an Entity references as follows:
In mathematics, a real-valued function f on the interval [a, b] is said to be singular if it has the following properties: * f is continuous on [a, b]. (**) * there exists a set N of measure 0 such that for all x outside of N the derivative f ′(x) exists and is zero, that is, the derivative of f vanishes almost everywhere. * f is non-constant on [a, b].