Number theory
Number theory is the branch of pure mathematics devoted to the study of the integers. Such a study involves an examination of the properties of that which integers are made of (namely, prime numbers) as well as the properties of objects made out of integers (such as rational numbers) or defined as generalisations of the integers (algebraic integers).
Origins
Given an equation or equations, can we find solutions that are integers? Solutions that are rational numbers? This is one of the basic questions of number theory. It seems to have been first addressed in ancient India (see Vedic number theory).
Hellenistic mathematicians had a keen interest in what would later be called number theory: Euclid devoted part of his Elements to prime numbers and factorization. Much later - in the third century CE - Diophantus would devote himself to the study of rational solutions to equations.