If we have a random unit vector on $\mathbb{C}^n$, drawn from the Fubini-Study metric, the marginal distribution of the squared absolute values of each of the coefficients in the vector is given by a Beta distribution with shape parameters $\alpha=1$, $\beta=n-1$. (See for example Zyczkowski and Sommers).

However, this doesn't immediately translate to a statement about the order statistics, since the variables are not independent (they are constrained to sum to 1, for example).

What is the distribution of the largest magnitude of the coefficients in the vector, that is, the first order statistic?


This problem was solved in Extreme statistics of complex random and quantum chaotic states (2007). The first two moments of the maximum absolute value of the elements of the random unit vector, $t=\max(|z_1|^2,|z_2|^2,\ldots |z_n|^2)$, are $$\langle t\rangle=\frac{H(n,1)}{n},\;\;\langle t^2\rangle=\frac{H^2(n,1)+H(n,2)}{n(n+1)}$$ where $H(n,k)$ is the $k$-th harmonic number. The cumulative density $F(t)$ in the large-$N$ limit is $$F(t)\rightarrow (1-e^{-nt})^n\approx\exp\left(-e^{-nt+\ln n}\right),\;\;\text{for}\;\;n\rightarrow\infty,$$ which is a Gumbel distribution.

  • 1
    $\begingroup$ Thank you very much for the reference! For clarity in case anyone else comes across it, the Gumbel distribution is true only in the limit of dimension tending to infinity, but the authors do provide an explicit CDF for the finite dimensional case. $\endgroup$ – A Simmons Apr 26 '18 at 9:11

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.