In the late 19th century, Karl Weierstrass invented a fractal-like function that was decried as nothing less than a “deplorable evil.” In time, it would transform the foundations of mathematics.

Giovanni Colombo, Antonio Marigonda, Peter R. Wolenski, The Clarke Generalized Gradient for Functions Whose Epigraph Has Positive Reach, Mathematics of Operations Research, Vol. 38, No. 3 (August 2013 ...