The theory of numbers is that branch of mathematics which deals with properties of the whole numbers, 1, 2, 3, 4, 5, ... also called the counting numbers, or positive integers. The positive integers are undoubtedly man's first mathematical creation. It is hardly possible to imagine human beings without the ability to count, at least within a limited range. Historical record shows that as early as 5700 BC the ancient Sumerians kept a calendar, so they must have developed some form of arithmetic.