What Kinds of Functions Do Deep Neural Networks Learn? Insights from Variational Spline Theory\ast

We develop a variational framework to understand the properties of functions learned by fitting deep neural networks with rectified linear unit (ReLU) activations to data. We propose a new function space, which is related to classical bounded variation-type spaces, that captures the compositional structure associated with deep neural networks. We derive a representer theorem showing that deep ReLU networks are solutions to regularized data-fitting problems over functions from this space. The function space consists of compositions of functions from the Banach space of second-order bounded variation in the Radon domain. This Banach space has a sparsity-promoting norm, giving insight into the role of sparsity in deep neural networks. The neural network solutions have skip connections and rank-bounded weight matrices, providing new theoretical support for these common architectural choices. The variational problem we study can be recast as a finite-dimensional neural network training problem with regularization schemes related to the notions of weight decay and path-norm regularization. Finally, our analysis builds on techniques from variational spline theory, providing new connections between deep neural networks and splines.