Optimizing tree decompositions in MSO

• Published in: Logical methods in computer science Vol. 18, Issue 1
• Main Authors: Bojańczyk, Mikołaj, Pilipczuk, Michał
• Format: Journal Article
• Language: English
• Published: Logical Methods in Computer Science e.V 03.02.2022
• Subjects: computer science - data structures and algorithms; computer science - discrete mathematics; computer science - logic in computer science
• ISSN: 1860-5974; 1860-5974
• DOI: 10.46298/lmcs-18(1:26)2022

The classic algorithm of Bodlaender and Kloks [J. Algorithms, 1996] solves the following problem in linear fixed-parameter time: given a tree decomposition of a graph of (possibly suboptimal) width k, compute an optimum-width tree decomposition of the graph. In this work, we prove that this problem can also be solved in mso in the following sense: for every positive integer k, there is an mso transduction from tree decompositions of width k to tree decompositions of optimum width. Together with our recent results [LICS 2016], this implies that for every k there exists an mso transduction which inputs a graph of treewidth k, and nondeterministically outputs its tree decomposition of optimum width. We also show that mso transductions can be implemented in linear fixed-parameter time, which enables us to derive the algorithmic result of Bodlaender and Kloks as a corollary of our main result.