函数的概念已经很清楚了，这里主要学习它的英文表述。
"y is a function of x" or "y equals f of x".
Independent variables: 自变量
Dependent variables: 应变量
严格的定义表述："A function from a set of D to a set Y is a rule that assigns a unique (single) element to each element ."
"The set D of all possible input values is called the domain of the function. The set of all values of f(x) as x varies throughout D is called the range of the function. The range may not include every element in the set Y."
"A function f have only one value f(x) for each x in its domain, so no vertical line can intersect the graph of a function more than once."
Piecewise-defined function: 分段函数
Greatest integer function or integer floor function
Least integer function or integer ceiling function
