## Introduction to Discrete Mathematics

Discrete mathematics is a branch of mathematics that deals with mathematical structures that are fundamentally discrete rather than continuous. It focuses on objects that can only take on distinct, separate values. This field of study is essential in various areas of computer science, cryptography, and theoretical mathematics.

## Understanding Subgroups in Discrete Mathematics

In the realm of abstract algebra, a subgroup is a subset of a group that retains the group’s structure and operations. In other words, a subgroup is a smaller group that is part of a larger group. To be considered a subgroup, it must satisfy certain conditions:

- The subset must contain the identity element of the original group.
- The subset must be closed under the group’s operation.
- Every element in the subset must have an inverse within the subset.

## Examples of Subgroups

Let’s explore a few examples to better understand subgroups in discrete mathematics:

## Example 1: Subgroup of Integers

Consider the group of integers under addition, denoted as ℤ. One example of a subgroup within this group is the set of even integers. This subset satisfies the conditions of a subgroup:

- The identity element, 0, is included in the set of even integers.
- When you add two even integers, the result is always an even integer, so the set is closed under addition.
- For every even integer x, its inverse (-x) is also within the set.

Therefore, the set of even integers forms a subgroup of the group of integers under addition.

## Example 2: Subgroup of Symmetric Group

The symmetric group, denoted as S_{n}, consists of all permutations of n distinct elements. Let’s consider the symmetric group S_{3} which contains the permutations of three elements: {1, 2, 3}.

One example of a subgroup within S_{3} is the subgroup generated by the permutation (1 2). This subgroup includes the following elements:

- (1 2)
- (1 2)(3)
- (1 2 3)
- (1)

This subset satisfies the conditions of a subgroup:

- The identity element, (1), is included in the subgroup.
- When you compose two permutations from the subgroup, the result is always another permutation within the subgroup, so it is closed under composition.
- For every permutation in the subgroup, its inverse is also within the subgroup.

Therefore, the subgroup generated by (1 2) forms a subgroup of the symmetric group S_{3}.

## Example 3: Subgroup of Matrix Group

Consider the group of 2×2 invertible matrices with real entries, denoted as GL_{2}(ℝ). One example of a subgroup within this group is the set of 2×2 rotation matrices.

A rotation matrix has the following form:

| cos(θ)-sin(θ) || sin(θ)cos(θ) |

This subset satisfies the conditions of a subgroup:

- The identity element, the 2×2 identity matrix, is included in the set of rotation matrices.
- When you multiply two rotation matrices, the result is always another rotation matrix, so the set is closed under matrix multiplication.
- For every rotation matrix, its inverse (which is the transpose of the matrix) is also within the set.

Therefore, the set of 2×2 rotation matrices forms a subgroup of the group of 2×2 invertible matrices with real entries.

## Conclusion

Subgroups play a crucial role in discrete mathematics and abstract algebra. They allow us to explore the structure and properties of groups by focusing on smaller, self-contained subsets. By understanding subgroups, we can better analyze and manipulate mathematical structures in various fields, including computer science, cryptography, and theoretical mathematics.