that non-winning plays eventually

switch to the second phase, where the controller then plays according tow. In case that the initial vertex of the game is winning, i.e.,([Q0]∗, p0)∈ WGP, we construct a controller

Chapter 7. Most general controllers for ω-regular objectives 7.3. Constructing most general controllers for Streett objectives

• a set Ann = {1, 2} for the two phases,

• the set of modes M = M1 ˙∪ Mw, which is the disjoint union of the modes of the first

phaseM1 =WGP∩ V P

ctrand of the second phaseMw,

• the initial mode m0= ([Q0]∗, p0)∈ M1 being the initial vertex of the game,

• the decision function template ∆ : M → 22Obs

with ∆(m) =

(

hO, 1i, hO, 2i : O ∈ ∆1(m)

form∈ M1  hO, 2i : O = ∆w(m) form∈ Mw,

where hO, ai denotes the set {hβ, ai : β ∈ O} and ∆1 : M1 → 22 Obs

is the deci- sion function template that provides all the choices that ensure staying in the winning region, i.e.,

∆1( (S, p) ) =



O : (S, p) ,→P (hS, Oi, p) and (hS, Oi, p) ∈ WGP

, • the next-mode function µ : M × Obs → M, with

µ(m,_{hβ, ai) =}          µ1(m, β) form∈ M1anda = 1 initw µ1(m, β)
∈ MG form∈ M1anda = 2 µw(m, β) form∈ MGanda = 2

and whereµ1 is the function that returns the unique β-successor of the vertex m in

GP as in the previous chapter.

• the fairness condition fair = (F1, . . . , Fk) for the Streett winning condition

ΩGP ={(R G 1, G G 1), . . . , (R G k, G G

k)} of GP such that, for16 i 6 k:

Fi = {(m, hO, 2i) : m∈ M1∩ RGi andhO, 2i ∈ ∆(m)}

∪ {(m, hO, 1i, (m, hO, 2i) : m = (S, p) ∈ M1, O∈ ∆1(m) and

(hS, Oi, p) ∈ GGi orm∈ G G i}.

The controller starts in the first phase, in the mode corresponding to the initial game vertex. In the first phase, it offers choices according to∆1, which ensures staying in the winning

region and is offered in two variants: one annotated with 1, the other annotated with 2. The controller stays in the first phase until an annotated observable with annotation2 is observed. In this case, the controller switches to phase two, the initial mode in the game strategyw for the corresponding vertex. In this second phase, the annotations become irrel- evant and are fixed arbitrarily at annotation2. Now the controller behaves according to ∆w

andµw, i.e., the game strategyw.

The fairness condition of the controller ensures that the controller may only stay in the first phase as long as the corresponding play satisfies the Streett winning condition in the game: If there is a Streett pair(RG_i, GG_i) where the mode (game vertex) infinitely often only occurs inRG_i but not inGG_i, the fairness condition requires that eventually a choice annotated with annotation2 is chosen, switching to the second phase. On the other hand, if for every Streett pair(RG_i, GG_i) where RG_i is visited infinitely often in the first phaseGG_i is visited infinitely often as well, it is fine to always schedule choices with annotation1, and the controller can stay in the first phase.

7.3. Constructing most general controllers for Streett objectives Chapter 7. Most general controllers for ω-regular objectives

Theorem 7.6 (Soundness of the constructed controller for_P).

(a) If([Q0]∗, p0) ∈ WGP then the strategySC induced by the controllerC constructed

above enforces_P.

(b) If there exists an admissible decision functiond that enforces_{P then} ([Q0]∗, p0)∈ WGP andd is an instance of SC.

Hence, if the objective specified by_{P is enforceable then S}C is a most general strategy

enforcingP. Proof.

ad (a). To show that SC enforces P, we have to show that SC is admissible and that

π _{∈ L}A(P) for all initial SC-pathsπ, i.e., that all instances d of SCare admissible and that

for all instancesd all d-paths are contained inLA(P).

