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 n=1n_\ell=1 for some 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.

Sources & referencesView supporting material

Primary source

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

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