Set Difference is not Associative

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Theorem

Let $R, S, T$ be sets.


The expression:

$\paren {R \setminus S} \setminus T = R \setminus \paren {S \setminus T}$

holds exactly when $R \cap T = \O$.

Here $R \setminus S$ denotes set difference.

Thus, set difference is not associative.


Proof

We assume a universal set $\mathbb U$ such that $R, S, T \subseteq \mathbb U$.


We have the identity Set Difference as Intersection with Complement:

$R \setminus S = R \cap \overline S$

where $\overline S$ is the set complement of $S$:

$\overline S = \relcomp {\Bbb U} S$


Thus we can represent the two expressions as follows:

\(\ds \paren {R \setminus S} \setminus T\) \(=\) \(\ds \paren {R \cap \overline S} \cap \overline T\) Set Difference as Intersection with Complement
\(\ds \) \(=\) \(\ds R \cap \overline S \cap \overline T\) Intersection is Associative


For the second:

\(\ds \) \(\) \(\ds R \setminus \paren {S \setminus T}\)
\(\ds \) \(=\) \(\ds R \cap \overline {\paren {S \cap \overline T} }\) Set Difference as Intersection with Complement
\(\ds \) \(=\) \(\ds R \cap \paren {\overline S \cup \overline {\paren {\overline T} } }\) De Morgan's Laws: Complement of Intersection
\(\ds \) \(=\) \(\ds R \cap \paren {\overline S \cup T}\) Complement of Complement
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S} \cup \paren {R \cap T}\) Intersection Distributes over Union
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap \mathbb U} \cup \paren {R \cap \mathbb U \cap T}\) Intersection with Universal Set
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap \paren {T \cup \overline T} } \cup \paren {R \cap \paren {S \cup \overline S} \cap T}\) Union with Complement
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap T} \cup \paren {R \cap \overline S \cap \overline T}\)
\(\, \ds \cup \, \) \(\ds \) \(\) \(\ds \paren {R \cap S \cap T} \cup \paren {R \cap \overline S \cap T}\) Intersection Distributes over Union
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap T} \cup \paren {R \cap \overline S \cap \overline T} \cup \paren {R \cap S \cap T}\) Set Union is Idempotent
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap \overline T} \cup \paren {R \cap \paren {S \cup \overline S} \cap T}\) Intersection Distributes over Union
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap \overline T} \cup \paren {R \cap \mathbb U \cap T}\) Union with Complement
\(\ds \) \(=\) \(\ds \paren {R \cap \overline S \cap \overline T} \cup \paren {R \cap T}\) Intersection with Universal Set


As can be seen, this full expansion of the second expression can be expressed:

$R \setminus \paren {S \setminus T} = \paren {\paren {R \setminus S} \setminus T} \cup \paren {R \cap T}$


It directly follows from Intersection with Empty Set that:

$R \setminus \paren {S \setminus T} = \paren {R \setminus S} \setminus T \iff R \cap T = \O$

$\blacksquare$


Also see


Sources

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