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Weighted Harmonic Bloch Spaces on the Ball

Weighted Harmonic Bloch Spaces on the Ball
Weighted Harmonic Bloch Spaces on the Ball

Complex Anal.Oper.Theory DOI10.1007/s11785-017-0645-9Complex Analysis and Operator Theory

Weighted Harmonic Bloch Spaces on the Ball

?mer Faruk Do?g an1·A.Ers˙?nüreyen2

Received:19April2016/Accepted:3February2017

?Springer International Publishing2017

Abstract We study the family of weighted harmonic Bloch spaces bα,α∈R,on the unit ball of R n.We provide characterizations in terms of partial and radial derivatives and certain radial differential operators that are more compatible with reproducing kernels of harmonic Bergman–Besov spaces.We consider a class of integral opera-tors related to harmonic Bergman projection and determine precisely when they are bounded on L∞α.We de?ne projections from L∞αto bαand as a consequence obtain integral representations.We solve the Gleason problem and provide atomic decom-position for all bα,α∈R.Finally we give an oscillatory characterization of bαwhen α>?1.

Keywords Harmonic Bloch space·Bergman space·Reproducing kernel·Radial fractional derivative·Bergman projection·Duality·Gleason problem·Atomic decomposition·Oscillatory characterization

Mathematics Subject Classi?cation Primary31B05·31B10;Secondary26A33·46E15

Communicated by H.Turgay Kaptanoglu.

B A.Ers˙?nüreyen

aeureyen@https://www.wendangku.net/doc/0115524035.html,.tr

?mer Faruk Do?g an

ofdogan@https://www.wendangku.net/doc/0115524035.html,.tr

1Department of Mathematics,Namik Kemal University,59030Tek˙?rda?g,Turkey

2Department of Mathematics,Anadolu University,26470Eskisehir,Turkey

?.F.Do?g an,A.E.üreyen

1Introduction

For n≥2,let B be the unit ball in R n and S be the unit sphere.We denote the space of complex-valued harmonic functions on B by h(B).The well-known harmonic Bloch space b is the space of all f∈h(B)such that

sup

x∈B

(1?|x|2)|?f(x)|<∞.

The space b is a member of the one-parameter family of weighted harmonic Bloch spaces bα,α∈R.The aim of this work is to investigate the properties of this family in a detailed,systematic and uni?ed way.The holomorphic counterpart of this family of spaces and the related little Bloch and Lipschitz spaces have been studied in[13,26].

To de?ne bαwe need to introduce more de?nitions.We denote by L∞the Lebesgue class of essentially bounded functions on B,and forα∈R we de?ne

L∞α={?:(1?|x|2)α?(x)∈L∞},

so that L∞0=L∞.The norm on L∞αis

? L∞

α

= (1?|x|2)α?(x) L∞.

We will also use the following subspaces of L∞α:

Cα={?∈L∞α:(1?|x|2)α?(x)is continuous on B},

Cα0={?∈Cα:(1?|x|2)α?(x)=0on?B}.

De?nition1.1Forα>0,the weighted harmonic Bloch space bαis h(B)∩L∞αand the weighted harmonic little Bloch space bα0is h(B)∩Cα0.

Obviously(forα>0),bα0={f∈bα:lim|x|→1?(1?|x|2)αf(x)=0}.The norm on bα(and bα0)is the norm inherited from L∞α.

To extend the above de?nition to the rangeα≤0,we need to consider growth rates of derivatives of f∈h(B).For this we will employ three different types of differentiation.For the usual partial derivatives we will write

?m f=

?|m|f

?x m11···?x m n n

,

where m=(m1,...,m n)is a multi-index,m1,...,m n are nonnegative integers and

|m|=m1+···+m n.

It is well-known that f∈h(B)has a homogeneous expansion f= ∞

k=0

f k,where

f k is a homogeneous harmonic polynomial of degree k and the series absolutely and uniformly converges on compact subsets of B(see[2]).The radial derivative R f of f∈h(B)is de?ned as

Weighted Harmonic Bloch Spaces on the Ball

R f(x)=x·?f(x)=

k=0

k f k(x),(1)

and

R N f(x)=RR N?1f(x)=

k=0

k N f k(x),N=2,3,....

In addition to partial and radial derivatives we will extensively use certain radial fractional differential operators D t s:h(B)→h(B),(s,t∈R)introduced in[7]and [8].These operators are de?ned in terms of reproducing kernels of harmonic Bergman–Besov spaces and are more convenient than partial or radial derivatives in studying harmonic function spaces.We will review properties of D t s in Sect.2.3.For now,we only note that t determines the order of the differentiation and s plays a minor role.

The following theorem will enable us to de?ne weighted harmonic Bloch space bαfor the whole rangeα∈R.We denote N={0,1,2,...}with0included. Theorem1.2Letα∈R and f∈h(B).The following are equivalent:

(a)For every N∈N withα+N>0,we have(1?|x|2)N?m f∈L∞αfor every

multi-index m with|m|=N.

(b)There exists an N∈N withα+N>0such that(1?|x|2)N?m f∈L∞αfor every

multi-index m with|m|=N.

(c)For every N∈N withα+N>0,we have(1?|x|2)N R N f∈L∞α.

(d)There exists an N∈N withα+N>0such that(1?|x|2)N R N f∈L∞α.

(e)For every s,t∈R withα+t>0,we have(1?|x|2)t D t s f∈L∞α.

(f)There exist s,t∈R withα+t>0such that(1?|x|2)t D t s f∈L∞α. Moreover,ifα+N>0andα+t>0,then

(1?|x|2)t D t s f L∞

α~|f(0)|+ (1?|x|2)N R N f L∞

α

|m|≤N?1

|(?m f)(0)|+

|m|=N

(1?|x|2)N?m f L∞

α

.(2)

A corresponding theorem holds when L∞αis replaced by Cα0.

Theorem1.3Letα∈R and f∈h(B).The following are equivalent:

(a)For every N∈N withα+N>0,we have(1?|x|2)N?m f∈Cα0for every

multi-index m with|m|=N.

