ScalingStacks

3.1. Measures with density [02DQ]

Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.

Complete original source context · Original author HTML

3.1. Measures with density

We now turn to the study of the complex Monge-Ampère equation

(ω+d​dc​φ)n=μ, when ​μ=f​ωn(\omega+dd^{c}\varphi)^{n}=\mu,\text{ when }\mu=f\omega^{n}

is a measure with density 0≤f∈Lp​(ωn)0\leq f\in L^{p}(\omega^{n}), p>1p>1.

Proposition 3.1.

Assume μ=f​ωn\mu=f\omega^{n} is a probability measure with density 0≤f∈Lp​(X)0\leq f\in L^{p}(X), for some p>1p>1. Then for any α>0\alpha>0, there exists Aα>0A_{\alpha}>0 such that μ\mu satisfies ℋ⁡(α,Aα,ω){\mathcal{H}}(\alpha,A_{\alpha},\omega).

Proof.

It is enough to establish ℋ⁡(α,Aα,ω){\mathcal{H}}(\alpha,A_{\alpha},\omega) for compact subsets, by regularity of μ\mu and C​a​pωCap_{\omega}. Let KK be a compact subset of XX. It follows from Hölder’s inequality that

0≤μ⁡(K)≤‖f‖Lp​(ωn)​[Volω​(K)]1/q,0\leq\mu(K)\leq||f||_{L^{p}(\omega^{n})}\left[\text{Vol}_{\omega}(K)\right]^{1/q},

where 1/p+1/q=11/p+1/q=1. Note that ‖f‖Lp​(ωn)=1||f||_{L^{p}(\omega^{n})}=1 since we assume μ\mu is a probability measure. We claim that

(4) Volω(K)≤Cωexp[−(Capω(K))−γω/n],\text{Vol}_{\omega}(K)\leq C_{\omega}\exp\left[-(Cap_{\omega}(K))^{-\gamma_{\omega}/n}\right],

for some constants Cω,γω>0C_{\omega},\gamma_{\omega}>0 that only depend on ω\omega. We will be done if we can prove (4) since we can then check by elementary computations that exp⁡(−x−δ)\exp(-x^{-\delta}) is dominated from above by Aα​xαA_{\alpha}x^{\alpha}, for all x∈[0,1]x\in[0,1].

The set of functions ℱ0:={φ∈PSH(X,ω)/supXφ=0}{\mathcal{F}}_{0}:=\{\varphi\in PSH(X,\omega)\,/\,\sup_{X}\varphi=0\} is compact in L1​(X)L^{1}(X) (see proposition 2.7, [GZ 1]). These functions have Lelong numbers ν⁡(φ,x)≤νω\nu(\varphi,x)\leq\nu_{\omega} bounded from above by a uniform constant. It follows therefore from Skoda’s uniform integrability theorem [Z], that

supφ∈ℱ0∫exp⁡[−2​φνω+1]​ωn≤C2<+∞.\sup_{\varphi\in{\mathcal{F}}_{0}}\int\exp\left[-\frac{2\varphi}{\nu_{\omega}+1}\right]\omega^{n}\leq C_{2}<+\infty.

Set γω:=2/(νω+1)>0\gamma_{\omega}:=2/(\nu_{\omega}+1)>0 and let

VK,ω∗(x):=(sup{φ(x)/φ∈PSH(X,ω),φ≤0 on K})∗V_{K,\omega}^{*}(x):=\left(\sup\{\varphi(x)\,/\,\varphi\in PSH(X,\omega),\,\varphi\leq 0\text{ on }K\}\right)^{*}

denote the Siciak extremal function of KK (see section 5.1 in [GZ 1]). Then

V​o​lω​(K)≤∫Xexp⁡(−γω​VK,ω∗)​ωn≤C2​Tω​(K)γω,Vol_{\omega}(K)\leq\int_{X}\exp\left(-\gamma_{\omega}V_{K,\omega}^{*}\right)\omega^{n}\leq C_{2}T_{\omega}(K)^{\gamma_{\omega}},

where Tω(K):=exp(−supXVK,ω∗)T_{\omega}(K):=\exp(-\sup_{X}V_{K,\omega}^{*}) denote the Alexander capacity of KK (see section 5.2 in [GZ 2]). It follows now from theorem 7.1 in [GZ 1] that

Tω(K)≤eexp[−Capω(K)−1/n],T_{\omega}(K)\leq e\exp\left[-Cap_{\omega}(K)^{-1/n}\right],

which yields (4). ∎

It follows therefore from theorem 2.1 that there exists a unique continuous function φ∈P​S​H​(X,ω)\varphi\in PSH(X,\omega) such that

μ=f​ωn=(ω+d​dc​φ)n, with ​supXφ=−1,\mu=f\omega^{n}=(\omega+dd^{c}\varphi)^{n},\text{ with }\sup_{X}\varphi=-1,

when 0≤f∈Lp​(ωn)0\leq f\in L^{p}(\omega^{n}), p>1p>1, with ∫Xf​ωn=1\int_{X}f\omega^{n}=1.

