R-function
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A mathematical function is called an R-function if its sign can change if and only if the sign of one of its arguments changes.
Typically, the function itself as well as its arguments are real-valued. Interpreting positive values as true and negative values as false, an R-function is transformed into an equivalent Boolean function (the two functions are termed friends). For instance, the R-function f(x,y,) = min(x,y) is one possible friend of the logical conjunction (AND). R-functions are used in the context of implicit functions and, in computer graphics, implicit surfaces.