## Search abd Phase Transitions## John Slaney, Sylvie Thiébaux and Toby Walsh
This is the abstract of the paper: Philip Kilby, John Slaney, Sylvie Thiébaux and Toby Walsh. We propose two new online methods for estimating the size of a backtracking search tree. The first method is based on a weighted sample of the branches visited by chronological backtracking. The second is a recursive method based on assuming that the unexplored part of the search tree will be similar to the part we have so far explored. We compare these methods against an old method due to Knuth based on random probing. We show that these methods can reliably estimate the size of search trees explored by both optimization and decision procedures. We also demonstrate that these methods for estimating search tree size can be used to select the algorithm likely to perform best on a particular problem instance.
This is the abstract of the paper: Philip Kilby, John Slaney and Toby Walsh. We study the backbone of the travelling salesperson optimization problem. We prove that it is intractable to approximate the backbone with any performance guarantee, assuming P ≠ NP and there is a limit on the number of edges falsely returned. Nevertheless, in practice, it appears that much of the backbone is present in close to optimal solutions. We can therefore often find much of the backbone using approximation methods based on good heuristics. We demonstrate that such backbone information can be used to guide the search for an optimal solution. However, the variance in runtimes when using a backbone guided heuristic is large. This suggests that we may need to combine such heuristics with randomization and restarts. In addition, though backbone guided heuristics are useful for finding optimal solutions, they are less help in proving optimality.
Backbones and Backdoors in SAT This is the abstract of the paper: Philip Kilby, John Slaney, Sylvie Thiébaux and Toby Walsh. We study the backbone and the backdoors of propositional satisfiability problems. We make a number of theoretical, algorithmic and experimental contributions. From a theoretical perspective, we prove that backbones are hard even to approximate. From an algorithmic perspective, we present a number of different procedures for computing backdoors. From an empirical perspective, we study the correlation between being in the backbone and in a backdoor. Experiments show that there tends to be very little overlap between backbones and backdoors. We also study problem hardness for the Davis Putnam procedure. Problem hardness appears to be correlated with the size of strong backdoors, and weakly correlated with the size of the backbone, but does not appear to be correlated to the size of weak backdoors nor their number. Finally, to isolate the effect of backdoors, we look at problems with no backbone.
This is the introduction to the paper: John Slaney and Toby Walsh. Combinatorial optimization problems (finding solutions that minimise a cost parameter) are closely related to the corresponding decision problems (deciding whether solutions within a given cost exist). Indeed, many algorithms for optimization essentially work by solving a sequence of decision problems. We might hope, therefore, that insights into decision problems gained by studying phase transition behavior could be useful in understanding similar behavior in optimization problems. For both theoretical and historical reasons, propositional satisfiability (or SAT) is the most intensively studied such decision problem. Accordingly, we begin our investigation with the simplest optimization versions of SAT: in the overconstrained case, MAXSAT and in the underconstrained case MAXONES. Given the insights gained from studying the 2+p-SAT decision problem, this paper look at optimization versions of random k+p-SAT.
Backbones in Optimization This is the abstract of the paper: John Slaney and Toby Walsh. We study the impact of backbones in optimization and approximation problems. We show that some optimization problems like graph coloring resemble decision problems, with problem hardness positively correlated with backbone size. For other optimization problems like blocks world planning and traveling salesperson problems, problem hardness is weakly and negatively correlated with backbone size, while the cost of finding optimal and approximate solutions is positively correlated with backbone size. A third class of optimization problems like number partitioning have regions of both types of behavior. We find that to observe the impact of backbone size on problem hardness, it is necessary to eliminate some symmetries, perform trivial reductions and factor out the effective problem size.
This is the abstract of the paper: John Slaney. Recent work on search has identified an intriguing feature dubbed the constrainedness knife-edge by Walsh (Proc. AAAI-98, 406-411) whereby overconstrained problems seem to become on average even more constrained as the search goes deeper, while underconstrained ones become less constrained. The present paper shows that while the knife-edge phenomenon itself is real, many of the claims that have been made about it are incorrect. It is not always associated with criticality, it is not a function of the problem so much as of the algorithm used to solve it, and it does not help to explain the peculiar hardness of problem instances near phase transitions. Despite the negative findings, the upshot is that Walsh's work has opened a fascinating line of research which will surely repay further investigation.
This is the abstract of the paper: S. Thiébaux, J. Slaney and P. Kilby. Estimating conputational cost is a fundamental issue in time-bounded computation. We present a method for estimating the hardness of optimisation problems (find a minimal cost solution to instance I) by observing that of the corresponding decision problems (has I a solution of cost less than threshold T). Provided T is not too close to the optimal, decision is typically much easier than optimisation, yet knowing the hardness of the former is useful for predicting the latter. The present paper reports an experimental investigation of this idea, with encouraging results. An investment of a few percent of the work required for optimisation suffices for estimation within a small factor, even using a very simple implementation of the method.
Hardness of Optimization and Decision This is the abstract of the paper: John Slaney and Sylvie Thiébaux.
Recent work on phase transition has detected apparently interesting phenomena
in the distribution of hard optimisation problems (find, on some measure, the
least
Dr J K Slaney Phone (Aus.): (026) 125 8607 Theory Group Phone (Int.): +61 26 125 8607 Research School of Computer Science Fax (Aus.): (026) 125 8651 Australian National University Fax (Int.): +61 26 125 8651 Canberra, ACT, 0200, AUSTRALIA |