🚧 Setup for 8.1

chapter_8/notes.md

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+**Definition**
+
+Let $R$ be a relation from $A$ to $B$. Define the inverse relation $R^{-1}$ from
+$B$ to $A$ as follows:
+
+$$ R^{-1} = \{(y, x) \in B \times A | (x, y) \in R\} $$
+
+---
+
+Page 513
+
+**Definition**
+
+A **relation on a set** A is a relation from $A$ to $A$.
+
+---
+
+Page 514
+
+**Definition**
+
+Given sets $A_1, A_2, \dots, A_n$ an **$n$-ary relation** $R$ on
+$A_1 \times A_2 \times \cdots \times A_n$ is a subset of
+$A_1 \times A_2 \times \cdots \times A_n$. The special cases of $2$-ary,
+$3$-ary, and $4$-ary relations are called **binary**, **ternary**, and
+**quarternary relations**, respectively.