= Normally ordered density correlation
{title2=$C^{(n)}=\langle\psi_1^\dagger\cdots\psi_n^\dagger\psi_n\cdots\psi_1\rangle$}
This correlation counts distinct detection events without the delta-function self-counting term in an un-ordered product of densities. It is nonnegative as the expectation of $A^\dagger A$ with $A=\psi_n\cdots\psi_1$. For a single occupied bosonic mode with sharp occupation $N$, the coincident $n$-point value contains the <falling factorial> $N(N-1)\cdots(N-n+1)$, rather than $N^n$.
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