Vector-valued Sprecher representation conjecture

Let n,mNn,m\in\mathbb{N} with m>1m>1, and let f:[0,1]nRmf:[0,1]^n\to\mathbb{R}^m be continuous. A Sprecher Network has architecture n[d1]mn\to[d_1]\to m with one hidden block of width d12n+1d_1\geq 2n+1 and an output block, using continuous splines ϕ(1),Φ(1),ϕ(2),Φ(2)\phi^{(1)},\Phi^{(1)},\phi^{(2)},\Phi^{(2)}, monotonic ϕ(1)\phi^{(1)} and ϕ(2)\phi^{(2)}, suitable parameters λ(1),η(1),λ(2),η(2)\lambda^{(1)},\eta^{(1)},\lambda^{(2)},\eta^{(2)}, and optionally lateral mixing parameters. Vector-valued Sprecher representation conjecture. For every ϵ>0\epsilon>0, there exists such a network whose output f^(x)\hat f(\mathbf{x}) satisfies

supx[0,1]nf(x)f^(x)Rm<ϵ.\sup_{\mathbf{x}\in[0,1]^n}\|f(\mathbf{x})-\hat f(\mathbf{x})\|_{\mathbb{R}^m}<\epsilon.

This extends the scalar Sprecher representation to vector-valued functions, but the supplied context states that universality of this specific construction is not directly covered by Sprecher's theorem. The status of the vector-valued extension therefore remains open.

Sources & referencesView supporting material

Primary source

Christian Hägg, Kathlén Kohn, Giovanni Luca Marchetti and Boris Shapiro, “Sprecher Networks: A Parameter-Efficient Kolmogorov-Arnold Architecture”, arXiv:2512.19367 (2026).

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