Abstract
Based on Harnack’s inequality and convex analysis we show that each plurisubharmonic function is locally BUO (bounded upper oscillation) with respect to polydiscs of finite type but not for arbitrary polydiscs. We also show that each function in the Lelong class is globally BUO with respect to all polydiscs. A dimension-free BUO estimate is obtained for the logarithm of the modulus of a complex polynomial. As an application we obtain an approximation formula for the Bergman kernel that preserves all directional Lelong numbers. For smooth plurisubharmonic functions we derive a new asymptotic identity for the Bergman kernel from Berndtsson’s complex Brunn–Minkowski theory, which also yields a slightly better version of the sharp Ohsawa–Takegoshi extension theorem in some special cases.
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1 Introduction
Let \(\Omega \) be a domain in \({\mathbb {C}}^n\) and \(PSH(\Omega )\) the set of plurisubharmonic (psh) functions on \(\Omega \). Recall that each \(\phi \in PSH(\Omega )\) satisfies the following mean-value inequality:
whenever S is a ball or a polydisc, with center z. Here |S| denotes the Lebesgue measure of S and \(\int _S\) means the Lebesgue integral. The above inequality implies \(\phi \in L^1_\mathrm{loc}(\Omega )\) and suggests to estimate the difference \(|\phi -\phi _S|\). The concept of BMO functions then enters naturally. Let \({{\mathcal {S}}}={{\mathcal {S}}}(\Omega )\) be a family of relatively compact open subsets in \(\Omega \). We say that \(\phi \in L^1_\mathrm{loc}(\Omega )\) has bounded mean oscillation (BMO) with respect to \({{\mathcal {S}}}\) if
Let \(BMO(\Omega ,{{\mathcal {S}}})\) denote the set of functions which are BMO with respect to \({{\mathcal {S}}}\). When \({{\mathcal {S}}}\) is the set of balls in \(\Omega \), this is the original definition of BMO functions due to John–Nirenberg [13]. A classical example of BMO functions is \(\log |z|\). It is also convenient to introduce local BMO functions as follows. For an open set \(\Omega _0\subset \subset \Omega \) we define \({{\mathcal {S}}}|_{\Omega _0}\) to be the sets of all \(S\in {{\mathcal {S}}}\) which are relatively compact in \(\Omega _0\). Let \(BMO_\mathrm{loc}(\Omega ,{{\mathcal {S}}})\) be the set of functions on \(\Omega \) which belong to \(BMO(\Omega _0,{{\mathcal {S}}}|_{\Omega _0})\) for every open set \(\Omega _0\subset \subset \Omega \).
By using pluripotential theory, Brudnyi [6] was able to show that each psh function is locally BMO with respect to balls (see also [7] for stronger results concerning subharmonic functions in the plane). Recently, the first author found another approach to local BMO properties of psh functions by using the Riesz decomposition theorem and some basic facts of psh functions (cf. [9]). Benelkourchi et al. [1] showed that every function in the Lelong class \({{\mathcal {L}}}\) is globally BMO with respect to balls. Recall that
In this paper we propose a new and simpler approach based on the following basic observation:
It is easier to look at the upper oscillation instead of the mean oscillation for psh functions.
To define the upper oscillation one simply uses \(\sup _S \phi \) instead of \(\phi _S\):
Note that \(-UO_S(-\phi )\) is exactly the lower oscillation introduced by Coiffman–Rochberg (cf. [10], see also [16] for further properties). Since
we see that bounded upper oscillation (BUO) implies BMO. One may define \(BUO(\Omega ,{{\mathcal {S}}})\) and \(BUO_\mathrm{loc}(\Omega ,{{\mathcal {S}}})\) analogously as the case of BMO.
Let \({{\mathcal {P}}}={{\mathcal {P}}}(\Omega )\) denote the set of relatively compact polydiscs in \(\Omega \) and \({{\mathcal {P}}}_N\) the set of polydiscs \(P\subset \subset \Omega \) of finite type N, i.e.,
where \(N>0\) and \(\{r_j\}_{1\le j\le n}\) is the polyradius of P.
