Kronecker delta

In algebra, the Kronecker delta is a notation $$\delta_{ij}$$ for a quantity depending on two subscripts i and j which is equal to one when i and j are equal, and zero then they are unequal.

If the subscripts are taken to vary from 1 to n then δ gives the entries of the n-by-n identity matrix. The invariance of this matrix under orthogonal change of coordinate makes δ a rank two tensor.