<P> In mathematics, an expression is called well - defined or unambiguous if its definition assigns it a unique interpretation or value . Otherwise, the expression is said to be not well - defined or ambiguous . A function is well - defined if it gives the same result when the representation of the input is changed without changing the value of the input . For instance if f takes real numbers as input, and if f (0.5) does not equal f (1 / 2) then f is not well - defined (and thus: not a function). The term well - defined is also used to indicate whether a logical statement is unambiguous . </P> <P> A function that is not well - defined is not the same as a function that is undefined . For example, if f (x) = 1 / x, then f (0) is undefined, but this has nothing to do with the question of whether f (x) = 1 / x is well - defined . It is; 0 is simply not in the domain of the function . </P>

When can you say that a set is well defined
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