Based on Harnack’s inequality and convex analysis, we are able to show the following
Theorem 1.1
-
(1)
\(PSH(\Omega )\subset BUO_\mathrm{loc}(\Omega ,{{\mathcal {P}}}_N) \subset BMO_\mathrm{loc}(\Omega ,{{\mathcal {P}}}_N)\);
-
(2)
\(PSH({\mathbb {D}}^n)\nsubseteq BMO_\mathrm{loc}({\mathbb {D}}^n,{{\mathcal {P}}})\) for \(n\ge 2\), where \({\mathbb {D}}^n\) is the unit polydisc;
-
(3)
\({{\mathcal {L}}}\subset BUO({\mathbb {C}}^n, {\mathcal {P}})\); more precisely, for every \(\phi \in PSH({\mathbb {C}}^n)\) with
$$\begin{aligned} \phi (z_1,\ldots , z_n) \le c+\max _{1\le j\le n} \log (1+|z_j|), \ \ \forall \ (z_1, \ldots , z_n)\in {\mathbb {C}}^n, \end{aligned}$$where c is a constant, we have \( UO_P(\phi ) < 3^n \) for all polydiscs P in \({\mathbb {C}}^n\).
For \((\mathrm{deg\,}p)^{-1}\log |p|\in {\mathcal {L}}\) where p is a complex polynomial, we even obtain a dimension-free BUO estimate with respect to all compact convex sets.
Theorem 1.2
For every non-empty compact convex set A in \({\mathbb {C}}^n\), we have
for all \(p\in {\mathbb {C}}[z_1, \ldots , z_n]\). Here the constant \(\gamma \in (1,2)\) is determined by
Remark
-
(i)
The above estimate is sharp, in fact, there exists a line segment A in \({\mathbb {C}}\) such that
$$\begin{aligned} UO_A(\log |z|)=\gamma . \end{aligned}$$ -
(ii)
In particular, if A is a compact convex set in \({\mathbb {R}}^n \subset {\mathbb {C}}^n\) and all coefficients of p are real, then we have
$$\begin{aligned} UO_A (\log |p|) \le \gamma \cdot \deg p <2\deg p, \end{aligned}$$which is closely related the classical Remez inequality for real polynomials. Theorem 1.2 also suggests to study the Remez inequality for complex polynomials (see [1] and [8] for related results).
-
(iii)
Notice that \( 1.278<\gamma <1.279. \) By (1.2) we have
$$\begin{aligned} MO_A (\log |p|) \le 2\gamma \cdot \deg p< 2.558\cdot \deg p. \end{aligned}$$Such dimension-free estimate (with a slightly better constant \(2+\log 2\approx 2.301\)) was first obtained by Nazarov et al. [15]. Our proof of Theorem 1.2 is elementary, however.
For \(\phi \in PSH(\Omega )\) we define the (weighted) Bergman kernel by
For a vector \(a=(a_1,\ldots ,a_n)\) with all \(a_j>0\) we set
It was shown in [9] that if \(\phi \) is psh on the closure of the unit ball \({\mathbb {B}}^n\) and \(a_0=(1,1/2,\ldots ,1/2)\) then
provided \(\varepsilon \ll 1\), where \(1+z=(1+z_1,z_2,\ldots ,z_n)\). The limit in RHS of the above inequality is called the \(a_0-\)directional Lelong number of \(\phi \) at \((1,0,\ldots ,0)\) (see [14]).
Here we will present an analogous but independent result, as an application of Theorem 1.1. For \(\phi \in PSH({\mathbb D}^n)\) and \(t\in {\mathbb {D}}^n\) we define
A fundamental result of Berndtsson [2] implies that
is psh on \({\mathbb {D}}^n \times {\mathbb {D}}^n\).
Theorem 1.3
For each \(a=(a_1,\ldots ,a_n)\) with all \(a_j>0\), there exists a number \(\varepsilon _0=\varepsilon _0(a,\phi ,\Omega )\) such that
holds for all \(\varepsilon \le \varepsilon _0\).
Although Theorem 1.3 makes sense only when \(\phi \) is singular at the origin, it is of independent interest to study the relation between \(F(\phi )\) and \(\phi \) for smooth \(\phi \).
