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Weighted constraint satisfaction problem : ウィキペディア英語版
Weighted constraint satisfaction problem
Whereas all constraints in a constraint satisfaction problem (CSP) must be satisfied, a Weighted Constraint Satisfaction Problem (WCSP) is a constraint satisfaction problem where constraints can be violated (according a violation degree) and in which preferences among solutions can be expressed. Many real problems can be represented as Constraint Satisfaction Problem. However, a wide range of problems are over-constrained (no solution can be found without violating at least one constraint) or have multiple solutions and the goal is to find the solution having minimal cost according to a cost function. This kind of Constraint Satisfaction Problem are called Weighted Constraint Satisfaction Problem (WCSP).
==Formal definition==
A Weighted Constraint Network (WCN) is a triplet \langle X,C,k \rangle where X is a finite set of variables, C is a finite set of soft constraints and k>0 is either a natural integer or \infty.
Each soft constraint c_S \in C involves an ordered set S of variables, called its scope, and is defined as a cost function from l(S) to \langle 0,...,k \rangle where l(S) is the set of possible instantiations of S . When an instantiation I \in l(S) is given the cost k, i.e., c_S(I)=k, it is said forbidden. Otherwise it is permitted with the corresponding cost (0 being completely satisfactory).
In WCSP, specific subclass of Valued CSP (VCSP), costs are combined with the specific operator \oplus defined as: \forall \alpha, \beta \in \langle 0,...,k \rangle, \alpha \oplus \beta = min(k,\alpha+\beta). The partial inverse of \oplus is \ominus defined by: if 0 \le \beta \le \alpha < k, \alpha \ominus \beta = \alpha - \beta and if 0 \le \beta , k \ominus \beta = k.
Without any loss of generality, the existence of a nullary constraint c_\empty (a cost) as well as the presence of a unary constraint c_x for every variable x is assumed.
The total cost of an instantiation I \in l(S) on a soft constraint c_S, includes the cost of I on c_S as well as the nullary cost c_ and the unary costs for I of the variables in S.
Considering a WCN, the usual (NP-hard) task of WCSP is to find a complete instantiation with a minimal cost.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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