We remark that in the same way, we can also form the extension of a mixed tensor Aμα
"(39)"
By the reduction of (38) with reference to the indices β and σ(inner multiplication with δβσ), we get a contravariant four-vector
On the account of the symmetry of
with reference to the indices β and κ, the third member of the right hand side vanishes when Aαβ is an antisymmetrical tensor, which we assume here; the second member can be transformed according to (29a); we therefore get
"(40)"