54,862 research outputs found
On the computational complexity of branch and bound search strategies
Many important problems in operations research, artificial intelligence, combinatorial algorithms, and other areas seem to require search in order to find an optimal solution. A branch and bound procedure, which imposes a tree structure on the search, is often the most efficient known means for solving these problems. While for some branch and bound algorithms a worst case complexity bound is known, the average case complexity is usually unknown despite the fact that it gives more information about the performance of the algorithm. In this dissertation the branch and bound method is discussed and a proabilistic model of its domain is given, namely a class of trees with an associated probability measure. The best bound first and depth-first search strategies are discusses and results on the expected time and space complexity of these strategies are presented and compared. The best-bound search strategy is shown to be optimal in both time and space. These results are illustrated by data from random traveling salesman problems. Evidence is presented which suggests that the asymmetric traveling salesman problem can be solved exactly in time O(nᄈlnᄇ(n)) on thePrepared for: National Science Foundation; Washington, D.C. 20550http://archive.org/details/oncomputationalc00smitNSF Grant MCS74-14445-A0
Analysis of the computational complexity of solving random satisfiability problems using branch and bound search algorithms
The computational complexity of solving random 3-Satisfiability (3-SAT)
problems is investigated. 3-SAT is a representative example of hard
computational tasks; it consists in knowing whether a set of alpha N randomly
drawn logical constraints involving N Boolean variables can be satisfied
altogether or not. Widely used solving procedures, as the
Davis-Putnam-Loveland-Logeman (DPLL) algorithm, perform a systematic search for
a solution, through a sequence of trials and errors represented by a search
tree. In the present study, we identify, using theory and numerical
experiments, easy (size of the search tree scaling polynomially with N) and
hard (exponential scaling) regimes as a function of the ratio alpha of
constraints per variable. The typical complexity is explicitly calculated in
the different regimes, in very good agreement with numerical simulations. Our
theoretical approach is based on the analysis of the growth of the branches in
the search tree under the operation of DPLL. On each branch, the initial 3-SAT
problem is dynamically turned into a more generic 2+p-SAT problem, where p and
1-p are the fractions of constraints involving three and two variables
respectively. The growth of each branch is monitored by the dynamical evolution
of alpha and p and is represented by a trajectory in the static phase diagram
of the random 2+p-SAT problem. Depending on whether or not the trajectories
cross the boundary between phases, single branches or full trees are generated
by DPLL, resulting in easy or hard resolutions.Comment: 37 RevTeX pages, 15 figures; submitted to Phys.Rev.
Expected performance of m-solution backtracking
This paper derives upper bounds on the expected number of search tree nodes visited during an m-solution backtracking search, a search which terminates after some preselected number m problem solutions are found. The search behavior is assumed to have a general probabilistic structure. The results are stated in terms of node expansion and contraction. A visited search tree node is said to be expanding if the mean number of its children visited by the search exceeds 1 and is contracting otherwise. It is shown that if every node expands, or if every node contracts, then the number of search tree nodes visited by a search has an upper bound which is linear in the depth of the tree, in the mean number of children a node has, and in the number of solutions sought. Also derived are bounds linear in the depth of the tree in some situations where an upper portion of the tree contracts (expands), while the lower portion expands (contracts). While previous analyses of 1-solution backtracking have concluded that the expected performance is always linear in the tree depth, the model allows superlinear expected performance
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