This is the expression of the divergence of a contravariant six-vector.
Divergence of the mixed tensor of the second rank.
Let us form the reduction of (39) with reference to the indices α and σ, we obtain remembering (29a)
"(41)"
If we introduce into the last term the contravariant tensor Aρσ = gρτ Aστ, it takes the form
If further Aρσ or is symmetrical it is reduced to
If instead of Aρσ, we introduce in a similar way the symmetrical co-variant tensor Aρσ = gρα gσβ Aαβ, then owing to (31) the last member can take the form