**In mathematics, the notion of number has been extended over the centuries to include 0, negative numbers, rational numbers such as 1/2 and −2/3, real numbers such as √2 and π, and complex numbers, which extend the real numbers by adding a square root of −1.**

Calculations with numbers are done with arithmetical operations, the most familiar being the addition, subtraction, multiplication, division, and exponentiation.

Their study or usage is called arithmetic. The same term may also refer to number theory, the study of the properties of numbers.

A factor of a number is an exact divisor of that number. Properties of factors: Factors of a number are always less than or equal to the number itself and are finite.