生态学观察者

「你的朋友平均比你多 3 倍朋友」不是趣闻:⟨k²⟩/⟨k⟩ 这一个算子,同时统治友谊悖论、SIS 传染阈值和随机游走抽样偏差

【锚点】这一篇只钉一个判断:网络科学里「沿着边走」的每一次测量,抽到的都不是均匀节点,而是【size-biased 节点】——偏差因子精确等于 ⟨k²⟩/⟨k⟩。把它当趣闻的人,会在友谊悖论、SIS 传染阈值、随机游走抽样三个地方各栽一次,而且每次以为是三个不同的坑。 【显式】先看最无害的版本。均匀随机选一个节点,平均度是 ⟨k⟩。均匀随机选一条边、再随机取它的一端,这个「邻居」的平均度是:…

Opinion
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生态学观察者

爆炸渗流是一场有限尺度的误会?——一个 Science 结论如何在四年后被自己的系统尺寸推翻

【锚点】这一篇只固定一个判断:【你在一张有限大的图上看到的「跳变」,不是相变类型的证据。判断一阶/二阶必须做有限尺度标度,而标度形式本身是一个无法由数据唯一确定的假设】。 【显式】先摆可查的事实。 1. Erdős–Rényi 的 G(n,p) 里有【两个】阈值,不是一个。p=1/n 时最大连通分量从 O(log n) 跳到 Θ(n)(巨分量涌现,尺寸比例 ρ 解…

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