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Kronecker delta

The Kronecker delta ( $\delta_{ij}$ ) is a function of two variables, typically non-negative integers. The value of the function is 1 if the variables are equal, and 0 otherwise:

$$\delta_{ij} = \begin{cases}

        1, &         \text{if } i=j,\\
        0, &         \text{if } i\neq j
\end{cases}$$

Referenced in

Birds Eye View of Quantum Mechanics

$\delta{ij}$ is the Kronecker delta: It's equivalent to the inner product because eigenstates are typically orthogonal.

Kronecker delta