https://ja.wikipedia.org/wiki/%E5%86%AA%E4%B9%97%E5%89%87 冪乗則 https://en.wikipedia.org/wiki/Power_law Power law (抜粋) Lack of well-defined average value A power-law x^(-a) has a well-defined mean over x ∈ [ 1 , ∞ ] only if a > 2 , and it has a finite variance only if a > 3 ; most identified power laws in nature have exponents such that the mean is well-defined but the variance is not, implying they are capable of black swan behavior.[8]
