Set of numbers
Sets of numbers have been formed in logical order. Natural numbers express quantity. When our ancestors noticed, that decrease of quantity can be expressed with negative numbers, they had the set of integers. Integers can be used, for example, to express the numbers roundings or time before and after present.
Integers were followed by rational numbers (or fractions). In the past, time may have been given as half of moon. Babylonians divided the circle and year to 360 parts, as it had many factors: 1, 2, 3, 4, 5, 6, 9, 10, 12,...This enabled them to solve many things exactly. Rational numbers can be either finite or infinite.
The circle gave new dimensions to the numbers. It was found, for example, that circumference of a circle cannot be solved exactly only with a radius and an angle. The missing part is not a rational numbers but infinite and non-recurring decimal number, that is irrational number. Rational numbers and irrational numbers are called as real numbers.
Mathematically set of numbers are symbolized as
Complex numbers are used in electrical engineering and in solving differential equations. Computationally, it can be seen, when, for example, the inner part of a square root is negative For complex numbers
