Kronecker delta

The Kronecker delta is a mathematical symbol, often written as δ(i, j), that indicates whether two values, i and j, are equal. If i and j are the same, δ(i, j) equals 1; if they are different, δ(i, j) equals 0. It is commonly used in mathematics, particularly in linear algebra and physics, to simplify expressions and manage sums over different variables. Essentially, it helps to "filter" values based on equality, allowing for more straightforward calculations in various applications.