Complex number: Complex number refers to a+bi, which can be written in the following form, where A and B are real numbers and I is imaginary unit (i.e.-1). It was first put forward by Italian Milan scholar Cardan in the16th century, and this concept was gradually accepted by mathematicians through the work of D'Alembert, De Moivre, Euler and Gauss. There are many representations of complex numbers, such as vector representation and triangle representation. Exponential representation, etc. It satisfies the properties of four operations. It is the most basic object and tool in complex variable function theory, analytic number theory, Fourier analysis, fractal, fluid mechanics, relativity, quantum mechanics and other disciplines. A complex set is a set of complex numbers.