Actually we will be interested in measures with LpL^{p}-density with respect to a positive definite volume form d​λd\lambda, while the smooth measure ωn\omega^{n} may vanish along a divisor. This does not make much difference, as follows from Hölder’s inequality:

Lemma 3.2.

Let VV be a nn-dimensional compact normal Kähler space and Ω\Omega be a smooth Kähler form on VV. Let π:X→V\pi:X\to V a resolution, ω=π∗​Ω\omega=\pi^{*}\Omega, and let d​λd\lambda be a positive definite smooth volume form on XX.

If μ=f1​d​λ\mu=f_{1}d\lambda, with f1∈Lp​(X,d​λ)f_{1}\in L^{p}(X,d\lambda) for some p>1p>1, then there exists p′>1p^{\prime}>1 such that μ=f​ωn\mu=f\omega^{n} and f∈Lp′​(X,ωn)f\in L^{p^{\prime}}(X,\omega^{n}).

Proof.

Observe that ωn=E​d​λ\omega^{n}=Ed\lambda for some smooth density E≥0E\geq 0 which vanishes along the exceptional divisor of π\pi, thus

μ=f1​d​λ=f​ωn, where ​f=f1/E.\mu=f_{1}d\lambda=f\omega^{n},\text{ where }f=f_{1}/E.

Fix local coordinates (zi)1≤i≤n(z^{i})_{1\leq i\leq n} on a polydisk 𝔻⊂X\mathbb{D}\subset X and a local embedding F:V→ℂmF:V\to\mathbb{C}^{m}. Note that EE is comparable to |∂F∂z1∧…∧∂F∂z1|2≃∑i=1r|fi|2|\frac{\partial F}{\partial z^{1}}\wedge\ldots\wedge\frac{\partial F}{\partial z^{1}}|^{2}\simeq\sum_{i=1}^{r}|f_{i}|^{2}, fif_{i} being holomorphic on 𝔻\mathbb{D}. Therefore E∈Ll​o​c∞​(𝔻)E\in L^{\infty}_{loc}(\mathbb{D}) and E−α∈Ll​o​c1​(𝔻,d​λ)E^{-\alpha}\in L_{loc}^{1}(\mathbb{D},d\lambda) for some 0<α<10<\alpha<1.

Choose 0<α′<α0<\alpha^{\prime}<\alpha such that 1p+1α=1α′\frac{1}{p}+\frac{1}{\alpha}=\frac{1}{\alpha^{\prime}}. Then fα′=f1α′​E−α′f^{\alpha^{\prime}}=f_{1}^{\alpha^{\prime}}E^{-\alpha^{\prime}} is the product of a function in Lp/α′​(d​λ)L^{p/\alpha^{\prime}}(d\lambda) and a function in Lα/α′​(d​λ)L^{\alpha/\alpha^{\prime}}(d\lambda), hence it is in Ll​o​c1​(𝔻,d​λ)L^{1}_{loc}(\mathbb{D},d\lambda) by Hölder’s inequality. A second application of Hölder’s inequality yields

∫𝔻f1+ε​ωn=∫𝔻fϵ​f1​𝑑λ≤(∫𝔻fϵ​q​𝑑λ)1/q​(∫𝔻f1p​𝑑λ)1/p<+∞,\int_{{\mathbb{D}}}f^{1+\varepsilon}\omega^{n}=\int_{\mathbb{D}}f^{\epsilon}f_{1}d\lambda\leq\left(\int_{\mathbb{D}}f^{\epsilon q}d\lambda\right)^{1/q}\left(\int_{\mathbb{D}}f_{1}^{p}d\lambda\right)^{1/p}<+\infty,

where qq denotes the conjugate exponent to pp. This shows that f∈Lp′​(ωn)f\in L^{p^{\prime}}(\omega^{n}) if p′=1+ε>1p^{\prime}=1+\varepsilon>1 is chosen so small that ε​q<α′\varepsilon q<\alpha^{\prime}. ∎

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.