用新算法让生成模型更准捕捉极端事件,无需知道分布尾巴形状。
Fine-Tuning Generative Models for Extreme Events via CVaR-Penalized Wasserstein Gradient Flows

- 基于CVaR惩罚的Wasserstein梯度流,让生成模型能学重尾分布。
- 在合成与真实数据上,尾部和全局准确率显著优于预训练基线。
- 不依赖模型结构,自动调节训练时间,适合金融等高风险场景。
我们提出一种鲁棒、尾部无关的生成粒子算法(CVaR-GPA),用于微调生成模型以学习重尾分布并捕捉极端事件,无需事先了解或估计目标分布的尾部特征。该方法是带有条件风险价值(CVaR)惩罚项的Lipschitz正则化KL散度的Wasserstein梯度流:Lipschitz正则化KL散度使模型在对目标分布假设极少的情况下仍能稳健学习,而CVaR惩罚项恢复了在欠采样尾部区域本会提前消失的速度。该惩罚流具有有界但非Lipschitz的速度场,不同于标准生成器的Lipschitz传输映射,后者会保留轻尾源分布的尾部特性,而此方法可实现向更重尾目标的传输。为在经验测度上定义该流,我们从Rockafellar-Uryasev表示中推导出CVaR的一阶变分次梯度,其有效性恰在传统密度公式失效处。CVaR-GPA可微调任意预训练模型的输出样本,无需访问其架构,并基于动能停止准则自适应设定训练时长,而非固定深度。在合成的各向同性与各向异性Student-t分布、Neal's funnel分布以及真实高维Fama-French 25投资组合数据集上,相比预训练基线,CVaR-GPA在重尾目标上的全局和尾部准确率均显著提升。
原文摘要 · Abstract (English)
We propose CVaR-penalized Generative Particle Algorithm (CVaR-GPA), a robust, tail-agnostic algorithm for fine-tuning generative models to learn heavy-tailed distributions and capture extreme events, requiring no prior knowledge or estimation of the target's tail characteristics. The method is the Wasserstein gradient flow of the Lipschitz-regularized Kullback-Leibler (KL) divergence penalized by a Conditional Value-at-Risk (CVaR) discrepancy term: the Lipschitz-regularized KL divergence enables robust learning under minimal assumptions on the target distribution, while the CVaR penalty restores the velocity that otherwise vanishes prematurely in the under-sampled tails. The penalized flow admits a bounded but non-Lipschitz velocity field. This departs from the Lipschitz transport maps of standard generators, which preserve the tail behavior of a light-tailed source, and enables transport toward heavier-tailed targets. To define this flow on empirical measures, we derive the first-variation subgradients of CVaR from its Rockafellar-Uryasev representation, valid precisely where the classical density-based formula fails. The particle algorithm CVaR-GPA fine-tunes the output samples of any pre-trained model, without access to its architecture, and runs on an adaptive time horizon set by a kinetic-energy stopping criterion rather than a preset depth. On synthetic isotropic and anisotropic Student-$t$ target distributions, Neal's funnel distribution, and the real-world high-dimensional Fama-French 25 portfolio dataset, CVaR-GPA dramatically improves global and tail accuracy on heavy-tailed targets over the pre-trained baseline.
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