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We show that some two-party Bell inequalities with two-valued observables are stronger than the CHSH inequality for 33 isotropic states in the sense that they are violated by some isotropic states in the 33 system that do not violate the CHSH inequality. These Bell inequalities are obtained by applying triangular elimination to the list of known facet inequalities of the cut polytope on nine points. This gives a partial solution to an open problem posed by Collins and Gisin. The results of numerical optimization suggest that they are candidates for being stronger than the I₃₃₂₂ Bell inequality for 33 isotropic states. On the other hand, we found no Bell inequalities stronger than the CHSH inequality for 22 isotropic states. In addition, we illustrate an inclusion relation among some Bell inequalities derived by triangular elimination.
Ito et al. (2006) studied this question.