Kronecker Delta

In mathematics, the Kronecker delta (named after Leopold Kronecker) is a function of two variables, usually just non-negative integers. The function is $1$ if the variables are equal, and $0$ otherwise:

$${\delta }_{i,j}=\{\begin{array}{ll}0,& \text{if }i\ne j\\ 1,& \text{if }i=j\end{array}$$