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arxiv: 2311.14344 · v3 · pith:TKW6EDSSnew · submitted 2023-11-24 · 🪐 quant-ph · cs.ET

Tensor-Network Formulation of the Traveling Salesman Problem and Variants

Pith reviewed 2026-05-24 05:35 UTC · model grok-4.3

classification 🪐 quant-ph cs.ET
keywords tensor networkstraveling salesman problemcombinatorial optimizationmarginal formulaBoltzmann weightingcounting filtersjob reassignment problemzero-temperature limit
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The pith

Tensor networks represent traveling salesman tours so their zero-temperature marginals select an optimal feasible path via sequential rules.

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

The paper constructs a tensor-network representation of candidate tours for the traveling salesman problem and its variants. Tours are encoded in network layers, weighted by Boltzmann factors according to their costs, and constrained by explicit counting filters that enforce validity. This setup produces an explicit marginal formula whose zero-temperature, exact-arithmetic limit extracts an optimal tour by sequential application of the marginal rule. The same construction adapts directly to generalizations such as the job reassignment problem. At finite temperature or finite precision the extraction functions as a heuristic whose outcome depends on numerical contrast and near-degeneracies.

Core claim

Candidate tours are represented with tensor-network layers, weighted by Boltzmann factors, and constrained through explicit counting filters. The construction yields an explicit tensor-network marginal formula whose zero-temperature, exact-arithmetic limit identifies an optimal feasible tour through a sequential marginal rule.

What carries the argument

The tensor-network marginal formula obtained from layers weighted by Boltzmann factors and constrained by counting filters; it supplies the sequential marginal rule that recovers the optimum in the zero-temperature exact limit.

If this is right

  • The same tensor-network layers and filters extend without change to several standard generalizations of the TSP.
  • The construction supplies a concrete tensor-network implementation for the job reassignment problem.
  • At any finite temperature the extraction procedure becomes a heuristic whose success depends on numerical contrast, calibration, and near-degeneracies.
  • The experiments remain deliberately small and serve only to illustrate the method against exact and heuristic references.

Where Pith is reading between the lines

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

  • If tensor-network contraction routines can be applied at scale, the same marginal formula might serve as a contraction-based solver for other routing or assignment problems.
  • The sequential marginal rule could be compared directly with belief-propagation or message-passing heuristics on the same constraint graphs.
  • Near-degeneracies that appear at finite temperature might be used to generate diverse near-optimal tours without additional sampling steps.

Load-bearing premise

The counting filters and Boltzmann weighting together produce a tensor network whose marginals remain numerically stable and contractible enough to extract the optimum without the extraction rule itself becoming equivalent to exhaustive search.

What would settle it

For any small TSP instance whose optimum is known by exhaustive enumeration, the sequential marginal rule applied to the exact-arithmetic tensor-network marginals fails to recover that optimum.

Figures

Figures reproduced from arXiv: 2311.14344 by Aitor Moreno Fdez. de Leceta, Alejandro Mata Ali, I\~nigo Perez Delgado.

Figure 1
Figure 1. Figure 1: TSP graph with the solution ⃗x = (0, 3, 1, 6, 4, 2, 9, 8, 13, 11, 7, 12, 10, 5). The blue edges represent possible paths that were not taken, and ar￾rowed red edges represent the path taken by the solu￾tion. Edges with weight considered infinite are not rep￾resented. With this formulation, the problem can be ex￾pressed as ⃗xopt = arg min ⃗x (C (⃗x)) xt ∈ [0, N − 1] ∀t ∈ [0, N − 1] xt ̸= xt ′ ∀t ̸= t ′ ∈ [0… view at source ↗
Figure 2
Figure 2. Figure 2: TSP tensor network, with the initialization and [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Notation for all indexes used in all tensors in [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Contraction scheme for 3 variables with the [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: MPO layer for 3 multiple bond index tensors. [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
read the original abstract

This work presents a tensor-network formulation of the Traveling Salesman Problem (TSP) and several of its variants. The approach represents candidate tours with tensor-network layers, weights them by Boltzmann factors, and enforces constraints through explicit counting filters. This formalism also yields an explicit tensor-network marginal formula whose zero-temperature, exact-arithmetic limit identifies an optimal feasible tour through a sequential marginal rule. At finite $\tau$ and finite precision, the implemented extraction is a heuristic whose behavior depends on numerical contrast, calibration, and near-degeneracies. We adapt the construction to several generalizations of the TSP and apply it to the Job Reassignment Problem, as a representative industrial integration. The experiments are deliberately small and illustrative; they contextualize the method against exact and heuristic references but do not establish general computational superiority over specialized classical solvers.

