arXiv:2604.18242math.STcs.LG2026-04

在哈达玛流形上定义了基于水平面深度的贝塞尔中位数,实现无基点、等距保持的统计分析。

Horospherical Depth and Busemann Median on Hadamard Manifolds

  • 用双曲极限距离函数构造水平面深度,替代传统半空间深度。
  • 证明深度区域嵌套且测地凸,中位数对任意概率分布存在且唯一(负曲率下)。
  • 对异常值鲁棒,样本深度一致收敛,适合非欧几何统计建模者使用。

我们引入了哈达玛流形上的内在统计深度——水平面深度,并将贝塞尔中位数定义为该深度的最大化集合。其构造基于一个事实:图基半空间深度中的线性泛函是归一化距离函数的极限;在哈达玛流形上,同样的极限过程产生贝塞尔函数,其下水平集即为水平球,是半空间的内在对应物。所得到的深度依赖于视觉边界参数,具有等距不变性,无需切空间线性化或指定基点。对于任意哈达玛流形,我们证明深度区域嵌套且测地凸,存在深度至少为 $1/(d+1)$ 的中心点,因此贝塞尔中位数对每个博雷尔概率测度均存在。在严格负截面曲率与温和正则性假设下,深度严格拟凹,中位数唯一。我们还建立了鲁棒性:深度在总变差扰动下稳定,且当污染质量逃逸至无穷时,极限中位数仅取决于逃逸方向,不随污染沿测地射线移动距离而改变,这与弗雷歇均值不同。最后,我们证明了样本深度的统一一致性,以及样本深度区域和样本贝塞尔中位数的收敛性;在非紧对称空间上,通过上水平球半空间的VC分析完成论证,而在一般哈达玛流形上,则基于紧凑性论证,前提是温和的非原子性假设。

原文摘要 · Abstract (English)

\We introduce the horospherical depth, an intrinsic notion of statistical depth on Hadamard manifolds, and define the Busemann median as the set of its maximizers. The construction exploits the fact that the linear functionals appearing in Tukey's half-space depth are themselves limits of renormalized distance functions; on a Hadamard manifold the same limiting procedure produces Busemann functions, whose sublevel sets are horoballs, the intrinsic replacements for halfspaces. The resulting depth is parametrized by the visual boundary, is isometry-equivariant, and requires neither tangent-space linearization nor a chosen base point. For arbitrary Hadamard manifolds, we prove that the depth regions are nested and geodesically convex, that a centerpoint of depth at least $1/(d+1)$ exists, and hence that the Busemann median exists for every Borel probability measure. Under strictly negative sectional curvature and mild regularity assumptions, the depth is strictly quasi-concave and the median is unique. We also establish robustness: the depth is stable under total-variation perturbations, and under contamination escaping to infinity the limiting median depends on the escape direction but not on how far the contaminating mass has moved along the geodesic ray, in contrast with the Fréchet mean. Finally, we establish uniform consistency of the sample depth and convergence of sample depth regions and sample Busemann medians; on symmetric spaces of noncompact type, the argument proceeds through a VC analysis of upper horospherical halfspaces, while on general Hadamard manifolds it follows from a compactness argument under a mild non-atomicity assumption.

统计深度哈达玛流形中位数几何统计

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