The Lagrangian finite strain tensor is defined as:
E_{i,j}=\frac{1}{2}\left(\frac{\partial x_k}{\partial X_i}\frac{\partial x_k}{\partial X_j}-\delta _{i,j}\right)
Is it in Einstein Notation so that there is a summation symbol missing, i.e. would it be the same thing if one wrote it as...