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

1 major / 1 minor

Summary. The manuscript presents a tensor-network formulation of the Traveling Salesman Problem (TSP) and variants. Candidate tours are represented via tensor-network layers, weighted by Boltzmann factors according to tour cost, and constrained by explicit counting-filter tensors (e.g., visit-once and degree-2 conditions). The central claim is that this construction yields an explicit tensor-network marginal formula whose zero-temperature, exact-arithmetic limit recovers an optimal feasible tour via a sequential marginal rule. The approach is adapted to several TSP generalizations and demonstrated on the Job Reassignment Problem; all experiments are small and illustrative, with finite-τ extraction explicitly qualified as heuristic.

Significance. If the unshown derivation of the marginal formula and its limiting behavior can be supplied and verified, the work would supply a novel, constraint-explicit encoding of combinatorial optimization inside the tensor-network formalism. This could open routes to contraction-based solvers or quantum-inspired algorithms. The manuscript's honesty about the heuristic status of finite-τ extraction and its deliberate choice of illustrative rather than competitive experiments are positive features that keep the claims proportionate.

major comments (1)
  1. [Abstract] Abstract (and the corresponding construction section): the central claim that the zero-temperature, exact-arithmetic limit of the tensor-network marginal formula identifies an optimal feasible tour through a sequential marginal rule is stated without any derivation steps, explicit marginal expression, or proof that the resulting marginals remain supported only on feasible tours and select a minimum-cost one. This derivation is load-bearing for the paper's primary contribution.
minor comments (1)
  1. The manuscript repeatedly notes that finite-τ behavior is heuristic and depends on numerical contrast and near-degeneracies, but does not supply a quantitative characterization (e.g., condition-number bounds or failure-mode examples) that would help readers assess when the heuristic is reliable.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading, balanced summary, and recognition that a verified marginal formula would constitute a novel contribution. We agree that the central claim requires explicit derivation and supporting proofs, which were omitted from the original manuscript.

read point-by-point responses
  1. Referee: [Abstract] Abstract (and the corresponding construction section): the central claim that the zero-temperature, exact-arithmetic limit of the tensor-network marginal formula identifies an optimal feasible tour through a sequential marginal rule is stated without any derivation steps, explicit marginal expression, or proof that the resulting marginals remain supported only on feasible tours and select a minimum-cost one. This derivation is load-bearing for the paper's primary contribution.

    Authors: We fully agree that the derivation, explicit marginal expression, and supporting proofs are missing and constitute a load-bearing gap. In the revised manuscript we will insert a dedicated subsection (placed after the tensor-network construction) that (i) writes the explicit marginal formula obtained by contracting all but one index of the weighted, filtered network, (ii) proves by induction on the filter tensors that every marginal is supported exclusively on feasible partial tours, and (iii) shows that, in the joint limit of exact arithmetic and τ→0, the Boltzmann weights concentrate on minimum-cost configurations, so that the sequential marginal rule recovers an optimal feasible tour. The added material will also clarify the precise sense in which finite-τ extraction remains heuristic. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained

full rationale

The paper defines a tensor-network encoding of TSP tours via explicit layers, Boltzmann weighting, and counting-filter tensors for constraints. The marginal formula and its zero-temperature limit are direct mathematical consequences of this network definition and the sequential marginal rule; they do not reduce to fitted parameters, renamed inputs, or load-bearing self-citations. No uniqueness theorem or ansatz is smuggled in via prior author work. The construction is presented as a fresh encoding whose formal properties follow from the tensor algebra without circular reduction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available; no explicit free parameters, axioms, or invented entities are stated. The construction implicitly assumes that the tensor network can be defined and contracted without introducing new fitted scales beyond the temperature parameter already named.

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Reference graph

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