TaPL Exercise 2.2.6

6 July, 2014

Suppose we are given a relation \(R\) on a set \(S\). Define the relation \(R^{\prime}\) as follows:

\[ R^{\prime} = R \cup \left\{ (s, s) | s \in S \right\} \]

That is, \(R^{\prime}\) contains all the pairs in \(R\) plus all pairs of the form \((s, s)\). Show that \(R^{\prime}\) is the reflexive closure of \(R\).

The reflexive closure is defined as the smallest reflexive relation that contains \(R\). So I need to prove:

  1. That the relation \(R^{\prime}\) contains \(R\);
  2. That it is reflexive;
  3. That it is the smallest relation satisfying the above two properties.

The first property is given in the definition of \(R^{\prime}\); \(R^{\prime}\) is formed by the union of \(R\) and some other relation, so it must contain \(R\).

To understand the second property we must consider what is meant by the term reflexive; luckily this is given to us by definition 2.2.1 in the text:

A binary relation \(R\) on a set \(S\) is reflexive if \(R\) relates every element of \(S\) to itself – that is, \(s R s\) (or \((s, s) \in R\)) for all \(s \in S\).

So, for \(R^{\prime}\) to be reflexive, the following must hold:

\[ \forall s \in S: (s, s) \in R \]

or in other words, it must contain the relations:

\[ \left\{ (s, s) | s \in S \right\} \]

which, as fortune would have it, is also given in the definition of \(R^{\prime}\), so the second property turns out to be as trivially true as the first.

That leaves only one thing left to prove, which is that \(R^{\prime}\) is the smallest relation satisfying the above two properties. Well, let’s consider a relation \(R^{\prime\prime}\) that satisfies the first two properties but is smaller than \(R^{\prime}\). In order for \(R^{\prime\prime}\) to be smaller than \(R^{\prime}\), there must be some \(x\) where \(x \in R^{\prime}\) and \(x \not \in R^{\prime\prime}\). But, in order to satisfy the first two properties, for every \(x\) in \(R^{\prime\prime}\), either \(x \in R\) or \(x \in \left\{ (s, s) | s \in S \right\}\). Since \(R^{\prime}\) is the union of these two sets by definition, there can be no \(x \not \in R^{\prime}\) which is in either set, thus there can be no \(R^{\prime\prime}\) smaller than \(R^{\prime}\) which is a reflexive relation containing \(R\).