“…Zheng, Sun, Li, and Xu (2012) present new sufficient conditions for verifying zero duality gap in nonconvex constrained quadratic programs and then show how the results specialize for UBQP. In related work, Sun, Liu, and Gao (2012) investigate the duality gap between UBQP and its semi-definite programming relaxation. Making the connection between the duality gap and the cell enumerations of hyperplane arrangement in discrete geometry, estimates of the duality gap can be derived, yielding improved lower bounds for UBQP.…”