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:
δ
i
j
=
{
0
if 
i
≠
j
,
1
if 
i
=
j
.
{\displaystyle \delta _{ij}={\begin{cases}0&{\text{if }}i\neq j,\\1&{\text{if }}i=j.\end{cases}}}
or with use of Iverson brackets:
δ
i
j
=
[
i
=
j
]
{\displaystyle \delta _{ij}=[i=j]\,}
For example,
δ
12
=
0
{\displaystyle \delta _{12}=0}
because
1
≠
2
{\displaystyle 1\neq 2}
, whereas
δ
33
=
1
{\displaystyle \delta _{33}=1}
because
3
=
3
{\displaystyle 3=3}
.
https://en.wikipedia.org/wiki/Kronecker_delta
