If a set of points is a function, the elements of the set must have the same number of elements.
For example, if the function is from A→B, the elements must be ordered pairs.
So Y is not a function.
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If Y is in fact {(2, 6), (3, 1), (4, 1), (5, 6)}
then Y is a function, because each of the first coordinates appears only once.
In functional notation, we have f(2)=6, f(3)=1 etc....
remark: if a first coordinate is repeated, for example if we have (2, 6) and also (2, 8)
this would mean f(2)=6, but also f(2)=8, which means f is not a function.