(b)There exists an N∈N withα+N>0such that(1?|x|2)N?m f∈Cα0for every

multi-index m with|m|=N.

(c)For every N∈N withα+N>0,we have(1?|x|2)N R N f∈Cα0.

(d)There exists an N∈N withα+N>0such that(1?|x|2)N R N f∈Cα0.

(e)For every s,t∈R withα+t>0,we have(1?|x|2)t D t s f∈Cα0.

(f)There exist s,t∈R withα+t>0such that(1?|x|2)t D t s f∈Cα0.

?.F.Do?g an,A.E.üreyen Forα≥0,equivalence of parts(a)–(d)of Theorems1.2and1.3are known and the main part of the above theorems is that they also hold forα<0.Forα=0,see[4, Theorem1.4]for the equivalence of parts(a)–(d)and an additional characterization with a different type of derivative.Forα>0,see[18,Theorem1.1]for the equivalence of parts(a)and(b)for the choices of N=0and N=1.

De?nition1.4Letα∈R.The weighted harmonic Bloch space bα(respectively weighted harmonic little Bloch space bα0)consists of those f∈h(B)such that any one of the equivalent conditions of Theorem1.2(respectively Theorem1.3)is satis?ed.

Ifα>0taking N=0in parts(b)of the above theorems shows that De?nition1.4 is consistent with De?nition1.1.Also,taking N=1in parts(b)of the above theorems shows that b0=b,the usual harmonic Bloch space and b00is the usual harmonic little Bloch space:b00={f∈h(B):lim|x|→1?(1?|x|2)|?f(x)|=0}.

We mention a few immediate consequences of De?nition1.4.First,for everyα∈R,we have bα0?bα.Also,if f∈h(B),then f∈bα0;in particular every bα0 (and bα)contains harmonic polynomials and therefore is non-trivial.It is also clear that

bα?bβ0?bβ(forα<β).(3) The above inclusions are in fact strict(see Remark4.9below)and therefore all these spaces are different.

Whenα>0we have a standard norm on bα,but whenα≤0we do not.For α∈R if we pick any N∈N withα+N>0or pick s,t∈R withα+t>0,each term in(2)is a norm on bα.Since all these norms are equivalent,there is no essential difference in choosing any one of them;and we will denote any one of these norms

by · b

αwithout indicating the dependence on N or s,t.

For s,t∈R and f∈h(B)we will write

I t s f(x):=(1?|x|2)t D t s f(x).

It is clear from Theorem1.2that givenα∈R,if t is chosen to satisfyα+t>0,then

f∈bαif and only if I t s f∈L∞αand I t s f L∞

αis a norm on bα.

Our next aim is to write bα(respectively bα0)as quotient spaces of L∞α(respectively Cαor Cα0)by using Bergman–Besov projections and obtain integral representations for elements of bα.For this we need more de?nitions.

Letνbe the volume measure on B normalized so thatν(B)=1.For q∈R we de?ne the weighted volume measures

dνq(x)=

1

V q

(1?|x|2)q dν(x).

These measures are?nite only when q>?1and in this case we choose V q so that νq(B)=1.For q≤?1,we set V q=1.For1≤p<∞,we denote the Lebesgue classes with respect toνq by L p q.

Weighted Harmonic Bloch Spaces on the Ball

For1≤p<∞and q>?1the weighted harmonic Bergman space b p q is h(B)∩L p q.It is well-known that the space b2q is a reproducing kernel Hilbert space with kernel R q(x,y).In[7,8]the spaces b p q and the reproducing kernels R q(x,y)are extended to the whole range q∈R.We will give a review of these in Sect.2.2.

De?nition1.5For s∈R,the harmonic Bergman–Besov projection is

Q s?(x)=

B

R s(x,y)?(y)dνs(y),

for suitable?.

Theorem1.6Letα∈R.The operator Q s:L∞α→bαis bounded if and only if

s>α?1.(4) For an s satisfying(4),if t satis?es

α+t>0,(5) then for f∈bα,

Q s I t s f=V s+t

V s

f,(6)

and therefore Q s is onto.Also,Q s:Cα→bα0or Q s:Cα0→bα0is bounded(and onto)if and only if(4)holds.

By(6)we have the following integral representation:For f∈bα,if(4)and(5) holds,then

f(x)=

V s

V s+t

B

R s(x,y)I t s f(y)dνs(y)=

B

R s(x,y)D t s f(y)dνs+t(y).(7)

This representation is very fruitful and we will use it many times in Sects.5and6.

The caseα=0of Theorem1.6is proved earlier in[4,11]and[14]where the

authors use different differential operators than our D t s.In[18]a different integral

representation valid forα>?1is given.We note that Theorem1.6covers allα∈R,gives a necessary and suf?cient condition for the boundedness of the projection operator Q s and provides a simple reproducing formula.

For the holomorphic analogue of Theorem1.6for the full range?∞<α<∞,

see[13,26].

This paper is organized as follows.In Sect.2we collect some known facts which

we will need in the sequel.In Sect.3we will de?ne a class of integral operators related

to harmonic Bergman projection and determine when they are bounded on L∞αand Cα0.In Sect.4we will prove Theorems1.2and1.3and derive basic properties of the spaces bαand bα0.We will also determine when R q(x,ζ),ζ∈S belongs to bα(or

?.F.Do?g an,A.E.üreyen bα0)and show that all bαand bα0are distinct.In Sect.5we will prove Theorem1.6 and as a consequence we will show that the dual of Bergman–Besov space b1q(for every q∈R)is bαand its pre-dual is bα0under suitable pairings.

Finally in Sect.6we will solve the Gleason problem and obtain atomic decom-position for allα∈R.We will also give an oscillatory characterization of bαfor α>?1.

2Preliminaries

For two positive expressions X and Y we will write X~Y if X/Y is bounded above and below by some positive constants.We will denote these constants whose exact values are inessential by a generic upper case C.We will also write X Y to mean X≤CY.

