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Space-Efficient Parameterized Algorithms on Graphs of Low Shrubdepth
Citation Link: https://doi.org/10.15480/882.14875
Publikationstyp
Journal Article
Date Issued
2025
Sprache
English
Author(s)
TORE-DOI
Citation
ACM Transactions on Computation Theory (2025)
ArXiv ID
Dynamic programming on various graph decompositions is one of the most fundamental techniques used in parameterized complexity. Unfortunately, even if we consider concepts as simple as path or tree decompositions, such dynamic programming uses space that is exponential in the decomposition’s width, and there are good reasons to believe that this is necessary. However, it has been shown that in graphs of low treedepth it is possible to design algorithms which achieve polynomial space complexity without requiring worse time complexity than their counterparts working on tree decompositions of bounded width. Here, treedepth is a graph parameter that, intuitively speaking, takes into account both the depth and the width of a tree decomposition of the graph, rather than the width alone.
Motivated by the above, we consider graphs that admit clique expressions with bounded depth and label count, or equivalently, graphs of low shrubdepth. Here, shrubdepth is a bounded-depth analogue of cliquewidth, in the same way as treedepth is a bounded-depth analogue of treewidth.
We show that also in this setting, bounding the depth of the decomposition is a deciding factor for improving the space complexity. More precisely, we prove that on n-vertex graphs equipped with a tree-model (a decomposition notion underlying shrubdepth) of depth d and using k labels,
- Independent Set can be solved in time 2O(dk) ·nO(1) using O(dk2logn) space; and
- Max Cut can be solved in time nO(dk) using O(dklogn) space; and
- Dominating Set can be solved in time 2O(dk) · nO(1) using nO(1) space via a randomized
algorithm.
We also establish a lower bound, conditional on a certain assumption about the complexity of Longest Common Subsequence, which shows that at least in the case of Independent Set the exponent of the parametric factor in the time complexity has to grow with d if one wishes to keep the space complexity polynomial.
Motivated by the above, we consider graphs that admit clique expressions with bounded depth and label count, or equivalently, graphs of low shrubdepth. Here, shrubdepth is a bounded-depth analogue of cliquewidth, in the same way as treedepth is a bounded-depth analogue of treewidth.
We show that also in this setting, bounding the depth of the decomposition is a deciding factor for improving the space complexity. More precisely, we prove that on n-vertex graphs equipped with a tree-model (a decomposition notion underlying shrubdepth) of depth d and using k labels,
- Independent Set can be solved in time 2O(dk) ·nO(1) using O(dk2logn) space; and
- Max Cut can be solved in time nO(dk) using O(dklogn) space; and
- Dominating Set can be solved in time 2O(dk) · nO(1) using nO(1) space via a randomized
algorithm.
We also establish a lower bound, conditional on a certain assumption about the complexity of Longest Common Subsequence, which shows that at least in the case of Independent Set the exponent of the parametric factor in the time complexity has to grow with d if one wishes to keep the space complexity polynomial.
Subjects
Parameterized complexity
shrubdepth
space complexity
algebraic methods
DDC Class
004: Computer Sciences
005.1: Programming
510: Mathematics
519: Applied Mathematics, Probabilities
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