The paper investigates topological expressivity of neural networks by examining Betti numbers of the network output rather than decision boundaries. It proves that a ReLU network with a specific architecture can represent spaces homotopy equivalent to wedge sums of spheres with arbitrary Betti numbers, and provides tighter bounds for spaces equivalent to products of spheres. The authors also conjecture an even stronger bound for networks representing a single sphere.
arXiv:2607. 07778v1 Announce Type: new Abstract: Bubeck, Li and Nagaraj conjectured that, for generic data, any two-layer neural network with $m$ neurons that fits $n$ noisy labels must have Lipschitz constant at least of order $\sqrt{n/m}$, with no restriction on the size of the weights.
arXiv:2607. 21651v1 Announce Type: new Abstract: We prove that the maximum of $n$ real numbers is exactly representable by a ReLU network with two hidden layers for every $n\le 10$.
The paper constructs a width‑4 ReLU network whose iterates simulate any deterministic Turing machine, with polynomial‑time construction and depth per iteration. It shows that reachability for such iterated networks is NP‑complete, NEXP‑complete, or undecidable depending on the time bound, and extends these hardness results to width‑6 networks with common smooth activations and to discretized Neural ODEs in dimension 5.
The paper investigates the geometry and closure properties of function spaces realized by ReLU neural networks with fixed architecture. It proves that the minimal number of convex pieces in continuous piecewise-linear functions is lower semicontinuous under pointwise convergence, and uses this to show that shallow scalar ReLU realization spaces are closed in the weighted supremum norm, while providing a counterexample that demonstrates non-closedness for a specific architecture. The work also characterizes the role of a blow‑up residue in non‑closedness and offers new geometric and functional‑analytic tools for analyzing ReLU realization spaces.
The paper proves that several decision and approximation problems for ReLU neural networks are computationally hard. For any number of layers λ≥2, deciding whether a network’s output is positive (and thus whether it is surjective) is W[ℓ−1]-hard when parameterized by the input dimension d. In particular, for two-layer networks, the related geometric problem of zonotope non‑containment is W[1]-hard in the ambient dimension, and computing or approximating the Lp‑Lipschitz constant is NP‑hard and W[ℓ−1]-hard with respect to d. The results also show that these problems remain hard when parameterized by the number of layers for constant d, implying that naive enumeration algorithms running in n^{(ℓ−1)d}·poly(N) time are essentially optimal under the Exponential Time Hypothesis.
arXiv:2510. 04060v3 Announce Type: replace-cross Abstract: We establish two related but logically distinct results for shallow ReLU$^k$ neural networks on the unit sphere $\SS^d$.
arXiv:2610.20229v1 Announce Type: cross Abstract: We investigate the minimal width of deep neural networks that can approximate all continuous functions from $\mathbb{R}^n$ to $\mathbb{R}^m$ in the t...
arXiv:2611.04074v2 Announce Type: replace-cross Abstract: We ask how hard it is to build a ReLU network when its parameter values are given in advance. An instance supplies an appropriate encoding of...
arXiv:2607. 06781v1 Announce Type: new Abstract: In this work, we investigate the fixed-architecture neural network approximation with explicit parameter bounds and elementary activations.
In this work, we investigate the fixed-architecture neural network approximation with explicit parameter bounds and elementary activations. While prior work demonstrated super-expressive approximation using fixed-size networks, they lack quantitative and non-asymptotic characterizations of parameter magnitude with respect to the approximation error.
arXiv:2606. 07728v1 Announce Type: new Abstract: It is well established that ReLU networks define continuous piecewise-linear functions, and that their linear regions are polyhedra in the input space.
arXiv:2608. 25221v1 Announce Type: new Abstract: We study exact representations of $\mathrm{MAX}_N(x)=\max{x_1,\ldots,x_N}$ using two-hidden-layer ReLU neural networks.