Example 6.4.2.Consider the system of two equations in the two unknowns X and Y,
X = a + bY
Y = b + aX.
We can see that for a solution of this system (if any),
{a, bb, baa, … } _ X, and
{b, aa, abb, … } _ Y.
Definition 6.4.3: for n-tuples of sets, s = < s1, s2, … , sn> and t = < t1, t2, … , tn>, we say that s is less or equal to t, written s « t, if si _ ti for each i, 1ŠiŠn.
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