Kronecker delta
From specialfunctionswiki
The Kronecker delta is the function $\delta \colon \mathbb{N}_0 \times \mathbb{N}_0 \rightarrow \{0,1\}$ defined by $$\delta(m,n)=\left\{ \begin{array}{ll} 0, & \quad m \neq n \\ 1, & \quad m=n. \end{array} \right.$$ Often this function is written as $\delta_{mn}$.
