P-Complete Approximation Problems
Journal of the ACM (JACM)
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We consider the Koopman's-Beckmann version of the Quadratic Assignment Problem, with a distance matrix satisfying the triangle inequality. It is shown that, unless P = NP, no polynomial heuristic can have a bounded performance ratio. This result holds when candidate locations can be arbitrary (distinct) points on the line, or in a metric space. It is not known to hold when candidate locations are regularly spaced points on the line - the Linear Arrangement Problem, grid points in the plane, or otherwise regularly spaced points in a metric space. Other open problems are the Triangle Inequality Asymmetric Travelling Salesman Problem, and the Unit Cost Triangle Inequality Quadratic Assignment Problem.