A function has two variables x and y in a certain change process, and the variable y changes with the variable x and depends on it. If the variable X takes a specific value and Y takes a corresponding value according to a certain relationship, then Y is said to be a function of X. This method was put forward by the French mathematician Riemann in the19th century, but it was first produced by the German mathematician Zebnitz. He and Newton were inventors of calculus. At the end of 17, the word "function" was used for the first time in his article. Translating into Chinese means "function". However, it doesn't have the same meaning as the word function we use today. Represents concepts such as power, coordinates, tangent length, etc.

Until18th century, French mathematician D'Alembert redefined the function in his research. He believes that the so-called function of variables refers to the analytical expression composed of these variables and constants, that is, the functional relationship is expressed by analytical expressions. Later, the Swiss mathematician Euler further standardized the definition of function. He thinks that a function is a curve that can be drawn. We often see images of linear function, quadratic function, proportional function and inverse proportional function. , are expressed in the form of images. If D'Alembert and Euler's methods are used to express the function relationship, they have their own advantages, but as the definition of function, they are still lacking. Because these two methods are still superficial phenomena, there is no hint of the essence of the function.

/kloc-In the middle of the 9th century, Li Jin, a French mathematician, absorbed the achievements of Leibniz, D'Alembert and Euler, and put forward the definition of function accurately for the first time: if a certain quantity depends on another quantity, so that when the latter quantity changes, the former quantity also changes, then the former quantity is called the function of the latter quantity. The biggest feature of Riemann's definition is that it highlights the relationship between dependence and change and embodies the essential attribute of the concept of function.

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