hi there

am having trouble understanding einstein summation as it applies to the multiplication of non square matrices.

if A is a 2 x 3 matrix and B is a 3 x 3

where A = 1 2 3 and B is 1 1 0

4 0 1 0 1 1

1 0 1

I understand that A12 = 2 (row 1, column 2)

if the matrix C = AB....... I am not understanding where the notation is derived where Cmn = Amj Bjn

and thus how to calculate C21 = A2jBj1