# Domain and Range

As you know from the definition of a function each x-value (from the domain) relates to exactly one y-value (from the range).

*Each x-value maps to exactly one y-value. The same y-value can be addressed multiple times.*

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### Attention

A function is

**single valued**. For each element x the function will give back a single element y.The following would not be a valid function, since $x_2$ (from the domain) is connected to two values from the range.