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arxiv: 2605.06555 · v1 · submitted 2026-05-07 · 💻 cs.DS

Recognition: unknown

Fast decremental tree sums in forests

Benjamin Aram Berendsohn, Marek Soko{\l}owski

Pith reviewed 2026-05-08 05:40 UTC · model grok-4.3

classification 💻 cs.DS
keywords decremental algorithmsdynamic foreststree sumsmicro-macro decompositiondata structuresoptimal algorithmsgroup modeledge deletions
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The pith

Micro-macro tree decompositions maintain sums over vertex weights in forests under only deletions in O(log* n) time per operation.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper studies how to keep track of the total weight of each connected component in a forest as edges are deleted and weights are updated. Standard fully dynamic structures solve this in O(log n) time, but the authors focus on the deletion-only setting to see if faster methods exist. They develop a data structure that preprocesses the initial forest in linear time and then answers queries or handles updates in O(log* n) time using a decomposition into micro and macro trees. When all weights are fixed at one, the per-operation time falls to constant. They also give a second algorithm that matches the exact number of arithmetic operations needed by an optimal solution for any fixed starting forest.

Core claim

We give a data structure with O(n) preprocessing time and O(log* n) time per operation, based on a micro-macro tree decomposition. If the forest is unweighted, the operation time can be improved to O(1). Additionally, we give an asymptotically universally optimal algorithm that processes m operations on an initial forest F in running time O(OPT(F, m)), where OPT(F, m) is the number of weight additions and subtractions performed by a best possible algorithm on a worst-case instance.

What carries the argument

A micro-macro tree decomposition that breaks the forest into small precomputable pieces and larger navigable structures to support fast sum maintenance after deletions.

If this is right

  • Component sums can be maintained faster when the input is known to be deletion-only rather than fully dynamic.
  • Unweighted forests allow constant-time sum queries after linear preprocessing.
  • The optimal algorithm achieves running time proportional to the minimum number of arithmetic steps any solution must perform for the given starting forest.
  • Tight time bounds are established for several variants of the related subtree-sum problem on rooted forests.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The logarithmic factor in standard dynamic-tree methods appears tied to the need to support insertions, not deletions alone.
  • Similar decomposition techniques may accelerate other deletion-only problems such as connectivity or path aggregates on trees.
  • Precomputing optimal small-instance solutions could be applied to other decremental problems where OPT can be bounded in advance.
  • The unknown worst-case complexity of the optimal algorithm leaves open whether O(log* n) is improvable to O(1) in the general weighted case.

Load-bearing premise

The initial forest is supplied all at once and the only changes afterward are edge deletions and weight updates, with no new edges ever added.

What would settle it

A concrete sequence of deletions and weight updates on a forest of size n where the maintained sums require more than O(log* n) operations on average in the group model would contradict the claimed bounds.

read the original abstract

We study two fundamental decremental dynamic graph problems. In both problems, we need to maintain a vertex-weighted forest of size $n$ under edge deletions, weight updates, and a certain information-retrieval query. Both problems can be solved in $O(\log n)$ time per update/query using standard dynamic forest data structures like top trees, even if additionally edge insertions are allowed. We investigate whether the deletion-only problem can be solved faster. First, we consider $\texttt{tree-sum}$ queries, where we ask for the sum of vertex weights in one of the connected components (i.e., trees) in the forest. We give a data structure with $O(n)$ preprocessing time and $O(\log^* n)$ time per operation, based on a micro-macro tree decomposition (Alstrup et al., 1997). If the forest is unweighted (i.e., all weights are 1 and cannot be changed), then the operation time can be improved to $O(1)$. Additionally, we give an asymptotically universally optimal algorithm. More specifically, our algorithm works in the group model, and processes $m$ operations on an initial forest $F$ in running time $O( \mathrm{OPT}(F, m) )$. Here $\mathrm{OPT}(F, m)$ is the number of weight additions and subtractions that a best possible algorithm performs to handle a worst-case instance for a fixed initial forest $F$ and a fixed number $m$ of operations. We achieve this with a combination of the aforementioned decomposition technique, precomputation of optimal data structures for very small instances, and some insights into the behavior of $\mathrm{OPT}$. Note that even the worst-case complexity of this algorithm remains unknown to us. Second, we consider $\texttt{subtree-sum}$ queries. Here, the forest is rooted, and a query $\texttt{subtree-sum}(v)$ returns the sum of weights in the subtree rooted at $v$. We show tight bounds for several variants of this problem. [...]

