Kronecker delta (${\delta }_{ij}$)

The Kronecker delta is a mathematical function that is typically denoted by the symbol \( \delta_{ij} \). It is defined as: \[ \delta_{ij} = \begin{cases} 1 & \text{if } i = j \\ 0 & \text{if } i \neq j \end{cases} \] In this definition, \( i \) and \( j \) are usually indices that can take integer values.