Sampling a target probability distribution with an unknown normalization constant is a fundamental challenge in computational science and engineering. Recent work shows that algorithms derived by considering gradient flows in the space of probability measures open up new avenues for algorithm development. This paper makes three contributions to this sampling approach by scrutinizing the design components of such gradient flows. Any instantiation of a gradient flow for sampling needs an energy functional and a metric to determine the flow, as well as numerical approximations of the flow to derive algorithms. Our first contribution is to show that the Kullback-Leibler divergence, as an energy functional, has the unique property (among all f-divergences) that gradient flows resulting from it do not depend on the normalization constant of the target distribution. Our second contribution is to study the choice of metric from the perspective of invariance. The Fisher-Rao metric is known as the unique choice (up to scaling) that is diffeomorphism invariant. As a computationally tractable alternative, we introduce a relaxed, affine invariance property for the metrics and gradient flows. In particular, we construct various affine invariant Wasserstein and Stein gradient flows. Affine invariant gradient flows are shown to behave more favorably than their non-affine-invariant counterparts when sampling highly anisotropic distributions, in theory and by using particle methods. Our third contribution is to study, and develop efficient algorithms based on Gaussian approximations of the gradient flows; this leads to an alternative to particle methods. We establish connections between various Gaussian approximate gradient flows, discuss their relation to gradient methods arising from parametric variational inference, and study their convergence properties both theoretically and numerically.

本文研究了基于梯度流的采样方法的设计要素，主要包括能量函数、度量、和用于算法推导的梯度流的数值近似。首先，我们展示了Kullback-Leibler散度作为能量函数的独特性质，即由它引导的梯度流与目标分布的标准化常数无关。其次，我们从不变性的角度研究了度量的选择，引入了一种放松的仿射不变性，构建了各种仿射不变的Wasserstein和Stein梯度流。最后，基于高斯近似的梯度流方法被提出，并与参数变分推断衍生的梯度方法建立了联系，理论和数值上研究了它们的收敛性。

概率测度空间中的梯度流采样