**Go backward to** Solution to part (a)

**Go up to** 6 Arc Consistency

## Solution to part (b)

Is the arc *<Y,X>* arc consistent?
**yes.**
Arc *<Y,X>* is arc consistent means that for every *Y* there
exists an *X* such that *r(X,Y)* is true.
The following table shows for each value of *Y* a value of *X* such
that *R(X,Y)* is true:

Thus the arc *<Y,X>* is arc consistent.
The most common mistake:
The relation *r(X,Y)* corresponds to two arcs, *<X,Y>*
and *<Y,X>*. You do not swap the arguments to *r(X,Y)*.

Computational
Intelligence online
material, ©David Poole, Alan Mackworth and Randy Goebel, 1999