In programming languages mathematical functions compute values of "functions" as defined in the science of mathematics. While functions in programming languages are subroutines that return values given certain parameters, functions in mathematics are defined as relations between elements in a definition set Df, and elements in a value set Dv, such that f: x --> y, where x belongs to Df and y to Dv, for each x there must be exactly one y, and this y must be the same whenever f is called. This can also be expressed in the more recognizable f(x) = y,