1.2.3: Special Properties of Relations
Some kinds of relations turn out to be so common that they have been given special names. For instance, \(\le\) and \(\subseteq\) both relate their respective domains (say, \(\Nat\) in the case of \(\le\) and \(\Pow{A}\) in the case of \(\subseteq\) ) in similar ways. To get at exactly how these relations are similar, and how they differ, we categorize them according to some special properties that relations can have. It turns out that (combinations of) some of these special properties are especially important: orders and equivalence relations.
A relation \(R \subseteq A^2\) is reflexive iff, for every \(x \in A\) , \(Rxx\) .
A relation \(R \subseteq A^2\) is transitive iff, whenever \(Rxy\) and \(Ryz\) , then also \(Rxz\) .
A relation \(R \subseteq A^2\) is symmetric iff, whenever \(Rxy\) , then also \(Ryx\) .
A relation \(R \subseteq A^2\) is anti-symmetric iff, whenever both \(Rxy\) and \(Ryx\) , then \(x=y\) (or, in other words: if \(x\neq y\) then either \(\lnot Rxy\) or \(\lnot Ryx\) ).
In a symmetric relation, \(Rxy\) and \(Ryx\) always hold together, or neither holds. In an anti-symmetric relation, the only way for \(Rxy\) and \(Ryx\) to hold together is if \(x = y\) . Note that this does not require that \(Rxy\) and \(Ryx\) holds when \(x = y\) , only that it isn’t ruled out. So an anti-symmetric relation can be reflexive, but it is not the case that every anti-symmetric relation is reflexive. Also note that being anti-symmetric and merely not being symmetric are different conditions. In fact, a relation can be both symmetric and anti-symmetric at the same time (e.g., the identity relation is).
A relation \(R \subseteq A^2\) is connected if for all \(x,y\in A\) , if \(x \neq y\) , then either \(Rxy\) or \(Ryx\) .
Give examples of relations that are (a) reflexive and symmetric but not transitive, (b) reflexive and anti-symmetric, (c) anti-symmetric, transitive, but not reflexive, and (d) reflexive, symmetric, and transitive. Do not use relations on numbers or sets.
A relation \(R \subseteq A^2\) is called irreflexive if, for all \(x \in A\) , not \(Rxx\) .
A relation \(R \subseteq A^2\) is called asymmetric if for no pair \(x,y\in A\) we have both \(Rxy\) and \(Ryx\) .
Note that if \(A \neq \emptyset\) , then no irreflexive relation on \(A\) is reflexive and every asymmetric relation on \(A\) is also anti-symmetric. However, there are \(R \subseteq A^2\) that are not reflexive and also not irreflexive, and there are anti-symmetric relations that are not asymmetric.