Z is to be defined by the formula f(
m/n ) = n for all integers m and n where n 
0.7. Given a formula for a relationship between x and f(x) we say that f is well defined if, and only if, the formula defines a function.
Here f: Q Z is to be defined by the formula f(
m/n ) = n for all integers m and n where n 0.
f is not well defined since f( 2/4 ) = 4 and f( 1/2 ) = 2, but 2/4 = 1/2. In other words, the same input value is related to two different outputs.