The Pochhammer symbol(a)b is de?ned by

(a)b= (a+b) (a)

when a and a+b are off the pole set?N of the gamma function.By Stirling formula

(a)c

c

~c a?b(c→∞).(8) For x∈B,y∈B,we will use the notation

[x,y]=

1?2x·y+|x|2|y|2.

It is easy to see that when x,y are nonzero

[x,y]=

|y|x?

y

|y|

=

|x|y?

x

|x|

,

and when y=ζ∈S,we have[x,ζ]=|x?ζ|.

2.1Zonal Harmonics

Let H k(R n)denote the space of all homogeneous harmonic polynomials on R n of degree k.The restriction of f k∈H k(R n)to the unit sphere S is called a spherical harmonic and the space of spherical harmonics of degree k is denoted by H k(S).The ?nite-dimensional space H k(S)?L2(S)is a reproducing kernel Hilbert space:For ζ∈S,there exists(real-valued)Z k(·,ζ)such that

f k(ζ)=

S

f k(η)Z k(η,ζ)dσ(η)(?f k∈H k(S)),

Weighted Harmonic Bloch Spaces on the Ball

where d σis normalized surface area measure on S .The spherical harmonic Z k (·,ζ)is called zonal harmonic of degree k with pole ζ.It can be extended to R n ×R n by making it homogeneous in each variable:If x =|x |η,y =|y |ζwith η,ζ∈S ,

Z k (x ,y )=|x |k |y |k Z k (η,ζ),k =1,2,...

For k =0,we set Z 0(x ,y )≡1.For future reference we state the following properties of Z k (for details see Chapter 5of [2]).Lemma 2.1The following properties hold:

(a)Z k (x ,y )is real-valued and symmetric in its variables.

(b)Z k (x ,0)=Z k (0,y )=0,for every x ,y ∈R n ,k =1,2,...

(c)For k ≥1and ζ∈S ,max η∈S |Z k (η,ζ)|=Z k (ζ,ζ)and Z k (ζ,ζ)~k n ?2.

Therefore |Z k (x ,y )| |x |k |y |k k n ?2.(d)If f k ∈H k (R n ),then f k (x )=

S

f k (η)Z k (x ,η)d σ(η).(e)If f k ∈H k (R n )and l =k,then S f k (η)Z l (x ,η)d σ(η)=0.2.2Harmonic Bergman–Besov Spaces and Reproducin

g Kernels

Let 1≤p <∞and q >?1.The weighted harmonic Bergman space b p

q consists of

all f ∈h (B )such that

f b p q

=

B

|f |p d νq

1/p

=

1

V q

B

|f (x )|p (1?|x |2)q d ν(x )

1/p

<∞.

It is well-known that the space b 2q is a reproducing kernel Hilbert space with repro-ducing kernel R q (x ,y ):

f (x )=

B

R q (x ,y )f (y )d νq (y ),?f ∈b 2

q (q >?1).

(9)

It is also well-known that R q (x ,y )has the series expansion (see [16])

R q (x ,y )=

∞ k =0

(1+n /2+q )k (n /2)k

Z k (x ,y )(q >?1),

where the series absolutely and uniformly converges on K ×B ,for any compact subset

K of B .R q (x ,y )is real-valued,symmetric in the variables x and y and harmonic with respect to each variable as these properties hold for Z k (x ,y ).

The family of weighted Bergman spaces can be extended to all q ∈R in the following way:Pick a nonnegative integer N such that

q +pN >?1.

(10)

?.F.Do?g an,A.E.üreyen The harmonic Bergman–Besov space b p q consists of all f∈h(B)such that

(1?|x|2)N?m f∈L p q,

for every multi-index m with|m|=N.When q>?1,choosing N=0shows that b p q=h(B)∩L p q is the usual weighted Bergman space.The harmonic Bergman–Besov spaces are studied in detail in[7,8]where it is shown that the choice of N is irrelevant as long as(10)is satis?ed.In[7,8]these spaces are called Besov spaces,whereas in the literature the spaces b p?n are usually called Besov spaces.

For every q∈R,the space b2q is a reproducing kernel Hilbert space with kernel

R q(x,y)=

k=0

γk(q)Z k(x,y),(11)

where(see[7,Theorem3.7],[8,Theorem1.3])

γk(q):=?

???

???

(1+n/2+q)k

k

,if q>?(1+n/2);

(k!)2

k k

,if q≤?(1+n/2).

(12)

For q>?1,we endow b2q with the canonical inner product f,g =

B f gdνq and

R q is the reproducing kernel with respect to this inner product.For q≤?1,there is no standard inner product,there are many possible choices each leading to a different reproducing kernel(see[8,Theorem5.2]for the inner product leading to above R q). The above choice of R q follows[3]and[12],where holomorphic Bergman–Besov spaces are studied.

We list a few simple properties that we will use later:For every q∈R we have γ0(q)=1and therefore by Lemma2.1(b),

R q(x,0)=R q(0,y)=1,?x,y∈B(?q∈R).(13) Checking the two cases in(12),we have by(8)

γk(q)~k1+q(k→∞).(14) For each x∈B,R q(x,·)is harmonic on B and if K?B is compact and m is a multi-index

|?m R q(x,y)| 1,?x∈K,y∈B,(15) where differentiation is performed with respect to x.

Weighted Harmonic Bloch Spaces on the Ball 2.3The Operators D t s

Let s ,t ∈R .The radial differential operator D t s

:h (B )→h (B )is de?ned in the following way (see [7,8]):If f = ∞

k =0f k is the homogeneous expansion,then

D t

s

f :=

∞ k =0

γk (s +t )k f k :=∞ k =0

d k (s ,t )f k .(16)

By (14),

d k (s ,t )=

γk (s +t )

γk (s )

~k t (k →∞),

(17)

and therefore roughly speaking D t s

multiplies the k th homogeneous term by k t .The exact form of D t s

is chosen in order to have the relation D t

s R s (x ,y )=R s +t (x ,y ),

(18)

where differentiation is performed on either of the variables x or y .For every s ∈

R ,D 0s

=I ,the identity.The additive property D u s +t D t s =D u +t

s

(19)

shows that every D t s is invertible with the two-sided inverse D ?t s +t

:D ?t s +t D t s =D t s D ?t s +t =I .