We will first relate SC-paths with the executions in the controller and the corresponding

plays in_GP.

Letπ be an initial SC-path and letπCbe one of the corresponding initialC ./A-paths (see

Lemma 4.2) withπC|A = π. Let

π0= m0 β1,a1 −−−→ m1 β2,a2 −−−→ m2 β3,a3 −−−→ . . . ∈ M × (Obs × M)∞

be the corresponding execution inC, i.e., with invisible actions removed, the states projected on the modes and with the annotated observables labeling the transitions.

Letd be a corresponding decision function and let O0₁O₂0O0₃. . . _{∈ (2}Obs)∞ be the corre- sponding sequence of annotated controller choices of an annotated instance corresponding to d, with O0_i ∈ ∆(mi) and hβi, aii ∈ Oi0 for 1 6 i 6 |π0| and satisfying the fairness

condition of the controller. This corresponds to a maximal, initial play in_GP

η = v0 ,→P v00 β1 ,_→P v1 ,→P v10 β2 ,_→P v2 ,→P v20 β3 ,_→P . . . , wherev0_i = (hS, Oi, p) ∈ VP

envforvi = (S, p) and where O = Oi0|Obs ∈ ∆(mi). By con-

struction of the controller, all the vertices in the play belong to _WGP. Furthermore,η is

winning forΩGP. In case that at some point the controller switches to the second phase, the

suffix ofη after the phase switch is winning because it is played according to w. But then the whole play is winning, because a play in a Streett game is winning if any suffix is winning. In the other case, where no switch to the second phase occurs, the playη is winning because it satisfies the fairness conditionfair. Assume by contradiction that η is not winning. Then there exists a Streett acceptance pair (RG, GG) ∈ ΩGP such thatη |= ♦R

G _{∧ ¬♦G}G_.

LetF be the corresponding element of the controller’s fairness condition fair. As there is no switch of phase, for alli> 1, the choices O0_iare annotated with1. As η|= ♦RG_{, there}

are infinitely many modesmi ∈ M1∩ RG, i.e., whereF(β1, . . . , βi) 6= ∅. On the other

hand, asη _{|= ♦¬G}G_{, from some indexj ∈ N on it is the case that O}0

Chapter 7. Most general controllers for ω-regular objectives 7.3. Constructing most general controllers for Streett objectives

for alli > j. But this violates condition (I2) in Definition 3.2 due to the fact that d is an instance ofC. We conclude that η is winning.

We now show thatSC is admissible. Suppose by contradiction thatSC is not admissible

by violating one of the conditions (A1), (A2) or (A3) of admissibility. By Lemma 5.9, this corresponds to the fact thatη reaches a vertex of the form fail(s) for some state s. But any maximal play that reaches a vertex of the form fail(s) is not winning for winning condition ΩGP as fail(s) 6∈ WGP, which contradicts the fact that all vertices of η belong toWGP.

It remains to show thatSC respectsFair[A]. Suppose by contradiction that SC does not

respectFair[A]. Then, there exists a Fi ∈ Fair[A] and an initial SC-pathπ such that obs(π)

is notFi-schedulable. But then the corresponding playη does not satisfy the Streett winning

condition(Rfair

i , Gfairi ), which contradicts the fact that η is winning.

Similarly, we show thatπ_{∈ L}A(P) for all initial SC-pathsπ, by showing that π∈ LA(P)

for all initiald-paths and for all instances d of SC.

We first consider the case that the pathπ and the observation σ = β1β2. . . βn are finite.

Then, the corresponding playη eventually always visits a vertex (stop(s), p) with s_|A =

last(π). As the play is winning, p ∈ Accstop and therefore π ∈ LA(P). Likewise, if

π is infinite then π is divergent and the corresponding play eventually stays in a vertex (div(s), p) with p_{∈ Acc}div, i.e.,π diverges from the state s|Aandπ∈ LA(P).

