The above is not a function because -2 is mapped to two elements 2 and -4.
What is a function?
A function relates an input to an output. The three main parts of a function are the inputs, outputs and the relationship.
The formal definition of a function is that it relates each element of a set with exactly one element of another set (possibly the same set).
In other words, one element must be mapped to another element.
Each element cannot be mapped to two elements.
Therefore, the above is not a function.
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