I need to use the partial derivative operator in Wolfram Mathematica within a summation, specifically to define the D'Alembertian operator of scalar fields. I am having trouble summing over the D operator.
This is how I have defined the function currently and this is giving me an error.
Edit : I made the suggested change of parenthesis and it solves the error. But I would appreciate any help in solving the summation issue, ie, to sum the two derivative operators over the 4 co-ordinates.