A relation as a function provided there is exactly one output for each input. It is NOT a function if at least one input has more than one output A function is to have a limit if it has a two-sided limit. A graph provides a visual method of determining the limit of a function. If the function has a limit as x approaches a, the branches of the graph will approach the same y-coordinate near x=a from the left and the right. Consider functions f :A (⊂ X) → Y where X and Y are metric spaces. Important cases are 1. f :A (⊂ R) → R, 2. f :A (⊂ Rn) → R, 3. f :A (⊂ Rn) → Rm Sometimes we can sketch the domain and the range of the function, perhaps also with a coordinate grid and its image.Sometimes we can represent a function by drawing a vector at various points in its domain to represent f(x). We can represent a real-valued function by drawing the level sets (contours) of the function. In other cases the best we can do is to represent the graph, or the domain and range of the function, in a highly idealized manner..Sometimes we can represent a function by drawing a vector at various points in its domain to represent f(x).