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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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Definition 19.3: A primitive constraint comprises of one or more<br />

constraint relation symbols together with their arguments, but cannot have a<br />

conjunction (Λ) in it.<br />

For example, x ≤2 , (x +y ) ≤ 2 are primitive constraints.<br />

Definition 19.4: A non-primitive or generic constraint is a conjunction<br />

(Λ) of primitive constraints.<br />

For example, (x+ y ≤ 2) Λ ( y ≤ 3 ) is a non-primitive constraint.<br />

Thus formally a constraint of the form C1 Λ C2 Λ ………….. Λ Cn, n>1 is a<br />

non-primitive constraint, when C1, C2, …………Cn all are primitive<br />

constraints [4].<br />

Definition 19.5: A valuation θ for the set of variables v is an assignment<br />

of the values from the constraint domain to the variables v [4] . An expression<br />

E having variables v is given a value θ(E), computed by replacing the<br />

variable by its assigned value in the expression.<br />

For example, let v= {v1 ,v 2}; the domain of v1 , v2 being 1 ≤ v1 , v2 ≤2 ; let<br />

E= v1 2 + v2 2 ; now θ(E) = [v1 2 + v2 2 ] v1 =x , v2 = y where 1 ≤ x, y ≤ 2.<br />

Definition 19.6: A constraint is called satisfiable, if there exists at least one<br />

solution satisfying the constraint.<br />

For instance, {(x+ y ≤2 ) ∧ ( 0 ≤x, y ≤2 ) } are satisfiable constraints for<br />

integer x, y as there exists a solution x = y =1 that satisfies the constraints.<br />

On the other h<strong>and</strong>, {(x+ y ≤ 2) ∧ ( x >2) ∧ (y>2)} does not have a solution for<br />

integer x, y. Consequently, constraint (x+ y ≤ 2) ∧ ( x > 2) ∧ (y > 2 ) is<br />

called unsatisfiable.<br />

Definition 19.7: Two constraints C1 <strong>and</strong> C2 having same solution set are<br />

called equivalent <strong>and</strong> the equivalent relation between C1 <strong>and</strong> C2 is denoted<br />

by<br />

C1 ↔ C2 .<br />

For example, C1 = { (x+ y ≤ 2) Λ (x=1) Λ (y0)}<br />

<strong>and</strong> C2= { (x+ y ≤ 2) Λ (0< x) Λ (x≤2) Λ (0

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