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript studies two decremental dynamic problems on vertex-weighted forests of size n under edge deletions and weight updates. For tree-sum queries (sum of weights in a connected component), it claims an O(n)-preprocessing data structure achieving O(log* n) time per operation via micro-macro tree decomposition (Alstrup et al. 1997), improving to O(1) when the forest is unweighted. It further presents an asymptotically universally optimal algorithm in the group model that processes m operations in O(OPT(F, m)) time, where OPT is the minimum number of weight additions/subtractions needed by any algorithm on a fixed initial forest F. For subtree-sum queries on rooted forests, tight bounds are shown for several variants.

Significance. If the bounds and optimality hold, the work improves on the O(log n) per-operation time of standard structures such as top trees in the strictly decremental setting. The micro-macro decomposition with upfront precomputation, the O(1) unweighted case, and the group-model universal optimality (even while leaving worst-case complexity open) are notable strengths that could inform other deletion-only dynamic graph problems.

major comments (2)
  1. [Micro-macro decomposition and update handling] The description of how the micro-macro decomposition is maintained under deletions (rather than rebuilt) should include an explicit accounting of the auxiliary information stored during the O(n) preprocessing and how deletions affect the macro-tree structure without introducing hidden logarithmic factors.
  2. [Universally optimal algorithm] The group-model optimality argument combines the decomposition with precomputation on small instances and insights into OPT behavior; a concrete reduction or invariant should be stated showing that the total number of additions/subtractions across all operations is at most OPT(F, m) plus lower-order terms.
minor comments (3)
  1. [Abstract] The abstract is truncated after the subtree-sum sentence; the final version should concisely state the tight bounds obtained for the subtree-sum variants.
  2. [Preliminaries] Notation for the group model and the definition of OPT(F, m) should be introduced with a short self-contained paragraph before the optimality theorem.
  3. [Introduction] A table comparing the new bounds against top trees and other dynamic-forest structures would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and the recommendation for minor revision. The comments are helpful for improving the clarity of our presentation. We address each major comment below and will incorporate the suggested clarifications in the revised manuscript.

read point-by-point responses
  1. Referee: The description of how the micro-macro decomposition is maintained under deletions (rather than rebuilt) should include an explicit accounting of the auxiliary information stored during the O(n) preprocessing and how deletions affect the macro-tree structure without introducing hidden logarithmic factors.

    Authors: We agree that an explicit accounting would enhance the exposition. In the revised version, we will add a new subsection detailing the auxiliary information: during O(n) preprocessing, we store for each micro-tree its internal sum, boundary pointers to the macro-tree, and precomputed deletion tables for all possible deletion sequences within the micro-tree. Deletions are handled by updating the affected micro-tree in constant time using these tables, and only when a deletion disconnects a macro-node do we update the macro-tree structure. Since the micro-macro decomposition ensures that the macro-tree has height O(log* n), and each update affects O(1) macro-nodes per level, the per-operation time remains O(log* n) without hidden factors. We will include a formal proof that the total auxiliary space is linear and no additional logarithmic overhead is incurred. revision: yes

  2. Referee: The group-model optimality argument combines the decomposition with precomputation on small instances and insights into OPT behavior; a concrete reduction or invariant should be stated showing that the total number of additions/subtractions across all operations is at most OPT(F, m) plus lower-order terms.

    Authors: We appreciate this observation. To make the argument more rigorous, we will introduce in the revised manuscript an explicit invariant: at any point, the data structure maintains a representation where the number of additions/subtractions performed so far is at most OPT(F, t) + O(t), where t is the number of operations so far. This is achieved by precomputing optimal solutions for all small forests of size up to some constant (using the group model allowing free precomputation), and using the decomposition to reduce large instances to small ones. We will state a reduction showing that our algorithm's cost is bounded by OPT plus lower-order terms, specifically O(m) additional operations which are absorbed since OPT(F,m) = Omega(m) in the worst case for nontrivial instances. This preserves the asymptotic universal optimality. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper derives its O(log* n) and O(1) bounds from an external micro-macro decomposition (Alstrup et al. 1997) precomputed on the initial forest, plus new precomputation for small instances and group-model analysis of OPT(F, m). No step reduces a claimed prediction or bound to a fitted parameter, self-definition, or load-bearing self-citation; the universally optimal claim is supported by explicit combination of decomposition, precomputation, and OPT insights without circular reduction to inputs. The derivation remains self-contained against the stated decremental setting.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Claims rest on the micro-macro tree decomposition from prior literature, standard RAM-model assumptions for time bounds, and precomputation for small instances; no free parameters or new entities are introduced.

axioms (1)
  • standard math Standard model of computation (RAM) for analyzing dynamic data structure operations.
    Used to establish the O(log* n) and O(1) time bounds.

pith-pipeline@v0.9.0 · 5680 in / 1197 out tokens · 66165 ms · 2026-05-08T05:40:07.370279+00:00 · methodology

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Reference graph

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