The 4-dimensional version of the Gradient, encountered frequently in
general relativity and special relativity, is

which can be written

where is the d'Alembertian Operator.

**References**

Morse, P. M. and Feshbach, H. ``The Differential Operator .'' §1.4 in
*Methods of Theoretical Physics, Part I.* New York: McGraw-Hill, pp. 31-44, 1953.

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1999-05-25