Number theory

Number theory, branch of mathematics that deals with the integers (or whole numbers), which include zero and the negative whole numbers. One of the important concepts of number theory is that of the prime numbers, those numbers greater than 1 divisible only by 1 and themselves. For instance, 2, 3, 5, and 7 are prime numbers, whereas 9 is not, since it is divisible by 3. One of the best-known theorems of number theory, called the fundamental theory of arithmetic, states that every positive integer can be expressed as the product of prime numbers that are unique. For example, the number 15 can be expressed as 3 × 5 and by no other product of primes (except for 5 × 3, which is regarded as the same thing). This theorem is attributed to the ancient Greek mathematician Euclid, who may be considered the founder of number theory.