You didn't tell us what the largest number in the range can be.  You just have
a + sign there. I'll assume it's at least 3.

{(0,1), (7,3), (2,1), (7,2)}

That's not a function because no two different ordered pairs in 
a function can have the same first coordinate, and (7,3) and (7,2)
have the same first coordinate 7.


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[Note:  two different ordered pairs in a function CAN have the same SECOND
coordinate, like (0,1) and (2,1) have the same second coordinate.  They
are not the reason the above is not a function.  (7,3) and (7,2) are the
only reason it's not a function.  In a function, the y's can occur twice, 
but the x's can't!]

Edwin