The contraction of a Tensor is obtained by setting unlike indices equal and summing according to the
Einstein Summation convention. Contraction reduces the Rank of a Tensor by 2.
For a second Rank Tensor,
Arfken, G. ``Contraction, Direct Product.'' §3.2 in
Mathematical Methods for Physicists, 3rd ed. Orlando, FL: Academic Press, pp. 124-126, 1985.