The truth set of the proposition \(\{1, 2\} \cap A = \emptyset\text{,}\) taken as a proposition over the power set of \(\{1, 2, 3, 4\}\) is \(\{\emptyset , \{3\}, \{4\}, \{3, 4\}\}\text{.}\)
Over the universe \(\mathbb{Z}\) (the integers), the truth set of \(4x^2- 3x = 0\) is \(\{0\}\text{.}\) If the universe is expanded to the rational numbers, the truth set becomes \(\{0, 3/4\}\text{.}\) The term solution set is often used for the truth set of an equation such as the one in this example.
\((s - 1)(s + 1) = s^2 - 1\) is a tautology over the rational numbers. \(x^2-2 = 0\) is a contradiction over the rationals.
The truth sets of compound propositions can be expressed in terms of the truth sets of simple propositions. For example, if \(a \in T_{p\land q}\) if and only if \(a\) makes \(p \land q\) true. This is true if and only if \(a\) makes both \(p\) and \(q\) true, which, in turn, is true if and only if \(a \in T_p\cap T_q\text{.}\) This explains why the truth set of the conjunction of two propositions equals the intersection of the truth sets of the two propositions. The following list summarizes the connection between compound and simple truth sets
Definition 3.6.9. Equivalence of propositions over a universe.
Two propositions, \(p\) and \(q\text{,}\) are equivalent if \(p \leftrightarrow q\) is a tautology. In terms of truth sets, this means that \(p\) and \(q\) are equivalent if \(T_p=T_q\) .