In undergraduate mathematics, we are confronted at an early stage with “Epsilon-Delta” definitions. For example, given a function of a real variable, we may ask what is the value of the function for a particular value . Maybe this is an easy question or maybe it is not.

The epsilon-delta concept can be subtle, and is sufficiently difficult that it has been used as a *filter* to weed out students who may not be considered smart enough to continue in maths (I know this from personal experience). The formulation of the epsilon-delta definitions is usually attributed to the German mathematician Karl Weierstrass. They must have caused him many sleepless nights.