Conjecture on identifiable parameters in ReLU architectures with width-one layers

Let A=(n0,…,nL+1)\mathcal{A}=(n_0,\ldots,n_{L+1}) be a ReLU network architecture, with parameter space ΘA\Theta_\mathcal{A}, quotient parameter space ΘA‾\overline{\Theta_\mathcal{A}}, and quotient map identifying parameters up to the relevant within-layer permutations. A parameter is finitely identifiable if its fiber in ΘA‾\overline{\Theta_\mathcal{A}} is a finite set.

Width-one identifiability conjecture.

  1. There exist architectures A=(n0,…,nL+1)\mathcal{A}=(n_0,\ldots,n_{L+1}) with nℓ=1n_\ell=1 for some ℓ≤L\ell\leq L for which no identifiable parameters exist.
  2. For every architecture, there exists a finitely identifiable parameter.

The first assertion predicts that width-one non-output layers can prevent even individual identifiable parameters, because the resulting activation image is at most one-dimensional and transverse breakpoint assignments may remain ambiguous. The second asserts that finite identifiability should nevertheless occur in every architecture. The source provides no resolution of either assertion.

References

Primary source

Moritz Grillo and Guido Montúfar, “Most ReLU Networks Admit Identifiable Parameters”, arXiv:2605.03601 (2026).

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