# How to sum over the variable in partial derivative operator?

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.

• You're using square brackets ([...]) after the partial derivative operator instead of parentheses ((...)). In Mathematica, expressions can only be grouped using parentheses. Square brackets are reserved for function calls – Lukas Lang Jan 9 at 14:43
• – Bob Hanlon Jan 9 at 16:57
• @LukasLang, using parentheses instead of square brackets has helped me obtain an output. Thank you! But I realize that my approach of summing over k and l is not giving me the intended result. What I intended is to take partial derivatives with respect to 3 space coordinates and 1 time coordinate. I used k and l hoping to run over these 4 coordinates, but that is not happening. Please give some suggestions on how to do this! – mv1996 Jan 9 at 18:06
• @BobHanlon I appreciate the info, since I am just beginning to use Mathematica. Thank you :) – mv1996 Jan 9 at 18:07