In algebra, the Kronecker delta is a notation for a quantity depending on two subscripts i and j which is equal to one when i and j are equal and zero when they are unequal:
Kronecker deltas appear frequently in summations where they act as a "filter". To clarify this we consider a simple example
that is, the element S4 is "sifted out" of the summation by δi,4.
In general, (i and a integers)