(20)

The following lemma is Theorem 3.2of [8].

Lemma 2.2Equip h (B )with the topology of uniform convergence on compact sub-sets.Then D t s

:h (B )→h (B )is continuous for every s ,t ∈R .In some cases we can write D t s

as an integral operator.To see this we ?rst show that we can push D t s

into some certain integrals.Lemma 2.3Let c ∈R and ?∈L 1c .For every s ,t ∈R ,

D t

s

B

R c (x ,y )?(y )d νc (y )=

B

D t s R c (x ,y )?(y )d νc (y ).

Proof Since,for ?xed x the series expansion (11)uniformly converges for y ∈B ,

B

R c (x ,y )?(y )d νc (y )=

∞ k =0

γk (c )

B

Z k (x ,y )?(y )d νc (y )=:

∞ k =0

γk (c )p k (x ).

?.F.Do?g an,A.E.üreyen As Z k(·,y)is a homogeneous harmonic polynomial of degree k,so is p k and the series on the right is homogeneous expansion.Therefore by(16),

D t s

B

R c(x,y)?(y)dνc(y)=

k=0

d k(s,t)γk(c)

B

Z k(x,y)?(y)dνc(y)

=

B

D t s R c(x,y)?(y)dνc(y),

where in the last equality we use uniform convergence of ∞

k=0

d k(s,t)γk(c)Z k(x,·)

(which follows from Lemma2.1(c),(14)and(17)).

If c=s,the following Corollary follows from(18).

Corollary2.4Let s∈R and?∈L1s.For every t∈R,

D t s

B

R s(x,y)?(y)dνs(y)=

B

R s+t(x,y)?(y)dνs(y).

Corollary2.5Let s>?1and f∈L1s∩h(B).For every t∈R,

D t s f(x)=

B

R s+t(x,y)f(y)dνs(y).(21)

Proof It is standard that the reproducing formula(9)remains true for all f∈b1q (q>?1).Therefore

f(x)=

B

R s(x,y)f(y)dνs(y).

We apply D t s to both sides and use the previous corollary.

The operator D t s as an integral operator as in(21)appears in[11].

2.4Estimates of Reproducing Kernels

For a j,b j>0(j=1,...J)and x∈B,y∈B,let

W(x,y)=

k=0

(a1+k)··· (a J+k)

(b1+k)··· (b J+k)Z k

(x,y).(22)

Note that by(12),R q(x,y)is of the form(22)for every q∈R.The following estimates for W(x,y)and its partial derivatives are taken from Section7of[8].

Weighted Harmonic Bloch Spaces on the Ball

Lemma2.6Let a j,b j>0(j=1,...J)and m be a multi-index.Set c=n?1+ (a1+···+a J)?(b1+···+b J)+|m|.Then for every x∈B,y∈B,

(?m W)(x,y)

?

???

??

???

??

1,if c<0;

1+log

1

[x,y]

,if c=0;

1

[x,y]c

,if c>0,

where differentiation is performed with respect to the?rst variable.

Checking the two cases of(12),one immediately obtains the following estimate for reproducing kernels.

Lemma2.7Let q∈R and m be a multi-index.Then for every x∈B,y∈B,

(?m R q)(x,y)

?

???

??

???

??

1,if q+|m|

1+log

1

[x,y]

,if q+|m|=?n;

1

[x,y]n+q+|m|

,if q+|m|>?n.

The q≥?1part of the above lemma is proved in many places including[4,11]. Since by(16),D t s R q(x,y)is also of the form(22),we have the following estimate. Lemma2.8Let q,s,t∈R and m be a multi-index.Then for every x∈B,y∈B,

?m(D t s R q)(x,y)

?

???

??

???

??

1,if q+t+|m|

1+log

1

[x,y]

,if q+t+|m|=?n;

1

[x,y]n+q+t+|m|

,if q+t+|m|>?n.

When y=ζ∈S and x=rζ,0≤r<1,the following two-sided estimate follows from part(c)of Lemma2.1and(14).

Lemma2.9Letζ∈S and0≤r<1.Then

|R q(rζ,ζ)|~?

???

?

???

?

1,if q

1

1?r2

,if q=?n;

1

(1?r2)q+n

,if q>?n.

For q>?1the following estimate on weighted integrals of R q is proved in various places.For the whole range q∈R,it is a special case of[8,Theorem1.5].

?.F.Do?g an,A.E.üreyen Lemma2.10Let q∈R and c>?1.Then for x∈B,

B |R q(x,y)|(1?|y|2)c dν(y)~

?

???

??

???

??

1,if q

1+log

1

1?|x|

,if q=c;

1

(1?|x|2)q?c

,if q>c.

We will also need the following integral estimate.For a proof see[15,Proposi-tion2.2]or[17,Lemma4.4].

Lemma2.11Let a>?1and c∈R.Then for x∈B,

B (1?|y|2)a

dν(y)~

?

???

??

???

??

1,if c<0;

1+log

1

1?|x|2

,if c=0;

1

(1?|x|2)c

,if c>0.

We mention one more integral estimate.

Lemma2.12Let a>?1,c>0and0≤r<1.Then

1 0

(1?t2)a

(1?r2t2)1+a+c dt

1

(1?r2)c

.

For a proof see,for example,[11,Lemma2.1].

3A Class of Integral Operators

In this section we will consider a class of integral operators and determine when they are bounded on L∞αor Cα0.