Theorem 1.4
Let \(\phi \) be a smooth psh function on \({\mathbb {D}}^n\). Then
In particular \(F(\phi )(t,0)\) is strictly psh at \(t=0\) if \(\phi \) is strictly psh at \(z=0\).
Remark
Since \(F(\phi )(t,0)\) depends only on \((|t_1|,\ldots , |t_n|)\), it follows from the psh property of \(F(\phi )\) that
Letting t tend to \((1,\ldots , 1)\), we obtain the sharp Ohsawa–Takegoshi estimate (cf. [5]; see also [4, 12]):
Theorem 1.4 suggests that one should have a better lower bound for \(K_{\phi , \,{\mathbb {D}}^n}\) in case \(\phi \) is strictly psh.
2 An enlightening example
To explain why BUO is easier than BMO, we will show that the upper oscillation of \(\log |z|\) with respect to discs is computable. Recall that
for every disc B in \({\mathbb {C}}\).
Lemma 2.0.1
Fix \(\hat{z}\in {\mathbb {C}}\) and set
Then we have
Proof
If \(c\le |{\hat{z}}|\) then \(\log |z|\) is harmonic in the disc \(\{z:|z-{\hat{z}}| < c\}\), so that \(I(c)=\log |{\hat{z}}|\), in view of the mean-value equality. For \(c>|{\hat{z}}|\) we may write
As \(\log |z|\) is harmonic in \(\{z:|z-c|< |{\hat{z}}|\}\), we get \(I(c)=\log c\). \(\square \)
Proposition 2.0.1
For any disc B we have
Moreover, the bound is sharp.
Proof
Suppose \(B=\{z:|z-{\hat{z}}|<b\}\). By Lemma 2.0.1 we have
and if \(b>|{\hat{z}}| \) then
It follows that
If \(b\le |{\hat{z}}|\) then
For \(b> |{\hat{z}}|\) we set \(x=|{\hat{z}}|/b\) and write \(UO_{B} (\log |z|)\) as
Since
we see that f is increasing on \([0, {\hat{x}}]\) and decreasing on \([{\hat{x}}, 1]\), where \( {\hat{x}}=\frac{\sqrt{5}-1}{2}. \) Notice that
Thus
and the equality holds if and only if
This finishes the proof. \(\square \)
3 Proof of Theorem 1.1
3.1 One dimensional case
Let \(\Omega \) be a domain in \({\mathbb {C}}\) and \(\phi \) a subharmonic function on \(\Omega \). Recall that
where \( B=\{z:|z-{\hat{z}}| <r\}\subset \Omega . \) The idea is to use Harnack’s inequality and a convexity lemma. Let us write
where
with \(\phi _{\partial B}\) being the mean-value of \(\phi \) over the boundary \(\partial B\). For each \(\tau >0\) we set
Applying Harnack’s inequality to the nonpositive subharmonic function \(\psi :=\phi -\sup _{B} \phi \), we get
i.e.,
Here the constant 1/3 comes from the Poisson kernel of the unit disc since
The following fact explains why we need such an estimate.
Fact 1 \( J_1:=\sup _{B} \phi - \sup _{\frac{1}{2} B} \phi \) is continuous in \({\hat{z}}\) and r respectively; moreover, it is increasing with respect to r.
