Four-gradient

Four-gradient

The four-gradient is the four-vector generalization of the gradient:

:partial_alpha stackrel{mathrm{def{=} left(frac{1}{c} frac{partial}{partial t}, abla ight)

and is sometimes also represented as "D".

The square of "D" is the four-Laplacian, which is called the d'Alembert operator:

:Dcdot D = partial_alpha partial^alpha = - frac{1}{c^2}frac{partial^2}{partial t^2} + abla^2.

As it is the dot product of two four-vectors, the d'Alembertian is a Lorentz invariant scalar.

It is also written Box

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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