Reduction of the two-spin state density matrix and evaluation of the left-hand side of the Bell-CHSH inequality

The Bell-Clauser-Horne-Shimony-Holt inequality is considered. The left-hand side of the inequality depends on four arbitrary vectors defined in three-dimensional space. They define the directions in which spins of particles forming a correlated pair are projected. It is necessary to find vectors such that the left-hand side of the inequality takes its maximum value. It is shown that this can be done with the help of a special reduction of the density matrix of a two-particle spin state.