Proof
Since \(\sup _{B} \phi \) is a convex function of \(\log r\) (see [11, Corollary 5.14]), it follows that \(J_1\) is a continuous increasing function of r. The continuity of \(J_1\) in \(\hat{z}\) is obvious. \(\square \)
Let \(\Omega _0\) be a relatively compact open subset in \(\Omega \). Let \(\delta _0\) denote the distance between \(\overline{\Omega _0}\) and \(\partial \Omega \). By the above fact we see that if the radius r of \(B\subset \Omega _0\) is less than \(\delta _0/2\) then
and if \(r\ge \delta _0/2\) then
To estimate \(I_2\), we need the following convexity lemma which was communicated to the second author by Bo Berndtsson:
Lemma 3.0.2
Let \(d\mu \) be a probability measure on a Borel measurable subset S in \({\mathbb {R}}^n\) with barycenter \({\hat{t}} \in {\mathbb {R}}^n\). Let f be a convex function on \({\mathbb {R}}^n\). Then
Proof
Since f is convex, there exists an affine function l such that \(f({\hat{t}})=l({\hat{t}})\) and \( f\ge l\) on \({\mathbb {R}}^n\), which implies
where the first equality follows from the definition of barycenter. \(\square \)
With \( f(t):=\phi _{\{z:|z-{\hat{z}}|=e^t r\}} \) we have
Since f(t) is convex and \(d (e^{2t})\) is a probability measure on \((-\infty , 0)\) with barycenter at \(t=-1/2\), it follows from Lemma 3.0.2 that
which implies
Since f is convex, we get an analogous conclusion as Fact 1:
Fact 2 \( J_2 \) is continuous in \({\hat{z}}\) and r respectively; moreover, it is increasing with respect to r.
By a similar argument as above, we may verify that
3.2 High dimensional case
The following result plays the role of Fact 1, 2.
Lemma 3.0.3
Let \(g(t)=g(t_1, \ldots , t_n)\) be a convex function on \((-\infty , 2)^n\) which is increasing in each variable. Then
where \(t-1:=(t_1-1, \ldots , t_n-1)\), \(N\ge 1\) and
Proof
A standard regularization process reduces to the case when g is smooth. Set
We have
where \(g_j:=\frac{\partial g}{\partial t_j}\). Notice that
and
is an increasing function of \(s\in (-\infty , 0)\) by convexity of g. Thus we have
which implies
For any \(t\in A_N\), we have \(t+a \in A_N\) (since \(a\le 0\)), so that
Thus
Since g is convex and increasing, we have
which finishes the proof. \(\square \)
Let
be a polydisc of type N, i.e.,
Similar as above, we write
where
and
is the Shilov boundary of P. Applying Harnack’s inequality (see [14, p. 186]) n-times, we get the following
Lemma 3.0.4
\(I_1\le 3^n J_1\), where \(J_1:=\sup _{P} \phi -\sup _{\frac{1}{2} P} \phi \).
Using (3.1) repeatedly we get
Lemma 3.0.5
\(I_2\le J_2\), where \(J_2:=f(0)-f(-1/2, \ldots , -1/2)\) with
Since both \(\sup _{P} \phi \) and \(\phi _{\partial P}\) are continuous in \({\hat{z}}_j\) and convex increasing with respect to \(\log r_j\) for all j, it follows from Lemma 3.0.3 (through a similar argument as the one-dimensional case) that
for every open set \(\Omega _0\subset \subset \Omega \), which finishes the proof of the first part of Theorem 1.1.
3.3 A counterexample
For the second part of Theorem 1.1, we need to construct a counterexample. For the sake of simplicity, we only consider the case \(n=2\). It suffices to verify the following
Theorem 3.1
Set \( \phi (z, w):=-\sqrt{(\log |z|+\log |w|)\log |w|}, \) \(z,w\in {\mathbb {D}}\). Then we have \(\phi \in PSH({\mathbb {D}}^2)\), while
where
The following lemma shows that Fact 1, 2 is no more true for general bidiscs.
Lemma 3.1.1
\(f(x,y):=-\sqrt{(x+y)y}\) is convex on \( (-\infty , 0)^2\) and increasing in each variable; moreover,
Proof
The first conclusion follows by a straightforward calculation. For (3.2) it suffices to note that
as \(x\rightarrow -\infty \). The proof is complete. \(\square \)
Let us first verify that \(\phi \notin BUO_\mathrm{loc}({\mathbb {D}}^2, {\mathcal {P}})\).
Lemma 3.1.2
\(\sup _{0<r_1, r_2<1} \sup _{{\mathbb {D}}^2_r}(\phi - \phi _{\mathbb D^2_r})=\infty \).
