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arXiv 2610.18992math.PRmath.STstat.TH
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无限可交换序列的刚性

On rigidity of infinite exchangeable sequences

University of Chicago(芝加哥大学)

Maximillian Newman

AI总结:

该研究探究无限可交换序列的概率分布由有限维边缘唯一确定的条件,通过概率核混合、指数核等情形推导最优坐标阈值,改进Freedman定理并确定ℓ_q-范数指数律的唯一性阈值。

摘要:

我们探究无限可交换序列的概率分布何时由其有限维边缘唯一确定。对于概率核的混合,弱局部化条件使得三个坐标即可确定其导向律;仅完备性条件就使得四个坐标足够,且一般而言这两个下界均无法降低。对于带有非负充分统计量的指数核,Müntz-Szász支撑条件意味着三个坐标足够,在标量情形下该结果是最优的。我们还通过加权Müntz密度条件刻画了完备性何时对所有标量混合律同时成立,并表明仅统计量支撑无法决定完备性。这些方法得到了关于可旋转序列的Freedman定理的最优有限维改进,即三个坐标的球对称性可使无限可交换序列成为可旋转的,而前两个坐标的对称性一般而言是不够的。最后,对于能量径向密度,我们给出了无限可扩展性的完全单调性准则,并确定了ℓ_q-范数指数律的精确唯一性阈值。

英文摘要:

We ask when the law of an infinite exchangeable sequence is uniquely determined by a finite-dimensional marginal. For mixtures of a probability kernel, a weak localization condition makes three coordinates determine the directing law. Completeness alone makes four coordinates sufficient. Neither order can be lowered in general. For exponential kernels with a nonnegative sufficient statistic, a Müntz-Szász support condition implies three coordinates suffice. In the scalar case, this result is sharp. We also characterize when completeness holds simultaneously for all scalar mixing laws by a weighted Müntz density condition and show that statistic support alone need not decide completeness. These methods yield sharp finite-dimensional refinements of Freedman's theorem on rotatable sequences, namely that spherical symmetry of three coordinates makes an infinite exchangeable sequence rotatable while symmetry of the first two coordinates is in general insufficient. Finally, for energy-radial densities we give a complete-monotonicity criterion for infinite extendibility and determine the exact uniqueness thresholds for $\ell_q$-norm exponential laws.

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