Kronecker delta

In mathematics, the Kronecker delta or Kronecker's delta, named after Leopold Kronecker, is a function of two variables, usually just positive integers. The function is 1 if the variables are equal, and 0 otherwise:

where the Kronecker delta d_ij is a piecewise function of variables i and j. For example, d_{1 2} = 0, whereas d_{3 3} = 1.