# Theory of Computation: Question Set – 02

#### What is a relation in mathematics?

In mathematics, a relation is a set of ordered pairs that describe the connection between two elements of a set. For example, consider the set of integers {1, 2, 3, 4} and the relation R defined by R = {(1,2), (2,3), (3,4)}. Here, the ordered pair (1,2) indicates that 1 is related to 2, the ordered pair (2,3) indicates that 2 is related to 3, and the ordered pair (3,4) indicates that 3 is related to 4.

#### What is the difference between a function and a relation?

A function is a special type of relation where each element of the domain is related to exactly one element of the range. In other words, for each input value, there is only one output value. A relation, on the other hand, does not have this requirement. In a relation, an element of the domain can be related to more than one element of the range, or even no element at all.

#### What is an equivalence relation?

An equivalence relation is a relation that is reflexive, symmetric, and transitive. That is, for a relation R defined on a set A, the relation is an equivalence relation if it satisfies the following properties:

- Reflexivity: For all a ∈ A, aRa (every element is related to itself)
- Symmetry: For all a, b ∈ A, if aRb, then bRa (if a is related to b, then b is related to a)
- Transitivity: For all a, b, c ∈ A, if aRb and bRc, then aRc (if a is related to b and b is related to c, then a is related to c)

#### What is a partial order relation?

A partial order relation is a relation that is reflexive, antisymmetric, and transitive. That is, for a relation R defined on a set A, the relation is a partial order if it satisfies the following properties:

- Reflexivity: For all a ∈ A, aRa (every element is related to itself)
- Antisymmetry: For all a, b ∈ A, if aRb and bRa, then a = b (if a is related to b and b is related to a, then a and b are the same element)
- Transitivity: For all a, b, c ∈ A, if aRb and bRc, then aRc (if a is related to b and b is related to c, then a is related to c)

#### What is the difference between a partial order and a total order relation?

A partial order relation is a binary relation that is reflexive, anti-symmetric, and transitive. A total order relation, on the other hand, is a partial order relation that also satisfies the property of comparability. That is, for any two distinct elements a and b in the set, either a <= b or b <= a holds.

#### What is a reflexive closure of a relation?

The reflexive closure of a relation R on a set A is the smallest reflexive relation on A that contains R. In other words, it is obtained by adding to R all pairs (a, a) where a belongs to A and (a, a) is not already in R.

#### What is the difference between an equivalence relation and a partial order relation?

An equivalence relation is a binary relation that is reflexive, symmetric, and transitive. A partial order relation, on the other hand, is a binary relation that is reflexive, anti-symmetric, and transitive. The main difference between the two is that an equivalence relation partitions the set into equivalence classes, while a partial order relation only partially orders the set.

#### What is a transitive closure of a relation?

The transitive closure of a relation R on a set A is the smallest transitive relation on A that contains R. In other words, it is obtained by adding to R all pairs (a,c) where a, b, and c belong to A, (a,b) and (b,c) are in R, and (a,c) is not already in R.

#### What is a binary relation?

A binary relation on a set A is a collection of ordered pairs of elements of A. It is often denoted by R and can be thought of as a subset of A x A. The elements of A are called the domain and the range of the relation.