They have been called "pathological"; Henri Poincaré spoke of "monsters," accusing them of being "strange functions that strive to resemble as little as possible the respectable functions that serve some purpose." Yet some discontinuous functions have left their mark on history.
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The introduction of series of functions, followed by theorems establishing when continuity or differentiability is preserved in the limit, made it possible to construct objects with unusual, counterintuitive properties. Some famous mathematicians had tremendous fun with them!
From Cauchy to Weierstrass, rigor steadily took hold in analysis throughout the 19th century. A variety of counterexamples swept away mistaken beliefs and forced mathematicians to define the relevant concepts more precisely. Some "monstrous" functions were introduced, fascinating to some and repellent to others.
Harmonic analysis, born of functional analysis, studies how functions can be represented as superpositions of basis functions. In signal theory, these basis functions, often polynomials, lie at the heart of sound and image processing in our digital world.
What could a quadratic polynomial possibly have in common with a vector in three-dimensional space? At first glance, nothing: they are different kinds of objects. Yet both have the same form—each is described by a triple of numbers. Better still, calculations with one correspond to calculations with the other!
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