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arxiv: 2604.10381 · v1 · submitted 2026-04-11 · 🧮 math.AT · math.RT

Recognition: unknown

Computing Homomorphisms of Poset Representations with Applications to Multiparameter Persistence

Jan Jendrysiak

Pith reviewed 2026-05-10 15:08 UTC · model grok-4.3

classification 🧮 math.AT math.RT
keywords poset representationshomomorphismsfree resolutionsmultiparameter persistencepersistent homologyalgorithmsZ^d
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The pith

Uniqueness of lifts along free resolutions enables improved algorithms to compute homomorphism spaces between poset representations.

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

The paper develops algorithms to compute the vector space of homomorphisms between finitely generated representations of a poset such as Z^d, with generalization to arbitrary posets. Its central theoretical result establishes uniqueness of lifts of homomorphisms through free resolutions. This uniqueness supports a new algorithm whose runtime depends on the maximal pointwise dimension of the target representation plus kernel computation time. The work also adapts a classical method from Green, Heath, and Struble and shows both approaches beat the naive bound when pointwise dimensions remain small. These algorithms are applied to strengthen the AIDA decomposition for multiparameter persistent homology and are implemented for benchmarking on bi-filtrations of immune-cell data.

Core claim

The main theoretical contribution is a uniqueness result for lifts of homomorphisms along free resolutions, which we use to obtain an algorithm running in O(n^4 (thick(Y) + thick(Omega^1 Y))^2 + T_ker(d,n)) time. We also apply and analyse a classical approach due to Green, Heath, and Struble, achieving O(n^3 thick(Y)^3 + n^4). Both improve on the naive O(n^6) bound when thick(Y) is small. Applied to the decomposition algorithm AIDA, the classical approach improves the asymptotic runtime the most.

What carries the argument

Uniqueness result for lifts of homomorphisms along free resolutions of finitely generated poset representations, which reduces Hom computation to solving a linear system after selecting one lift.

If this is right

  • The classical approach most improves the asymptotic runtime of the AIDA decomposition algorithm for multiparameter persistence.
  • Both new and classical algorithms improve on the naive O(n^6) bound whenever the maximal pointwise dimension is small.
  • The classical method strengthens existing runtime results for uniquely graded modules.
  • Practical implementations allow direct benchmarking against persistent homology computations on density-alpha bi-filtrations.

Where Pith is reading between the lines

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

  • The observed practical speed of the lifting algorithm on 2-parameter modules indicates it may be the method of choice in applications even when its worst-case complexity is not the best.
  • The general-poset formulation could support custom poset structures arising in data beyond the standard Z^d grid.
  • The lift-uniqueness technique may adapt to compute related functors such as Ext groups in the same persistence setting.

Load-bearing premise

The representations are finitely generated and the uniqueness of lifts along free resolutions holds for the given poset.

What would settle it

A concrete counterexample of two distinct lifts of the same homomorphism along a free resolution for a finitely generated Z^d-representation would falsify the uniqueness claim and break the derived algorithm.

Figures

Figures reproduced from arXiv: 2604.10381 by Jan Jendrysiak.

Figure 1
Figure 1. Figure 1: Sheets of metal and paper - San Diego Air & Space Museum Archives [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Hilbert function of H1 of a density-delaunay filtration of the Large Hypoxic Regions (FOXP3) dataset from [46] on a 200×200 grid (left). Hilbert functions of its 4 largest indecomposable summands (right). This module has 5917 generators, and 74% of these belong to an interval-summand A few summands have many generators and a large area of support. At small scale-values, many short lived homology classes ap… view at source ↗
Figure 3
Figure 3. Figure 3: A homomorphism. The red module has thickness 1, the blue module thickness 2. [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Left: The free/projective module A[−(1, 1)] ≃ F (1, 1). Middle: The injective module I (2, 2). Right: the blue module from [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: A presentation of the blue module in [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: A chain map lifting a homomorphism of persistence modules in [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7 [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: From left to right: X, Y X1, Y1 Ω 1 (X), Ω 1 (Y ) The Restricted Computation of Homomorphisms of Presentations Algorithm A: Restricted Computation of Homomorphisms of Presentations. Input: Presentation matrices M ∈ k G×R, N ∈ k G′×R ′ Output: Graded matrices {Qi}i∈I ∈ k G′×G which descend to a basis of Hom(X, Y ) 1 for α ∈ deg(G) do 2 dα, G′ α ← Algorithm 5(N, α) 3 for g ∈ G with deg(g) = α do 4 Qg ′ ,g ← … view at source ↗
Figure 9
Figure 9. Figure 9: Layer-thickness (y) and |b0 + b1| (x) for H0 and H1. The thickness of indecomposable summands also correlates strongly with the cardinality of their graded Betti-numbers in this dataset as visible in [PITH_FULL_IMAGE:figures/full_fig_p019_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Estimates for exponents of runtime on this dataset. Some runtimes of [PITH_FULL_IMAGE:figures/full_fig_p019_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Thickness (y) and |b0 + b1| (x) of all non-interval indecomposables in H1. 26 [PITH_FULL_IMAGE:figures/full_fig_p026_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Runtimes (log-scale) to compute End(X) for non-interval indecomposables in the dataset. 27 [PITH_FULL_IMAGE:figures/full_fig_p027_12.png] view at source ↗
read the original abstract