For a,c∈R we de?ne

T a,c?(x)=(1?|x|2)a

B

R a+c(x,y)?(y)(1?|y|2)c dν(y)

S a,c?(x)=(1?|x|2)a

B

R a+c(x,y)

?(y)(1?|y|2)c dν(y)

E a,c?(x)=(1?|x|2)a

B

1

?(y)(1?|y|2)c dν(y)

The following theorem determines exactly when the above operators are bounded from L∞αto L∞α.Later,we will invoke this theorem many times.

Theorem3.1Letα,a,c∈R.The following are equivalent:

(a)T a,c is bounded on L∞α.

Weighted Harmonic Bloch Spaces on the Ball

(b)S a,c is bounded on L∞α.

(c)E a,c is bounded on L∞α.

(d)a+α>0and c>α?1.

Before proving this theorem we?rst show the following lemma.Recall that by(13), R q(0,y)=1for every q∈R and y∈B.The lemma below shows that if x stays close to0,then R q(x,y)is uniformly away from0for every y∈B.

Lemma3.2Let q∈R.There exists >0such that for all|x|< and for all y∈B, we have R q(x,y)≥1/2.

Proof Sinceγ0(q)=1and Z0(x,y)≡1,we have

R q(x,y)=

k=0

γk(q)Z k(x,y)=1+

k=1

γk(q)Z k(x,y).

By(14)and Lemma2.1(c),for|x|≤1/2,

k=1

γk(q)Z k(x,y)

k=1

k1+q k n?2|x|k|y|k |x|

k=1

k n+q?1

1

2

k?1

|x|.

Hence,for small enough , ∞

k=1

γk(q)Z k(x,y)

<1/2for|x|< and the lemma

follows. Proof of Theorem3.1We?rst show the equivalence(a)?(b)?(d).

(b)?(a):This is immediate by the inequality|T a,c?(x)|≤S a,c(|?|)(x). (a)?(d):We?rst show that c>α?1.Let?(x)=(1?|x|2)?α.Then?∈L∞αand with as in Lemma3.2,for|x|< we have

T a,c?(x)≥(1?|x|2)a

B

1

2

(1?|y|2)c?αdν(y).

If c≤α?1,the last integral will be divergent and T a,c?couldn’t be in L∞α.

To see that a+α>0,we again let?(x)=(1?|x|2)?αand integrate in polar coordinates to obtain

T a,c?(x)=(1?|x|2)a

B

R a+c(x,y)(1?|y|2)c?αdν(y)

=(1?|x|2)a 1

nρn?1(1?ρ2)c?α

S

R a+c(x,ρη)dσ(η)dρ.

By mean-value property the integral over S is R a+b(x,0)which is1by(13).So,

T a,c?(x)= (n/2+1) (c?α+1)

(n/2+c?α+1)

(1?|x|2)a=C(1?|x|2)a.

?.F.Do?g an,A.E.üreyen

Since T a ,c ?∈L ∞αwe must have a +α≥0.What remains is to show that a +α=0

is not possible.So,suppose a +α=0.For x 0∈B ,let

?x 0(y )=

?

??

(1?|y |2)?α|R a +c (x 0,y )|R a +c (x 0,y )if R a +c (x 0

,y )=0;

(1?|y |2)?αif R a +c (x 0,y )=0.

Clearly, ?x 0 L ∞α

=1.On the other hand by Lemma 2.10we have T a ,c ?x 0(x 0)=(1?|x 0|2)

a

B |R a +c (x 0,y )|(1?|y |2)c ?αd ν(y )

~(1?|x 0|2)a

1+log

11?|x 0|2

.This implies,by continuity of T a ,c ?x 0,that

T a ,c ?x 0 L ∞α

= (1?|x |2)αT a ,c ?x 0(x ) L ∞≥(1?|x 0|2)αT a ,c ?x 0(x 0) 1+log 1

1?|x 0|2

.

Since ?x 0 L ∞α

=1,we get a contradiction with boundedness of T a ,c .(d)?(b):Suppose a +α>0and c >α?1.Let ?∈L ∞α.Then almost everywhere

|?(y )|≤ ? L ∞

α

(1?|y |2)?αand it follows from Lemma 2.10that |S a ,c ?(x )|≤(1?|x |2)

a

B

|R a +c (x ,y )||?(y )|(1?|y |2)c d ν(y )≤ ? L ∞α(1?|x |2)a

B

|R a +c (x ,y )|(1?|y |2)c ?αd ν(y ) ? L ∞α

(1?|x |2)a 1

(1?|x |2)a +α

.

Hence S a ,c ? L ∞α ? L ∞α

.We next show (c)?(d).

(c)?(d):To see that c >α?1,we let ?(

x )=(1?|x |2)?α.Note that for |x |<1/2we have 1/2≤[x ,y ]= |x |y ?y /|y | ≤3/2.Therefore,for |x |<1/2,

E a ,c ?(x ) (1?|x |2)a

B

(1?|y |2)c ?αd ν(y ).

Since E a ,c ?∈L ∞α,we must have c ?α>?1.That a +α≤0is not possible follows from Lemma 2.11:Letting again ?(y )=(1?|y |2)?α,we have

E a ,c ?(x )=(1?|x |2)

a

B

(1?|y |2)c ?α

[x ,y ]n +a +c

d ν(y ).

Weighted Harmonic Bloch Spaces on the Ball

If a +α<0,then by Lemma 2.11,the above integral is ~1and if a +α=0,it is ~1+log (1?|x |2)?1.In each case E a ,c ?cannot belong to L ∞α.(d)?(c):This part easily follows from Lemma 2.11. Remark 3.3Theorem 3.1remains true when L ∞αis replaced with C α0.This can be veri?ed by repeating the above proof with making appropriate modi?cations (for example,we change ?(x )=(1?|x |2)?αwith ?(x )=(1?|x |2)?α/ 1+log (1?

|x |2)?1

,etc.).We omit the details.

4Proofs of Theorems 1.2and 1.3

Before dealing with the general case α∈R ,we will ?rst consider the case α>0.As is mentioned before when α≥0the equivalence of parts (a)–(d)of Theorems 1.2and 1.3are known.Nevertheless for the convenience of the reader and to make this work self-contained we will not refer to other sources and give a complete proof.

