where δm,n is Kronecker’s delta.

Kronecker's delta is a mathematical symbol that is used to represent the equality or inequality of two variables. It is denoted by the symbol δ and is defined as follows:

δm,n =
{
1 if m = n
0 if m ≠ n
}

In other words, Kronecker's delta is equal to 1 if the variables m and n are equal, and 0 if they are not equal. It is commonly used in linear algebra, calculus, and other branches of mathematics.