Proof
With \(x=\log r_1\) and \(y=\log r_2\), we get
Integrate by parts with respect to t and s successively, we may write
where
and
Obviously, \(I_2(x,-1)\) is bounded on \((-\infty , 0]\), but \(I_1(x,-1)\rightarrow \infty \) as \(x\rightarrow -\infty \), from which the assertion immediately follows. \(\square \)
Proof of Theorem 3.1
By Lemma 3.0.2 we have (still with \(x=\log r_1,\,y=\log r_2\))
which yields
By a similar argument as Lemma 3.1.2, we conclude the proof of Theorem 3.1. \(\square \)
3.4 Lelong class
In this section we shall prove the third part of Theorem 1.1. The key ingredient is the following counterpart of Lemma 3.0.3.
Lemma 3.1.3
Let \(g(t)=g(t_1, \ldots , t_n)\) be a convex function on \(\mathbb R^n\) which is increasing in each variable. Assume that
Then for every \(M>0\) we have
where \(t-M:=(t_1-M, \ldots , t_n-M)\).
Proof
For fixed t, we consider the following convex increasing function
on \({\mathbb {R}}\). Convexity of f gives
By the assumption, we have
for every \(s\ge 0\), so that
The proof is complete. \(\square \)
Proof of the third part of Theorem 1.1
Again for any polydisc
we may write
where
By Lemma 3.0.4 we have
Put
and \(f_1(t):=\sup _{P_t} \phi \). Since \(\phi \in {{\mathcal {L}}}\), we know that for some constant \(c_1\gg 1\) the function \(f_1-c_1\) satisfies the assumption in Lemma 3.1.3, so that
which in turn implies
Moreover, we infer from Lemma 3.0.5 that
Applying Lemma 3.1.3 in a similar way as above, we have
Thus
which finishes the proof. \(\square \)
4 Proof of Theorem 1.2
The starting point is the following
Definition 4.0.1
(\(\gamma \)-constant) We shall define the constant \(\gamma \) as the BUO norm of \(\log |z|\) on \({\mathbb {C}}\) with respect to all line segments. More precisely,
where [a, b] denotes the line segment connecting a and b, and the upper oscillation is defined by
The key step is to show the following
Lemma 4.0.4
\(1<\gamma <2\) is determined by
Proof
For each pair \(a, b \in {\mathbb {C}}\), we shall compute
Since \(\log |z|\) is \(S^1\)-invariant, by a rotation of z, we may assume that
Thus
is independent of a. Since
with equality holds if and only if \(a\in {\mathbb {R}}\). Thus it suffices to verify (4.1) for
Consider \(\log |z|-\log b\) instead of \(\log |z|\), one may further assume that
which implies
We divide into two cases. (i) \(0\le a<1\). Then we have
(ii) \(-1<a<0\). Then we have
Thus
It suffices to verify that \(\gamma \) satisfies (4.1). To see this, put
and write
Since
it follows that \(f'(t)=0\) if and only if
i.e.,
Thus we have
where \(a_0\) is determined by
which gives
It is clear that (4.2) is equivalent to (4.1). \(\square \)
Since a translation of a line segment is still a line segment, we know that \(\log |z-z_0|\) and \(\log |z|\) have the same line segment BUO norm. This fact can be used to estimate the line segment BUO norm of \(\log |p|\) for general polynomials p. In fact, if we write
then
and
Thus
This combined with the fact \(UO_{[a,b]}(\log |z-a_j|) \le \gamma \) gives
for all polynomials p and all \(a,b\in {\mathbb {C}}\).
Now we may conclude the proof of Theorem 1.2 as follows. Since A is compact, we may choose \(z_0\in A\) such that
For every ray (half line), say L, starting from \(z_0\), we see that \(A\cap L\) is a line segment in view of convexity of A. Let \(L_{{\mathbb {C}}}\) be the complex line containing L. Apply (4.3) to \(p|_{L_{{\mathbb {C}}}}\), we have
which gives
since \(UO_{A} (\log |p|) \) is a certain average of \(UO_{A\cap L} (\log |p|)\) for all L starting from \(z_0\): in fact, since \(z_0\) is a maximum point of \(\log |p|\) on A and L contains \(z_0\), we always have
together with (4.3) it gives
Thus
where \(d\mu \) is a certain measure on the unit sphere \(S_{2n-1}\) and we identify the set of rays L starting from \(z_0\) with \(S_{2n-1}\). Notice that the above inequality gives
from which the assertion immediately follows.
