Applied Math

IMG
Peilin Zhong*, Yuchen Mo*, Chang Xiao*, Pengyu Chen, and Changxi Zheng
Rethinking Generative Coverage: A Pointwise Guaranteed Approach.
arXiv:1902.04697, Feb. 2019
(*equal contribution)

Paper Abstract
All generative models have to combat missing modes. The conventional wisdom is by reducing a statistical distance (such as f-divergence) between the generated distribution and the provided data distribution through training. We defy this wisdom. We show that even a small statistical distance does not imply a plausible mode coverage, because this distance measures a global similarity between two distributions, but not their similarity in local regions--which is needed to ensure a complete mode coverage.

From a starkly different perspective, we view the battle against missing modes as a two-player game, between a player choosing a data point and an adversary choosing a generator aiming to cover that data point. Enlightened by von Neumann's minimax theorem, we see that if a generative model can approximate a data distribution moderately well under a global statistical distance measure, then we should be able to find a mixture of generators which collectively covers every data point and thus every mode with a lower-bounded probability density. A constructive realization of this minimax duality--that is, our proposed algorithm of finding the mixture of generators--is connected to a multiplicative weights update rule. We prove the pointwise coverage guarantee of our algorithm, and our experiments on real and synthetic data confirm better mode coverage over recent approaches that also use a mixture of generators but focus on global statistical distances.
IMG
Chang Xiao, Peilin Zhong, and Changxi Zheng
BourGAN: Generative Networks with Metric Embeddings.
Advances in Neural Information Processing Systems (NeurIPS), 2018
(Spotlight presentation)

Paper (PDF) Abstract Source Code Bibtex
This paper addresses the mode collapse for generative adversarial networks (GANs). We view modes as a geometric structure of data distribution in a metric space. Under this geometric lens, we embed subsamples of the dataset from an arbitrary metric space into the L2 space, while preserving their pairwise distance distribution. Not only does this metric embedding determine the dimensionality of the latent space automatically, it also enables us to construct a mixture of Gaussians to draw latent space random vectors. We use the Gaussian mixture model in tandem with a simple augmentation of the objective function to train GANs. Every major step of our method is supported by theoretical analysis, and our experiments on real and synthetic data confirm that the generator is able to produce samples spreading over most of the modes while avoiding unwanted samples, outperforming several recent GAN variants on a number of metrics and offering new features.
@incollection{Xiao18Bourgan,
  title = {BourGAN: Generative Networks with Metric Embeddings},
  author = {Xiao, Chang and Zhong, Peilin and Zheng, Changxi},
  booktitle = {Advances in Neural Information Processing Systems (NeurIPS) 31},
  pages = {2275--2286},
  year = {2018},
  publisher = {Curran Associates, Inc.},
}
IMG
Alexander Vladimirsky and Changxi Zheng
A fast implicit method for time-dependent Hamilton-Jacobi PDEs.
arXiv:1306.3506, Jun. 2013

Paper Project Page Abstract Source Code
We present a new efficient computational approach for time-dependent first-order Hamilton-Jacobi-Bellman PDEs. Since our method is based on a time-implicit Eulerian discretization, the numerical scheme is unconditionally stable, but discretized equations for each time-slice are coupled and non-linear. We show that the same system can be re-interpreted as a discretization of a static Hamilton-Jacobi-Bellman PDE on the same physical domain. The latter was shown to be "causal" in [Vladimirsky 2006], making fast (non-iterative) methods applicable. The implicit discretization results in higher computational cost per time slice compared to the explicit time marching. However, the latter is subject to a CFL-stability condition, and the implicit approach becomes significantly more efficient whenever the accuracy demands on the time-step are less restrictive than the stability. We also present a hybrid method, which aims to combine the advantages of both the explicit and implicit discretizations. We demonstrate the efficiency of our approach using several examples in optimal control of isotropic fixed-horizon processes.