# 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 $$R$$.

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