Indicator Function

Definition

Let $u\in \mathbb{R}$ be a constant value and consider the function $1_{x\ge u}:{\mathbb{R}}^n\to \{0,1\}$ defined, for every $x\in \mathbb{R}$, as

1,&x\geq u\\ 0,&x< u\;.\\ \end{array}\right.$$ is called as _indicator function_.