 If we assign the number \(-3\) to \(v_1\) and the number \(-6\) to \(v_2\), the formula works out as true in \(\mathbb{Z}\). We express this by saying that the pair \((-3,-6)\) satisfies this formula in \(\mathbf{Z}\). Likewise (15,30) and (0,0) satisfy it, but \((2,-4)\) and (3,3) don’t. Thus the formula defines a binary relation on the integers, namely the set of pairs of integers that satisfy it. A relation defined in this way in a structure \(A\) is called a first-order definable relation in \(A\). A useful generalisation is to allow the defining formula to use added names for some specific elements of \(A\); these elements are called parameters and the relation is then definable with parameters. 