5 Proof of Theorem 1.3
The starting point is the following
Proposition 5.0.1
(John–Nirenberg inequality) Suppose \(\phi \in PSH(\Omega )\) and \(\Omega _0\subset \subset \Omega \) is open. For each \(a=(a_1,\ldots ,a_n)\) with all \(a_j>0\) there exists \(\varepsilon _0=\varepsilon (a,\phi ,\Omega _0,\Omega ) >0\) such that
for every \(\varepsilon \le \varepsilon _0\). Here
Although the argument is fairly standard, we will provide a proof in Appendix, because the result cannot be found in literature explicitly.
Lemma 5.0.5
Let \(\psi \) be a psh function on \(\Omega \) which satisfies \(\sup _{\Omega } \psi <\infty \) and \(\int _\Omega e^{-\psi } <\infty \). Suppose \(\Omega \) is circular, i.e., \(\zeta z\in \Omega \) for every \(\zeta \in {\mathbb {C}}\), \(|\zeta |\le 1\), and \(z\in \Omega \). Then
Proof
The extremal property of the Bergman kernel implies that
and the first inequality in (5.1) holds. On the other hand, as \(\Omega \) is circular, it is easy to verify that
for all \(f\in {{\mathcal {O}}}(\Omega )\). Thus we have
so that the second inequality in (5.1) also holds. \(\square \)
Proof of Theorem 1.3
Since
it follows that
where
Thus we have
This combined with Lemma 5.0.5 gives
By Proposition 5.0.1, we conclude the proof. \(\square \)
6 Proof of Theorem 1.4
Recall that
By Proposition 2.2 in [3], we have
where \(K_{\phi ^t, \,{\mathbb {D}}^n}(z,0)\) satisfies the following reproducing property
for all \(L^2\) holomorphic functions f on \({\mathbb {D}}^n\). In particular, if \(f=zK_{\phi ^t, \,{\mathbb {D}}^n}(z,0)\) then
and since \( \frac{\partial \phi ^t}{\partial t_j}|_{t=0}=z_j\phi _{z_j}(0), \) we get
for all \(t\in {\mathbb {D}}^n\). Thus we may write (6.1) as
In particular,
Thus we can further write (6.1) as
which implies
Since
and
we get
Notice that
and
our assertion follows.
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Acknowledgements
Open Access funding provided by NTNU Norwegian University of Science and Technology (incl St. Olavs Hospital - Trondheim University Hospital) The authors would like to thank Ahmed Zeriahi for bringing their attention to the reference [1]. The second author would like to thank Bo Berndtsson for numerous useful discussions about the topics of this paper.
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Bo-Yong Chen is supported by NSF Grant 11771089 and Gaofeng grant from School of Mathematical Sciences, Fudan University.
Appendix
Appendix
In this section we provide a proof of Proposition 5.0.1. Let us first recall a few basic facts in real-variable theory, by following Stein [17]. A quasi-distance defined on \({{\mathbb {R}}}^m\) means a nonnegative continuous function \(\rho \) on \({{\mathbb {R}}}^m\times {\mathbb {R}}^m\) for which there exists a constant \(c>0\) such that
-
1.
\(\rho (x,y)=0\) iff \(x=y\);
-
2.
\(\rho (x,y)\le c\rho (y,x)\);
-
3.
\(\rho (x,y)\le c(\rho (x,z)+\rho (y,z))\).
Given such a \(\rho \), we define “balls”
One can verify that there exists a constant \(c_1>1\) such that for all x, y and r,
In the case of Proposition 5.0.1, we define
It is easy to verify that \(\rho \) is a quasi-distance on \({\mathbb C}^n\) and
Besides (7.1), the following properties also hold for \(B(\hat{z},r)\):
Fix a pair of positive constants \(c^*\) and \(c^{**}\) with \(1<c^*<c^{**}\). For \(B=B(\hat{z},r)\) we define \(B^*=B(\hat{z},c^*r)\) and \(B^{**}=B(\hat{z},c^{**} r)\). Then we have
Lemma 7.0.6
(cf. [17, p. 15–16]) Choose \(c^*=4c_1^2\) and \(c^{**}=16c_1^2\). Given a closed nonempty set \(F\subset {{\mathbb {C}}}^n\), there exists a collection of balls \(\{B_k\}\) such that
-
(1)
The \(B_k\) are pairwise disjoint;
-
(2)
\(\bigcup _k B_k^*= F^c:={\mathbb {C}}^n\backslash F\);
-
(3)
\(B_k^{**}\cap F\ne \emptyset \) for each k.
