Number theory, a branch within the field of discrete mathematics, is the study of the properties and relationships of numbers, primarily the integers. As a foundational pillar of mathematics, number theory delves into various aspects of numbers including their divisibility, the structure of integers, and the distribution of primes.

In number theory, some core concepts include:

1. Divisibility and Factoring: Exploring what integers divide other integers without leaving a remainder. The fundamental theorem of arithmetic states that every integer greater than 1 can be uniquely factored into prime numbers.

   \[
   n = p_1^{e_1} p_2^{e_2} \cdots p_k^{e_k}
   \]

   where \( n \) is a positive integer, \( p_1, p_2, \ldots, p_k \) are primes, and \( e_1, e_2, \ldots, e_k \) are their respective positive integer exponents.

2. Primes and Prime Factorization: Investigating the nature and distribution of prime numbers, those greater than 1 and having no positive divisors other than 1 and themselves. The concept of prime factorization is crucial for understanding the structure of integers.

3. Greatest Common Divisor (GCD) and Least Common Multiple (LCM): Understanding how to determine the largest integer that divides two or more integers, and conversely, the smallest integer divisible by two or more integers. The Euclidean algorithm is commonly used to find the GCD.

4. Congruences and Modular Arithmetic: Studying equivalences between integers where numbers are said to be congruent modulo \( n \) if they leave the same remainder when divided by \( n \). This is written as:

   \[
   a \equiv b \ (\text{mod} \ n)
   \]

5. Diophantine Equations: Solving polynomial equations where the solutions are required to be integers. An example of such an equation is the linear Diophantine equation,

   \[
   ax + by = c
   \]

   where \( a \), \( b \), and \( c \) are given integers, and \( x \) and \( y \) are the unknown integers to be determined.

6. Quadratic Residues: Investigating the solutions to congruences of the form:

   \[
   x^2 \equiv a \ (\text{mod} \ p)
   \]

   where \( p \) is a prime. This leads to the Law of Quadratic Reciprocity, a crucial theorem in number theory.

7. Cryptographic Applications: Utilizing properties of numbers in practical applications such as cryptography. Algorithms like RSA encryption rely on the difficulty of factoring large composite numbers into their prime components.

Number theory intersects with various other fields, such as algebra, analysis, and even computer science, due to its importance in algorithms and cryptographic systems. It remains a vibrant area of research with significant historical roots, stretching back to ancient mathematicians like Euclid and Diophantus and evolving through modern developments.