For future reference we record the following simple lemma which is a special case of the reproducing formula (7).

Lemma 4.1Let α>0and s >α?1.If f ∈b α,then

f (x )= B

R s (x ,y )f (y )d νs (y )=1

V s

B R s (x ,y )f (y )(1?|y |2)s d ν(y ).

Proof The conditions imply f ∈b 1s

and the lemma follows from the reproducing formula (9)which is well-known to be true when f ∈b 1q

. We begin the proof of Theorem 1.2with the following lemma.This lemma is

standard and can be proved by more elementary techniques.We include a proof for completeness and to illustrate how it follows from the reproducing formula,the kernel estimates and Theorem https://www.wendangku.net/doc/0115524035.html,ter,we will employ this technique many times.Lemma 4.2Let α>0and f ∈h (B ).The following are equivalent:(a)f ∈b α.

(b)(1?|x |2)|?f (x )|∈L ∞α.

(c)(1?|x |2)R f (x )∈L ∞α.Moreover,

f ?f (0) b α~ (1?|x |2)|?f (x )| L ∞

α~ (1?|x |2)R f (x ) L ∞α

.(23)

Proof (a)?(b):Let f ∈b α.Pick s >α?1.By Lemma 4.1,

f (x )?f (0)=

1

V s

B R s (x ,y ) f (y )?f (0) (1?|y |2)s d ν(y ).Taking partial derivative we obtain

?f ?x i (x )=1

V s

B ??x i

R s (x ,y ) f (y )?f (0) (1?|y |2)s d ν(y ),

?.F.Do?g an,A.E.üreyen where changing the order of the derivative and integral is easily justi?ed using(15). Applying Lemma2.7,we get

(1?|x|2) ?f

?x i

(x)

(1?|x|2)

B

1

[x,y]n+s+1

f(y)?f(0)

(1?|y|2)s dν(y),

and part(b)now follows from Theorem3.1.

(b)?(c):This immediately follows from(1).

(c)?(a):Let M:= (1?|x|2)R f(x) L∞α.Then

|R f(x)|≤M

(1?|x|2)α+1

,for x∈B.(24) By calculus and(1),

f(x)?f(0)=

1

0x·?f(tx)dt=

1/2

R f(tx)

t

dt+

1

1/2

R f(tx)

t

dt=:I1+I2.

To estimate I1note that Cauchy’s estimate and(24)implies,for|x|≤1/2,

|?R f(x)|≤C sup

|y|=3/4

|R f(y)| M.(25)

Since R f(0)=0,we have R f(x)= 1

x·?R f(tx)dt and using(25)we deduce

|R f(x)| M|x|for|x|≤1/2.Therefore

|I1|≤ 1/2

|R f(tx)|

t

dt

1/2

Mt|x|

t

dt M≤

M

α

.

For the second integral I2we use(24)and Lemma2.12to obtain

|I2|≤ 1

1/2

|R f(tx)|

t

dt

1

1/2

|R f(tx)|dt

1

M

(1?t2|x|2)α+1dt

M

(1?|x|2)α

.

Hence f?f(0) b

α (1?|x|2)R f(x) L∞

α

.

We note that we can write(23)in the following form:

f bα~|f(0)|+ (1?|x|2)|?f(x)| L∞

α~|f(0)|+ (1?|x|2)R f(x) L∞

α

.

It is straightforward to extend the previous lemma to higher order derivatives. Lemma4.3Letα>0and f∈h(B).The following are equivalent:

(a)f∈bα.

(b)For every N∈N,we have(1?|x|2)N?m f∈L∞αfor every multi-index m with

|m|=N.

Weighted Harmonic Bloch Spaces on the Ball

(c)There exists N ∈N such that (1?|x |2)N ?m f ∈L ∞αfor every multi-index m with

|m |=N.

(d)For every N ∈N ,we have (1?|x |2)N R N f ∈L ∞α.

(e)There exists N ∈N such that (1?|x |2)N R N f ∈L ∞α.Moreover,

f b α~

|m |≤N ?1

|(?m

f )(0)|+

|m |=N

(1?|x |2)N ?m f L ∞

α

~|f (0)|+ (1?|x |2)N R N f L ∞α

.(26)

Proof We show (a)?(b)?(c).The equivalence (a)?(d)?(e)can be justi?ed

similarly.

(a)?(b):Suppose f ∈b α.By Lemma 4.2,?f

i

∈b α+1for every i =1,2,...,n .

Applying Lemma 4.2again we obtain

?2f

j i

∈b α+2for every i ,j =1,2,...,n .We continue until we obtain ?m f ∈b α+N for every m with |m |=N .(b)?(c):This part is clear.

(c)?(a):Suppose (1?|x |2)N ?m f ∈L ∞α,that is ?m f ∈b α+N for every multi-index m with |m |=N .Let m be a multi-index with |m |=N ?1.Then ??x i

?m

f ∈

b α+N for every i =1,2,...,n and Lemma 4.2implies ?m

f ∈b α+N ?1.We repeat the same argument suf?ciently many times until we obtain f ∈b α.It is not hard to verify (26)and we omit the details.

We next show that instead of partial or radial derivatives we can use the operators D t s

.We remain in the region α>0.

Lemma 4.4Let α>0and f ∈h (B ).The following are equivalent:(a)f ∈b α.

(b)For every s ,t ∈R with α+t >0,we have (1?|x |2)t D t s f ∈L ∞α.(c)There exist s ,t ∈R with α+t >0such that (1?|x |2)t D t s f ∈L ∞α.

Moreover, f b α~ (1?|x |2)t D t s f L ∞α

.Proof (a)?(b):Suppose f ∈b α.Pick c >α?1.By Lemma 4.1,

f (x )=

B

R c (x ,y )f (y )d νc (y ).

