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.
