Game Development Reference
In-Depth Information
u and v are the initial vertices of G u and G v , and T u and T v are the sets
of target vertices of G u and G v , respectively. Consider a game G obtained
by taking the disjoint union of G u and G v extended with a fresh stochastic
vertex s with two outgoing transitions s
−−→ u and s 0 . 5
0 . 5
−−→ v . The set of target
vertices of G is T u ∪ T v , and the initial vertex is s . Since val ( u ) = 0 and
val ( v ) = 1, we obtain that val ( s )= 2 . Now consider the temporal objective
P 0 . 5 F t . First, assume that player has a winning strategy σ in s . Since
player has no optimal maximising strategy in v , there is a constant P σ < 2
such that
( σ,π )
s
P σ
P
( Reach ( T v ))
for every π
Π. Since val ( u ) = 0, there
( σ,π )
s
( Reach ( T u )) < 2
P σ
is a strategy π of player
such that
P
for every
( σ,π )
s
T v )) < 2
σ
Σ. Hence,
P
( Reach ( T u
which contradicts the assumption
that σ is a winning strategy for player
. Similarly, one can show that there
is no winning strategy for player
which would achieve the negated objective
P < 0 . 5 F t against every strategy of player
.
Now let us assume that G is finitely branching, and let us fix a vertex v
of G . For technical convenience, assume that every target vertex t has only
one outgoing transition t → t . The second part of Proposition 5.9 is not
completely trivial, because player does not necessarily have an optimal
maximising strategy in v even if G is finitely branching. Observe that if
>val ( v ), then player
has a ( P F t )-winning strategy in v (he may use,
e.g., an ε -optimal maximising strategy where ε =(
val ( v )) / 2). Similarly, if
¬ P F t )-winning strategy in v . Now assume
that = val ( v ). Obviously, it su ces to show that if player
<val ( v ), then player
has a (
does not have
¬ P F t )-winning strategy in v , then player
has a ( P F t )-winning
a(
strategy in v . This means to show that
σ,π
v
π
Π
σ
Σ:
P
( Reach ( T ))
(5.8)
implies
σ,π
v
σ
Σ
π
Π:
P
( Reach ( T ))
.
(5.9)
Observe that if is > , then (5.8) does not hold because player ♦ has an
optimal minimising strategy by Proposition 5.6. For the constraint
0, the
statement is trivial. Hence, it su ces to consider the case when
is
and
= val ( v ) > 0. Assume that (5.8) holds. We say that a vertex u
V is good
if
σ, u ( Reach ( T ))
π
Π
σ
Σ:
P
val ( u ) .
(5.10)
u of G is optimal if either u
Further, we say that a transition u
V ,or
V and val ( u )= val ( u ). Observe that for every u
u
V
V
V there
u , because G is finitely branching.
is at least one optimal transition u
 
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