Rvachev function
In mathematics, a function is called an R-function or Rvachev function if its sign can change if and only if the sign of one of its arguments changes, that is, if its sign is determined solely by its arguments.
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. They also appear in certain boundary-value problems, and are also popular in certain artificial intelligence applications, where they are used in pattern recognition.
R-functions were first proposed by V. L. Rvachev in 1963, and elaborated on by Kravchenko. They are sometimes called Rvachev's atomic functions or Kravchenko-Rvachev functions.