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Covariant derivative of a vector

Mathematica Asked by John Greger on February 21, 2021

I have a code that gives me the Christoffel symbols of a metric. How do I take the covariant derivative of a vector?

It does not necessarily have to build upon my code, but this is what I have used so far that gives me the affine connactions:

n = 4 
coord = {T, R, [Theta], [Phi]}

metric = {{-1, 0, 0, 0}, {0, 1, 0, 0}, {0, 0, R^2, 0}, {0, 0, 0, R^2 Sin[[Theta]]^2}}

inversemetric = Simplify[Inverse[metric]]

affine := affine = Simplify[Table[(1/2)*Sum[(inversemetric[[i, s]])*
       (D[metric[[s, j]], coord[[k]] ] +
         D[metric[[s, k]], coord[[j]] ] - 
         D[metric[[j, k]], coord[[s]] ]), {s, 1, n}],
    {i, 1, n}, {j, 1, n}, {k, 1, n}] ]

listaffine := 
 Table[If[UnsameQ[affine[[i, j, k]], 
    0], {ToString[[CapitalGamma][i, j, k]], affine[[i, j, k]]}] , {i,
    1, n}, {j, 1, n}, {k, 1, j}]

TableForm[Partition[DeleteCases[Flatten[listaffine], Null], 2], 
 TableSpacing -> {2, 2}]```

One Answer

If you google something like "covariant derivative mathematica" you will get some hits that lead to mathematica.stackexchange.com where similar questions are answered. One example is Covariant derivative given Christoffel symbols which I think gives you the answer you are looking for. Be aware that the covariant derivative of a "vector" depends on whether you mean a covariant or contravariant vector - see e.g. https://mathworld.wolfram.com/CovariantDerivative.html for the difference.

Answered by Sofie on February 21, 2021

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