A single-valued function is an emphatic term for a mathematical function in the usual sense. That is, each element of the function's domain maps to a single, well-defined element of its range. This contrasts with a general binary relation, which can be viewed as being a multi-valued function.
In complex analysis, each complex number other than 0 has three distinct cube roots. Therefore, the function f(z) = z1/3 can be naturally treated as a multi-valued function in which each number other than zero maps to three distinct values. The theory of Riemann surfaces gives a way to study the behavior of this function and other multi-valued functions on the complex numbers.
A function is injective (also called "one-to-one") if each element of the range arises from a single element of the domain. In contrast, a function is single-valued if each element of the domain maps to a single element of the range.
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