-A variable that is related to another variable, and each value of a variable has another definite value.
Independent variable, a variable whose function is related to another quantity. Any value in this quantity can find a corresponding fixed value in another quantity.
A correspondence rule between two sets such that a unique element in the second set is assigned to each element in the first set.
The law of one-to-one correspondence between two groups of elements of a function, each element in the first group has only a unique corresponding quantity in the second group.
The concept of function is the most basic for every branch of mathematics and quantity.
Definition of ~ ‖ function: Let x and y be two variables, and D be a subset of real number set. If for every value x in D, the variable Y has a definite value corresponding to it according to certain rules, it is called a function of the variable X, and it is denoted as y=f(x).
The number set d is called the domain of function, which is determined by the corresponding function law or the requirements of practical problems. The integer value of the corresponding function is called the range of the function, and the corresponding rule and the defined range are two elements of the function.