The four-gradient is the four-vector generalization of the gradient:
and is sometimes also represented as D.
The square of D is the four-Laplacian, which is called the d'Alembert operator:
.
As it is the dot product of two four-vectors, the d'Alembertian is a Lorentz invariant scalar.
It is also written 
References
- S. Hildebrandt, "Analysis II" (Calculus II), ISBN 3-540-43970-6, 2003
- L.C. Evans, "Partial differential equations", A.M.Society, Grad.Studies Vol.19, 1988
- J.D. Jackson, "Classical Electrodynamics" Chapter 11, Wiley ISBN 0-471-30932-X
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