We apply D t s

to both sides,push it into the integral by Lemma 2.3and then use Lemma 2.8(with n +c +t >n +α?1+t >n ?1>0)to obtain

(1?|x |2)t |D t s f (x )| (1?|x |2)t

B 1|f (y )|(1?|y |2)c d ν(y ).Theorem 3.1now implies (1?|x |2)t D t s f (x ) L ∞α f L ∞α

and part (b)follows.

?.F.Do?g an,A.E.üreyen With(b)?(c)being clear,we show(c)?(a):Suppose(1?|x|2)t D t s f(x)∈L∞α, that is D t s f∈bα+t.Pick c with c>α+t?1.By Lemma4.1,

D t s f(x)=

B

R c(x,y)D t s f(y)dνc(y).

We apply D?t s+t to both sides,use(20)on the left,push D?t s+t into the integral by Lemma2.3and obtain

f(x)=

B

D?t s+t R c(x,y)D t s f(y)dνc(y).

Applying Lemma2.8shows(with n+c?t>n+α+t?1?t>n?1>0)

|f(x)|

B

1

[x,y]n+c?t

(1?|x|2)t|D t s f(y)|(1?|y|2)c?t dν(y).

It now follows from Theorem3.1that f L∞

α (1?|x|2)t D t s f(x) L∞

α

.

Before proving Theorem1.2for allα∈R we mention one last elementary lemma. We include a proof for completeness.

Lemma4.5Let N≥1be an integer.Then

R N=

1≤|m|≤N

p m?m,

where p m is a polynomial with degree equal to|m|.

Proof Let f be a smooth function.Then R f(x)=x·?f(x)= n

i=1

x i?f/?x i,so

the lemma is true for N=1.For N=2we compute

R2f(x)=

n

j=1

x j

?

?x j

n

i=1

x i

?f

?x i

=

n

i,j=1

x i x j

?2f

?x j?x i

+

n

j=1

x j

?f

?x j

and the lemma is true for N=2.The general case follows from induction.

We are now ready to deal with the main part of Theorem1.2,i.e.extending the previous lemmas to allα∈R.

Proof of Theorem1.2We will show(a)?(b)?(c)?(d)?(e)?(f)?(a);the implications(a)?(b),(c)?(d)and(e)?(f)being clear.We will refer many times to Lemmas4.3and4.4and in these cases we will make sure that the subscript of b is always greater than0.

(b)?(c):Suppose there exists N0withα+N0>0such that(1?|x|2)N0?m f∈L∞α,that is?m f∈bα+N

for every multi-index m with|m|=N0.

Weighted Harmonic Bloch Spaces on the Ball

We ?rst show that if m is a multi-index with |m |

f is also in b α+N 0:

For |m |=N 0,by Lemma 4.3,?m

?m f ∈b α+N 0+|m |.Since by (3),b α+N 0+|m |?

b α+2N 0,we deduce that ?m ?m

f ∈b α+2N 0for every multi-index m with |m |=N 0

and it follows from Lemma 4.3that ?m

f ∈b α+N 0.Applyin

g Lemma 4.5now shows

R N 0f ∈b α+N 0.

(27)

Suppose N ∈N is such that α+N >0.If N >N 0,Lemma 4.3and (27)implies R N

f =R N ?N 0(R N 0f )∈b α+N 0+(N ?N 0)=b α+N .Similarly,if N

(d)?(e):Suppose there exists N 0∈N with α+N 0>0such that

(1?|x |2)N 0R N 0f ∈L ∞α,that is R

N 0f ∈b α+N 0.Take any s ,t ∈R such that α+t >0.Then by Lemma 4.4,we have D t s (R N 0f )∈b α+N 0+t .By considering their

actions on homogeneous expansions it is clear that D t s and R N 0commute.Therefore

R N 0(D t s f )∈b α+N 0+t and we conclude by Lemma 4.3that D t s f ∈b α+t .

(f)?(a):Suppose there exists s 0,t 0∈R with α+t 0>0such that (1?

|x |2)t 0D t 0s 0f ∈L ∞α,that is D t 0s 0∈b α+t 0.Pick c >α+t 0?1.Then by Lemma 4.1,

D t 0

s 0

f (x )=

B

R c (x ,y )D t 0

s 0

f (y )d νc (y ).Applyin

g D ?t 0s 0+t 0to bot

h sides,using (20)on the left and pushing D ?t 0

s 0+t 0into the integral by Lemma 2.3,we obtain

f (x )=

B

D ?t 0s 0+t 0R c (x ,y )D t 0s 0

f (y )d νc (y ).Take N ∈N with α+N >0and let m be a multi-index with |m |=N .Then

?m

f (x )=?m

B D ?t 0s 0+t 0R c (x ,y )D t 0

s 0

f (y )d νc (y )= B

?m D ?t 0s 0+t 0R c (x ,y )

D t 0

s 0f (y )d νc (y ).Applying Lemma 2.8(with n +c ?t 0+N >n +α+N ?1>n ?1>0),we get (1?|x |2)N

|?m

f (x )| (1?|x |2)

N

B

(1?|y |2)t 0|D t 0

s 0f (y )|[x ,y ]n +c ?t 0+N

(1?|y |2)c ?t 0

d ν(y ).

Theorem 3.1now implies that (1?|x |2)N ?m f ∈L ∞α.

By retracing the above proof it is not hard to see that (2)holds.

Proof of Theorem 1.3is similar to the proof of Theorem 1.2;the main difference

is we refer to Remark 3.3instead of Theorem 3.1.We omit the details.

?.F.Do?g an,A.E.üreyen We now show the basic properties of the spaces bαand bα0.As mentioned before, by(2)we can endow bα(and its subspace bα0)with many equivalent norms.In the sequel we will mainly use the norms induced by D t s:Givenα∈R,pick any s,t with

α+t>0,then (1?|x|2)t D t s f L∞

α= I t s f L∞

α

is a norm on bα;all these norms

are equivalent and we will denote any one of them by · b

αwithout indicating the

dependence on s and t.

We?rst show that all bα(resp.bα0)are isomorphic.We emphasize that the propo-sition below is true for every t∈R without any restriction.