Proposition 7.0.2
(Calderón–Zygmund decomposition) Let \(B_0\) be a ball in \({\mathbb {C}}^n\) and \(f\in L^1(B_0)\). There is a constant \(c=c(c_1,c_2)>0\) such that given a positive number \(\alpha \), there exists a sequences of balls \(\{B_k^*\}\) in \(B_0\) such that
-
(1)
\(|f(z)|\le \alpha \), for a.e. \(z\in B_0\backslash \bigcup _k B_k^*\);
-
(2)
\( \int _{B_k^*} |f|\le c\alpha |B_k^*|, \) for each k;
-
(3)
\(\sum _k |B_k^*| \le \frac{c}{\alpha } \int _{B_0} |f|\).
Proof
We extend f to an integrable function on \({{\mathbb {C}}}^n\) by setting \(f=0\) outside \(B_0\). Recall the following two types of Hardy–Littlewood maximal functions:
where the supremum is taken over all balls B containing z. The relationship between Mf and \(\widetilde{M}f\) is as follows:
Notice that
is an open set since \(\widetilde{M}f\) is lower semicontinuous, and
in view of (7.5) and [17, p. 13, Theorem 1]. Here and in what follows c will denote a generic positive constant depending only on \(c_1,c_2\). With \(F:={\mathbb {C}}^n\backslash E_\alpha \) we choose balls \(\{B_k\}\), \(\{B_k^*\}\) and \(\{B_k^{**}\}\) according to Lemma 7.0.6. Then we have
Since \(B_k^{**}\cap F\ne \emptyset \) for each k, we have
Finally, by (7.5) and [17, p. 13, Corollary], we know that \(|f(z)|\le \widetilde{M}f(z)\) for a.e. z, from which (1) immediately follows. \(\square \)
Proof of Proposition 5.0.1
By Theorem 1.1, we know that
Assume without loss of generality \(M=1\). Fix a ball \(B_0\subset \Omega _0\). It suffices to show
for certain \(\varepsilon \ll 1\). With c as Proposition 7.0.2 we choose
Applying Proposition 7.0.2 with \( f= |\phi -\phi _{B_0}|, \) we have a sequence of balls \(\{B_k^{(1)}\}\) in \(B_0\) such that
and
Applying Proposition 7.0.2 with \( f= |\phi -\phi _{B_k^{(1)}}| \) for each k, we obtain a sequence of balls \(\{B_k^{(2)}\}\) in \(\bigcup _k B_k^{(1)}\) such that
and
which in turn implies
Continue this process. For each j there exists a sequence of balls \(\{B_k^{(j)}\}\) in \(\bigcup _k B_k^{(j-1)}\) such that
Thus
For any t there exists an integer j such that \(t\in [j\cdot c\alpha ,(j+1)\cdot c\alpha )\). It follows that
from which (7.6) immediately follows. Now we have
which gives
By Theorem 1.1, \(\sup _{B_0}\{\sup _{B_0}\phi -\phi _{B_0}\} <\infty \), thus Proposition 5.0.1 follows. \(\square \)
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Chen, BY., Wang, X. Bergman kernel and oscillation theory of plurisubharmonic functions. Math. Z. 297, 1507–1527 (2021). https://doi.org/10.1007/s00209-020-02567-9
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DOI: https://doi.org/10.1007/s00209-020-02567-9
Keywords
- BUO
- Plurisubharmonic function
- Bergman kernel
- Remez inequality
- Directional Lelong number
- Complex Brunn–Minkowski theory
- Ohsawa–Takegoshi theorem