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The sectional matrix is the best way to achieve a strong contact point in Class II restorations with composite resin in the posterior dental sector. The pre-wedging is essential to get a separation between teeth which avoid the matrix deformation during its insertion. This article describes the clinical technique for restoring Class II cavities using a sectional matrix.

We report the discovery of a partial conservation law obeyed by a schematic Hamiltonian of two protons and two neutrons in a [Formula: see text] shell. In our Hamiltonian, the interaction matrix element of two nucleons with combined angular momentum [Formula: see text] is linear in [Formula: see text] for even [Formula: see text] and constant for odd [Formula: see text]. It turns out that in some stationary states, the sum of the angular momenta [Formula: see text] and [Formula: see text] of the proton and neutron pairs is conserved. The energies of these states are given by a linear function of [Formula: see text]. The systematics of their occurrence is described and explained.