Proposition4.6Letα∈R.For any s,t∈R,the map D t s:bα→bα+t(resp.

D t s:bα0→b(α+t)0)is an isomorphism and is an isometry when appropriate norms are used.

Proof Pick u such thatα+t+u>0.We endow bαwith the norm f b

α=

I t+u

s f L∞

α

and bα+t with the norm g b

α+t

= I u s+t g L∞

α+t

.By(19),

D t s f bα+t= I u s+t D t s f L∞

α+t

= (1?|x|2)u D u s+t(D t s f) L∞

α+t

= (1?|x|2)u D u+t s f L∞

α+t

= I u+t

s

f L∞

α

= f bα.

For0

Corollary4.7Letα∈R.The following properties hold:

(a)bαand bα0are complete spaces.

(b)Let f∈bα.Then f r→f(as r→1?)in bαif and only if f∈bα0.

(c)bα0is closure of polynomials in bα.

(d)bα0is separable whereas bαis inseparable.

Proof It is well known that these properties hold for b0and b00(it is also elementary to verify them forα>0).The general case then follows from the isomorphism in Proposition4.6,the fact that D t s maps polynomials to polynomials and the simple identity D t s(f r)=(D t s f)r.

Fixζ∈S.Then for any q∈R,we have R q(·,ζ)∈h(B).In the next theorem we determine when R q(·,ζ)belongs to bα(or bα0)and therefore provide non-trivial(i.e. non-polynomial)examples of elements of bα(or bα0).This theorem will also allow us to distinguish between these spaces.

Theorem4.8Let q,α∈R andζ∈S.Then

(i)R q(·,ζ)∈bαif and only ifα≥n+q.

(ii)R q(·,ζ)∈bα0if and only ifα>n+q.

Proof Pick t large enough thatα+t>0and n+q+t>0.By(18),we have

I t q R q(x,ζ)=(1?|x|2)t D t q R q(x,ζ)=(1?|x|2)t R q+t(x,ζ),

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XX民航机场集团有限责任公司 发展战略 二〇一一年三月 目录战略设计的总体架构

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谐波和无功功率的产生(Generation of harmonic and reactive power)

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医学超声谐波成像技术研究进展

第36卷 第5期2004年5月  哈 尔 滨 工 业 大 学 学 报 JOURNA L OF H ARBI N I NSTIT UTE OF TECH NO LOGY   V ol 136N o 15M ay ,2004 医学超声谐波成像技术研究进展 刘贵栋,沈 毅,王 艳 (哈尔滨工业大学航天学院,黑龙江哈尔滨,150001,E 2mail :gtomasd @https://www.wendangku.net/doc/0115524035.html, ) 摘 要:对目前所采用的谐波成像技术作了简要的叙述,并探讨了应用前景.由于在组织和造影剂成像中利用了谐波频率,明显地改善了超声图像质量.超声中的谐波是由组织和造影剂产生.造影谐波来源于所注入的造影剂对超声的反射,与组织的反射无关.当不使用造影剂时,谐波是由非线性传播产生的.组织和造影剂的谐波成像在图像分辨力和对比度之间的折衷使得非线性信号大打折扣.关键词:医学超声;对比谐波成像;组织谐波成像;脉冲反相谐波成像中图分类号:R312 文献标识码:A 文章编号:0367-6234(2004)05-0599-04 The technical progress of medical ultrasonic harmonic imaging LI U G ui 2dong ,SHE N Y i ,W ANG Y an (S ch ool o f As tr onautics ,H arbin Ins titute o f T echn ology ,H arbin 150001,China ,E 2m ail :g tom asd @https://www.wendangku.net/doc/0115524035.html, ) Abstract :Medical ultras ound scanners are widely used in hospitals all over the w orld for diagnostic purposes.While many technological im provements have been achieved over the years that resulted in better images ,a large number of patients are still difficult to image due to inhom ogeneous skin layers and limited penetration.In recent years ,harm on 2ic frenquency is adopted in tissue imaging and contrast agents imaging ,which im proves the image quality.Harm onics in ultras ound are generated by tissue or by contrast agents.C ontrast 2agent harm onics are generated by reflections from the injected contrast agent and not from reflections from tissue.When no contrast is em ployed ,harm onics are generated by tissue itself as a result of nonlinear propagation.Harm onic imaging of tissues or contrast agent forces an inherent com promise between image res olution and contrast that limits its sensitivity to nonlinear signals.This paper describes the technical progress of harm onic imaging briefly.Its clinical application prospect has been discussed al 2s o. K ey w ords :medical ultras ound ;contast harm onic imaging ;tT issue harm onic imaging ;pulse inversion harm onic imaging 收稿日期:2003-05-29. 基金项目:跨世纪优秀人才培养计划资助项目;高等学校骨干教 师资助计划资助项目;哈尔滨工业大学校基金资助项目(HIT.2002.11). 作者简介:刘贵栋(1976-),男,博士研究生; 沈 毅(1965-),男,博士,教授,博士生导师. 医学超声在医学诊断中起着十分重要的作用.但是,医学超声所包含的诊断技术,无论是B 型成 像还是血流检测,都沿用了线性声学的规律.但是线性是相对的、局部的,非线性是绝对的、全面的.实际上医学超声中存在着非线性现象[1].过去它处 于次要地位而被忽略,但是,随着人们对超声研究的深入,研究医学超声中非线性现象将有助于人们 进一步提高现有的诊断水平.近年来产生的谐波成像技术就是非线性声学在超声诊断中的一项卓有成效的新技术.传统的超声影像设备是接收和发射频率相同的回波信号成像,称为基波成像(funda 2mental imaging ).实际上回波信号受到人体组织的非线性调制后产生基波的二次三次等高次谐波,其中二次谐波幅值最强,为此利用人体回声的二次等高次谐波构成人体器官的图像,可提高图像清晰分辨率.这种用回波的二次等高次谐波成像的方法叫

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