From Approximation Rates to Loss-Landscape Barrier Decay in Shallow ReLU Networks

We study pathwise connectivity of sublevel sets for one-hidden-layer ReLU networks with constrained first-layer weights and an ℓ1 penalty on the output layer. The data term is assumed convex and globally Lipschitz in the scalar logit. We first give a finite-width construction that connects any two points of a common sublevel through a path controlled by a loss-consistent compression functional and a first-order perturbation term. The proof replaces the quadratic perturbation estimate in the Freeman--Bruna mechanism by a direct Lipschitz bound. Positive homogeneity is then used in a direction that is compatible with the penalty: every active atom is moved monotonically from the unit ball to the unit sphere while its output coefficient is reduced. Sphere covering and cluster merging consequently give O(m-1/(n-1)) fixed-level thickening for n≥2, while the one-dimensional two-ray dictionary gives exact connectivity for every m≥4. We also prove internally that the regularized approximation values satisfy e(l)-e_∞=O(l-1/2). More generally, a rate O(l-s) transfers to a near-optimal barrier rate O(m-s/((n-1)s+1)); under the standing assumptions, this yields the explicit rate O(m-1/(n+1)). A theorem-aligned finite-distribution experiment complements the analysis. The primary Huber run yields a maximal best certified upper gap 1.66×10-5 over 720 recorded pairs at widths m≥16; a matched binary-cross-entropy rerun and a 720-endpoint dense-representation stress test probe loss robustness and the active cluster-merging mechanism.