Delta function

Delta function may refer to the distribution:

• Dirac delta function, $\langle \delta, \varphi \rangle = \varphi(0)$

or the indexed matrix:

• Kronecker delta, $\delta_{ij} = \left\{\begin{matrix} 1, & \mbox{if } i=j \\ 0, & \mbox{if } i \ne j \end{matrix}\right.$