Moderately beyond clique-width: reduced component max-leaf and related parameters
Pith reviewed 2026-05-10 02:05 UTC · model grok-4.3
The pith
Reduced component max-leaf is a graph parameter strictly between clique-width and reduced bandwidth that supports polynomial algorithms for induced subgraph problems when a contraction sequence is given.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Reduced component max-leaf, obtained by taking the maximum leaf count over spanning trees of components after a sequence of contractions, sits strictly between clique-width and reduced bandwidth. Graphs supplied with a witnessing contraction sequence of bounded reduced component max-leaf admit polynomial algorithms for Maximum Induced d-Regular Subgraph and Induced Disjoint Paths. In any sparse class of bounded reduced component max-leaf, bounded maximum degree implies bounded treewidth while the absence of a large biclique implies strongly sublinear treewidth; the former follows from the existence of balanced separators dominated by a bounded number of vertices. Most reduced parameters, the
What carries the argument
A contraction sequence witnessing bounded reduced component max-leaf, where component max-leaf is the largest number of leaves in any spanning tree of a connected component.
If this is right
- Bounded degree together with bounded reduced component max-leaf forces bounded treewidth.
- K_{t,t}-subgraph-free graphs with bounded reduced component max-leaf have strongly sublinear treewidth.
- Graphs with bounded reduced component max-leaf possess balanced separators dominated by a bounded number of vertices.
- The class of graphs with bounded reduced component max-leaf is closed under first-order transductions.
- Three-dimensional grids have unbounded reduced bandwidth, so planar graphs cannot first-order transduce them.
Where Pith is reading between the lines
- The separator property may extend algorithmic tractability to additional problems already known to be tractable on unit interval graphs.
- The closure under transductions suggests that non-transducibility proofs for other grid-like structures could be obtained by showing unbounded reduced bandwidth on those structures.
- The same contraction-sequence technique might unify tractability results for further intermediate parameters between clique-width and bandwidth.
Load-bearing premise
The input graphs are supplied together with an explicit contraction sequence that bounds the reduced component max-leaf value.
What would settle it
A single planar graph on which every contraction sequence has reduced component max-leaf growing with the size of the graph, or a graph with a bounded reduced component max-leaf sequence for which Induced Disjoint Paths cannot be solved in polynomial time.
Figures
read the original abstract
Reduced parameters [BKW, JCTB '26; BKRT, SODA '22] are defined via contraction sequences. Based on this framework, we introduce the reduced component max-leaf, denoted by $\operatorname{cml}^\downarrow$, where component max-leaf is the maximum number of leaves in any spanning tree of any connected component. Reduced component max-leaf is strictly sandwiched between clique-width and reduced bandwidth, it is bounded in unit interval graphs, and unbounded in planar graphs. We design polynomial-time algorithms for problems such as \textsc{Maximum Induced $d$-Regular Subgraph} and \textsc{Induced Disjoint Paths} in graphs given with a contraction sequence witnessing low $\operatorname{cml}^\downarrow$, unifying and extending tractability results for classes of bounded clique-width and unit interval graphs. We get the following collapses in sparse classes of bounded $\operatorname{cml}^\downarrow$: bounded maximum degree implies bounded treewidth, whereas $K_{t,t}$-subgraph-freeness implies strongly sublinear treewidth; we show the latter, more generally, for classes of bounded reduced cutwidth. We establish the former result by showing that graphs with bounded $\operatorname{cml}^\downarrow$ admit balanced separators dominated by a bounded number of vertices. We then showcase an application of the reduced parameters to establishing non-transducibility results. We prove that for most reduced parameters $p^\downarrow$ (including reduced bandwidth), the family of classes of bounded $p^\downarrow$ is closed under first-order transductions. We then answer a question of [BKW '26] by showing that the 3-dimensional grids have unbounded reduced bandwidth. As the class of planar graphs (or any class of bounded genus) has bounded reduced bandwidth [BKW '26], this reproves a recent result [GPP, LICS '25] that planar graphs do not first-order transduce the 3-dimensional grids.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces the reduced component max-leaf parameter (cml↓) within the reduced-parameters framework based on contraction sequences. It establishes that cml↓ is strictly sandwiched between clique-width and reduced bandwidth, bounded on unit interval graphs, and unbounded on planar graphs. Polynomial-time algorithms are given for Maximum Induced d-Regular Subgraph and Induced Disjoint Paths (among others) when a witnessing contraction sequence of low cml↓ is provided as input. Structural results include a balanced-separator lemma implying that bounded cml↓ plus bounded degree yields bounded treewidth (and, more generally, K_{t,t}-freeness yields strongly sublinear treewidth, also shown for reduced cutwidth). The paper further proves that classes of bounded reduced parameters are closed under first-order transductions and applies this to show that 3-dimensional grids have unbounded reduced bandwidth, reproving that planar graphs do not FO-transduce 3D grids.
Significance. If the results hold, the work extends the reduced-parameters program with a new intermediate parameter that unifies tractability for clique-width and unit-interval graphs on induced-subgraph problems. The balanced-separator lemma and transducibility-closure results supply reusable tools for sparse classes and logical definability, while the 3D-grid application directly resolves a question from prior work on non-transducibility.
minor comments (2)
- In the algorithmic sections, the dependence on an explicit contraction sequence as input is clearly stated, but a short discussion of whether low cml↓ can be recognized in polynomial time (or is NP-hard) would strengthen the presentation of the results' applicability.
- The notation for component max-leaf and its reduced variant should be introduced with a self-contained definition before the sandwiching theorems to improve readability for readers unfamiliar with the prior reduced-parameters papers.
Simulated Author's Rebuttal
We thank the referee for their positive summary, significance assessment, and recommendation of minor revision. The report accurately captures the main contributions of the paper. As no specific major comments were listed under the MAJOR COMMENTS section, we have no point-by-point responses to address. We will incorporate any minor editorial or polishing changes in the revised version.
Circularity Check
No significant circularity
full rationale
The paper introduces the new parameter cml↓ by direct definition from the contraction-sequence framework of the cited prior works and then supplies independent proofs for its sandwiching between clique-width and reduced bandwidth, its boundedness on unit interval graphs, the balanced-separator lemma, the polynomial-time algorithms on explicit contraction sequences, and the FO-transduction closure for most reduced parameters. These steps rely on explicit constructions and lemmas rather than any equation or claim that reduces by construction to a fitted input, self-definition, or unverified self-citation chain. The 3D-grid unboundedness result is a fresh construction that answers an open question from the cited prior paper, and the reproof for planar graphs simply invokes an externally stated fact from that prior work. No load-bearing derivation collapses to its own inputs.
Axiom & Free-Parameter Ledger
axioms (1)
- standard math Standard definitions and properties of clique-width, bandwidth, contraction sequences, and first-order transductions from the cited literature [BKW, JCTB '26; BKRT, SODA '22; GPP, LICS '25].
invented entities (1)
-
reduced component max-leaf (cml↓)
no independent evidence
Reference graph
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