It remains to show thatπ ∈ LA(P) in the case that both π and σ are infinite, which is the

case if observationσ is accepted by_{P. We show that σ is accepted by P if the play η is} winning. Note that the runξ = p0p1p2, . . . for σ inP corresponds to the states pi in the

playη. Let (R_jG, GG_j) be one of the Streett pairs in the winning condition ΩP of the game

GP and let(Rj, Gj) be the corresponding Streett pair in the acceptance condition Acc ofP.

By construction ofΩGP, for alli> 0,

vi = (Si, pi)∈ RG_j ⇐⇒ pi ∈ Rj and vi= (Si, pi)∈ GG_j ⇐⇒ pi ∈ Gj.

Therefore, ifRG_j is visited infinitely often in the playη then Rj is visited infinitely often in

the runξ. Also, if GG_j is visited infinitely often in the playη then Gj is visited infinitely

often in the runξ. As this is the case for all Streett pairs in the acceptance condition ofP, the runξ in_{P for observation σ is accepted by P iff the play η is winning. As we know that} η is winning, π_{∈ L}A(P). We conclude that SCenforcesP for (A, Fair[A]).

ad (b). We show that if there exists an admissible decision functiond that enforcesP then the initial vertex ofGP is winning,([Q0]∗, p0)∈ WGP, andd is an instance of SC.

Claim 1. Let d be an admissible decision function that enforces P. Then, ([Q0]∗, p0)∈ WGP.

7.3. Constructing most general controllers for Streett objectives Chapter 7. Most general controllers for ω-regular objectives

As usual, w.l.o.g, we assume thatd only schedules schedulable observables. We show that ([Q0]∗, p0) ∈ WGP by showing the stronger claim that all verticesv ∈ V

P _{that are reach-}

able by an initial, maximal play inGP _{played according to decision functiond, i.e., where}

the player 1 choices are determined by d, are winning, v _{∈ W}GP. As the initial vertex

([Q0]∗, p0) is trivially reachable, it is then surely contained inWGP.

Assume by contradiction that there is an infinite, initial playη inGP where the successor

chosen by player 1 is determined byd depending on the history of the play with at least one of the vertices inη not contained in_WGP.

Then, the playη either consists entirely of alternating VP

ctr andVenvP vertices or it reaches

some vertex from_VtermP . In the first case, the playη has the form η = v0 ,→P v0₀ β1 ,→P v1 ,→P v0₁ β2 ,→P v2 ,→P v0₂ β3 ,→P . . . ,

starting inv0 = ([Q0]∗, p0), with vi = (Si, pi) ∈ VctrP andvi0 = (hSi, d(β1. . . βi)i, pi) ∈

VP

envfor someSiandpiandi> 0.

In the second case, the playη has the form η = v0 ,→P v00

β1

,→P v1 ,→P v10 . . . ,→P vk ,→P vk0 ,→P vt ,→P vt ,→P . . . ,

starting inv0 = ([Q0]∗, p0), with vi = (Si, pi) ∈ VctrP andvi0 = (hSi, d(β1. . . βi)i, pi) ∈

VP

envfor someSiandpiand06 i 6 k and with vt∈ VtermP .

By the assumption, not all vertices inη are in_WGP. We first consider the case that the play

reaches a vertex fromVP

termand thatvtis the first and only vertex inη that is not contained

inWGP.

By the construction of the game winning conditionΩGP,vt∈ W/ GP implies thatvt∈ R term_,

as otherwise the infinite play consisting solely ofvtvertices would be winning for player 1.

It can not be the case thatvtis of the form(fail(s), p), as then d would be non-admissible

(Lemma 5.9). Ifvtis of the form(div(s), p), then Lemma 5.8 provides the existence of an

initial d-path π with observation obs(π) = β1. . . βk. As vtis not winning, p /∈ Accdiv.

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