We present algorithms to compute the vector space of homomorphisms Hom(X,Y) between finitely generated representations of the partially ordered set Z^d. Our results generalise to any partially ordered set. Our main theoretical contribution is a uniqueness result for lifts of homomorphisms along free resolutions, which we use to obtain an algorithm running in O(n^4 (thick(Y) + thick(Omega^1 Y))^2 + T_ker(d,n)) time, where thick(Y) denotes the maximal pointwise dimension of Y and T_ker is the time it takes to compute the kernel of a map between projective Z^d-modules. We also apply and analyse a classical approach due to Green, Heath, and Struble (J. Symbolic Comput., 2001), achieving O(n^3 thick(Y)^3 + n^4). Both improve on the naive O(n^6) bound when thick(Y) is small. Applied to the decomposition algorithm AIDA (Dey-J-Kerber, SoCG '25), the classical approach improves the asymptotic runtime the most, strengthening the result of Dey and Xin (J. Appl. Comput. Topology, 2022) for uniquely graded modules. We implement all algorithms in the Persistence Algebra C++ library and benchmark them on the persistent homology of density-alpha bi-filtrations of immune-cell locations. The classical approach has the best worst-case complexity, yet for 2-parameter modules, the lifting algorithm is fastest in practice.

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

0 major / 3 minor

Summary. The paper presents algorithms to compute the vector space Hom(X,Y) of homomorphisms between finitely generated representations of the poset Z^d (with generalization to arbitrary posets). The central theoretical contribution is a uniqueness result for lifts of homomorphisms along free resolutions, which yields an algorithm with complexity O(n^4 (thick(Y) + thick(Omega^1 Y))^2 + T_ker(d,n)). The authors also analyze and apply a classical method of Green-Heath-Struble achieving O(n^3 thick(Y)^3 + n^4), both improving on the naive O(n^6) bound for small thick(Y). These are applied to improve the runtime of the AIDA decomposition algorithm, implemented in the Persistence Algebra C++ library, and benchmarked on persistent homology of density-alpha bifiltrations from immune-cell data, where the lifting algorithm is fastest in practice for 2-parameter modules despite the classical method's superior worst-case complexity.

Significance. If the uniqueness result for lifts holds and the complexity analyses are correct, the work provides meaningful improvements to the computational toolkit for multiparameter persistence, a core task in topological data analysis. The explicit application to strengthening the AIDA algorithm and the independent implementation with external benchmarks on real data are strengths that increase the practical impact. The generalization to arbitrary posets further broadens potential use cases.

minor comments (3)
  1. The abstract introduces the notation thick(Y) and thick(Omega^1 Y) without a brief inline definition or pointer to the preliminaries; adding this would improve immediate readability for readers outside the subfield.
  2. The complexity expression includes T_ker(d,n) (time to compute kernels of maps between projective Z^d-modules); a short sentence clarifying its dependence on d and n in the introduction would help contextualize the bound.
  3. Section 4 (or wherever the uniqueness theorem is stated) would benefit from an explicit statement of the precise hypotheses on the poset and the finite-generation assumption to make the scope of the lift-uniqueness result immediately clear.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of our manuscript, as well as for recommending minor revision. We appreciate the recognition of the theoretical contribution, the complexity improvements, the strengthening of the AIDA algorithm, the implementation, and the benchmarks on real data.

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained

full rationale

The paper presents a new uniqueness result for lifts of homomorphisms along free resolutions of finitely generated poset representations as its main theoretical contribution, which is then used to derive the stated algorithm and complexity bound. This result is not imported via self-citation but claimed and (presumably) proven within the manuscript. The classical Green-Heath-Struble method is cited from an external 2001 reference and analyzed separately with its own complexity. Application to the AIDA algorithm is an extension rather than a load-bearing premise for the core claims. Implementation in an open library and benchmarking on external immune-cell data provide independent verification outside any fitted parameters or self-generated inputs. No self-definitional equations, fitted inputs renamed as predictions, or ansatzes smuggled through citations appear in the derivation chain.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard assumptions from representation theory of posets and module theory; no free parameters or invented entities are introduced beyond the algorithmic framework.

axioms (2)
  • domain assumption Representations of the poset Z^d (or general posets) are finitely generated modules over the associated ring
    Explicitly stated as the setting for the algorithms and uniqueness result.
  • domain assumption Free resolutions exist and lifts of homomorphisms along them satisfy the claimed uniqueness property
    This is the main theoretical contribution used to derive the algorithm.

pith-pipeline@v0.9.0 · 5561 in / 1417 out tokens · 63546 ms · 2026-05-10T15:08:53.984706+00:00 · methodology

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Reference graph

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