# Combinatorics

Combinatorics is a field of mathematics that mainly focuses on numbers as a way to generate results, as well as specific properties of finite structures. It is closely connected to many other fields, from logic to mathematical mechanics, from evolutionary biology to computer science, and so on.

There is no consensus on the entire spectrum of combinatorics. Following H.J. Ryser, it is difficult to describe the subject since there are so many mathematical subdivisions. As long as a field can be defined by the types of problems related, combinatorics include the enumeration (counting) in the most general sense of specific structures. It is often known as arrangements or configurations, connected with final systems, facilities that meet specific requirements, and the construction of such networks and optimization.

Two sets A and B, suppose.

One must note the basic rules of combinatorics:

*The Rule of Product:*

The product rules imply that if X and Y are given several ways of choosing one element from B, X and y are selected for two features, one of A and one of B.

*The Rule of Sum:*

In the sum rule, when X numbers are present in choosing one element from A and Y numbers, there will be an X+Y number of ways for selecting a component from B, which can either be A or B. The sum rule is simple.

**Subfields of Combinatorics**

*Enumerative combinatorics*

Enumerative combinatorics is the most traditional area which focuses on counting such combinatorial objects. Although it is a very general mathematical problem to count the number of elements in a sequence, many of the applications' problems have a relatively simple combinatory definition. Fibonacci numbers are an actual example of a combinatorial problem. The twelvefold way provides a unified framework for counting permutations, combinations, and partitions.

*Graph theory*

Graphs in combinatorics are necessary artifacts. Considerations in graph theory ranging from enumeration (for example, number of n vertices with k-length) to existing structures (e.g., hamilton cycles). Even to algebraic representations (for instance, graph G and two numbers x and y). ). While graph theory and combinatorics are very closely related, they are often considered individual subjects. Although combinatorial methods are used for many of the problems of graph theory, both disciplines are used in general to find solutions to various types of issues.

*Design Theory*

Design theory is a study of designs combining sub-components with some intersectional characteristics. Block designs are a unique form of combinatory methods. This area is one of the oldest cross-sections suggested in 1850 at Kirkman's schoolgirl issue. The problem is overcome by a Steiner scheme, which has a significant role in the classification of finite simple groups. The zone has additional links to the theory of coding and grommets.

*Probabilistic Theory*

The following form in probabilistic combinatorics: what is the likelihood of specific properties for a random discrete object, like a random graph? For example, how many random graphs are the average numbers?

Probabilistic programs are often used to estimate the attendance (for whom explicit models can be challenging to find) of combinatorial objects. They have specific prescribed properties by merely observing that a chance of selecting an item with such properties is greater than 0. In applications to extreme combinatorics and graph theory, this approach (often called the probabilistic method) has proven extremely successful. The analysis of finite Markov chains is closely related, in particular on combinatorial artifacts. Again, probabilistic instruments are used to measure the time of mixing.