Two remarks on rectangular mixed finite elements for elasticity. (arXiv:1012.1768v1 [math.NA])

Two remarks on rectangular mixed finite elements for elasticity. (arXiv:1012.1768v1 [math.NA]): "

The lowest order nonconforming rectangular element in three dimensions
involves 54 degrees of freedom for the stress and 12 degrees of freedom for the
displacement. With a modest increase in the number of degrees of freedom (24
for the stress), we obtain a conforming rectangular element for linear
elasticity in three dimensions. Moreover, unlike the conforming plane
rectangular or simplicial elements, this element does not involve any vertex
degrees of freedom. Second, we remark that further low order elements can be
constructed by approximating the displacement with rigid body motions. This
results in a pair of conforming elements with 72 degrees of freedom for the
stress and 6 degrees of freedom for the displacement.

"