A binary relation is a mathematical concept that defines a relationship between two sets. Formally, given two sets $A$ and $B$, a binary relation $R$ from $A$ to $B$ is a subset of the Cartesian product $A \times B$. That is:

$R \subseteq A \times B$

Each pair $(a, b) \in R$ indicates that the element $a \in A$ is related to the element $b \in B$ by the relation $R$.

Key Terminology:

1. Domain: The set $A$ is called the domain of the relation.
2. Codomain: The set $B$ is called the codomain of the relation.
3. Range: The range of $R$ is the set of elements in $B$ that are related to some element in $A$.
4. Graph of a Relation: The graph of a relation is the set of ordered pairs $(a, b) \in A \times B$ for which the relation holds.

Special Types of Binary Relations:

1. Reflexive Relation: A relation $R$ on a set $A$ is reflexive if every element is related to itself. That is, $(a, a) \in R$ for all $a \in A$.
2. Symmetric Relation: A relation $R$ on a set $A$ is symmetric if whenever $(a, b) \in R$, then $(b, a) \in R$.
3. Antisymmetric Relation: A relation $R$ is antisymmetric if whenever $(a, b) \in R$ and $(b, a) \in R$, then $a = b$.
4. Transitive Relation: A relation $R$ is transitive if whenever $(a, b) \in R$ and $(b, c) \in R$, then $(a, c) \in R$.

Example:

Consider the sets $A = \{1, 2\}$ and $B = \{3, 4\}$. A binary relation $R$ from $A$ to $B$ could be: $R = \{(1, 3), (2, 4)\}$ This means that:

• $1$ in $A$ is related to $3$ in $B$,
• $2$ in $A$ is related to $4$ in $B$.

Would you like more details or have any specific questions?

Here are some related questions:

1. What are Cartesian products and how are they used in relations?
2. What is the difference between functions and binary relations?
3. How can binary relations be represented as matrices or graphs?
4. What properties can a binary relation have (e.g., reflexive, symmetric)?
5. What is an equivalence relation, and how does it relate to binary relations?

Tip: A binary relation is not always a function! For a relation to be a function, every element in the domain must relate to exactly one element in the codomain.