Figure 1: Maximal Extremum. An example of a unique mathematical maximal extremum that we would observe in the protein–protein interaction if cooperativity was in operation. In calculus, the rules of mathematical extrema state that if a function ƒ is defined on an interval I and c is a number in I, then the function ƒ(c) is the maximum value of ƒ if ƒ(x) ≤ ƒ(c) for every x in I.