Sorting out Lipschitz function approximation
Authors: Cem Anil, James Lucas, Roger B. Grosse
Keywords: deep learning, lipschitz neural networks, generalization, universal approximation, adversarial examples, generative models, optimal transport, adversarial robustness
TL;DR: We identify pathologies in existing activation functions when learning neural networks with Lipschitz constraints and use these insights to design neural networks which are universal Lipschitz function approximators.
Abstract: Training neural networks subject to a Lipschitz constraint is useful for generalization bounds, provable adversarial robustness, interpretable gradients, and Wasserstein distance estimation. By the composition property of Lipschitz functions, it suffices to ensure that each individual affine transformation or nonlinear activation function is 1-Lipschitz. The challenge is to do this while maintaining the expressive power.  We identify a necessary property for such an architecture: each of the layers must preserve the gradient norm during backpropagation. Based on this, we propose to combine a gradient norm preserving activation function, GroupSort, with norm-constrained weight matrices. We show that norm-constrained GroupSort architectures are universal Lipschitz function approximators. Empirically, we show that norm-constrained GroupSort networks achieve tighter estimates of Wasserstein distance than their ReLU counterparts and can achieve provable adversarial robustness guarantees with little cost to accuracy.