z = g(y)；

Then the functional relationship between z and x is z=g(f(x)) is a coincidence function.

Is to turn a certain relationship into two commonly used functional relationships. The relationship between these two functions is that the value of one function serves as the independent variable of the other function. Of course, they can also be combined in other ways.

But not simple addition, subtraction, multiplication and division.

A compound function is also a function, which only decomposes the original complex relationship into two or more simple functional relationships.