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Lemma 23.1.33264893FAE204

Lemma 23.1.33.264893FAE204 The positivity criterion (23.1.8) holds in every dimension $n\geq 3$, and

D216F8E7C5AA 23.1.34
\begin{equation} J_ n(V;x,w)=\frac{n(n-1)}6 (\mu +V_ x)^{n-3}J_3(V;x,w). \end{equation}

Here $J_3$ is evaluated for the same function $V$ on the threefold.

Proof. Choose $n-2$ horizontal angular vectors, one horizontal radial vector and the vertical vector $\partial _ w$. In the normalization of the proof of lemma 23.1.7, each angular entry is $A=(\mu +V_ x)/\mu $, and the radial–vertical block is

\[ K_ V=\begin{pmatrix} (\mu +V_{xx})/[\mu (1-\mu )] & (\mu +V_{x\bar w})/\sqrt{\mu (1-\mu )} \\ (\mu +V_{wx})/\sqrt{\mu (1-\mu )} & 1+V_{w\bar w} \end{pmatrix}. \]

This follows by differentiating $s=w+\bar w+\log (1+\mathrm e^ x)$: the radial part of $\partial \bar\partial x$ is zero, whereas its angular part contributes $V_ x/\mu $. The Schur complement of the last entry gives the stated positivity criterion. At $\xi =0$ the same endpoint argument as in dimension three applies.

Set $d=n-1$. The radial Fubini–Study probability density is $d\mu ^{d-1}\mathrm d\mu =d\mu ^ d(1-\mu )\mathrm dx$; it follows from the polar-coordinate determinant of $\mathrm{dd}^{\mathrm{c}}\log (1+|\xi |^2)$ and its integral is one. The background block $K_0$ has determinant one. Using the fiber mass two, therefore,

\[ J_ n(V)=2nd\mu ^ d(1-\mu )A^{n-2}\det K_ V. \]

For $n=3$ this is $12\mu ^2(1-\mu )A\det K_ V$. Comparing the two expressions proves (23.1.34). $\square$

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