d914a6c6d93c8df063b9099a076a488c-AuthorFeedback.pdf

Forinstance, one candefine anewupper bounding5 function that returns the minimum of two other upper bounding functions. The difficulty is in finding other upper6 bounding functions that are (1) tighter than the bound we use, (2) efficient to compute, and (3) empirically require7 few calls toRefine. Unfortunately,thefastcimplementation ofthe11 bound provided by [1] is numerically unstable for sparse matrices with 0 entries and the numerically stable matlab12 implementation is prohibitively slow. This difficulty could be overcome by rewriting an efficient implementation.13 This14 bound is computed by solving an optimization problem, but unfortunately we do not know of an efficient solution.15