Atomic Decomposition of Hardy Spaces

Exercises

6.6 Atomic Decomposition

6.6.4 Atomic Decomposition of Hardy Spaces

We now pass to one of the main theorems of this chapter, the atomic decomposition of H^p(Rⁿ)for 0<p≤1. We begin by defining atoms for H^p.

Definition 6.6.8. Let 1<q≤∞. A function A is called an L^q-atom for H^p(Rⁿ)if there exists a cube Q such that

(a) A is supported in Q;

(b)A

L^q ≤ |Q|^1q−1p; (c)

x^γA(x)dx=0 for all multi-indicesγ with|γ| ≤[ⁿ_p−n].

Notice that any L^r-atom for H^pis also an L^q-atom for H^pwhenever 0<p≤1 and 1<q<r≤∞. It is also simple to verify that an L^q-atom A for H^pis in fact in H^p. We prove this result in the next theorem for q=2, and we refer the reader to Exercise 6.6.4 for the case of a general q.

Theorem 6.6.9. Let 0<p≤1. There is a constant Cn,p<∞such that every L²-atom A for H^p(Rⁿ)satisfies

A

H^p ≤Cn,p.

Proof. We could prove this theorem either by showing that the smooth maximal function M(A;Φ⁾is in L^por by showing that the square function

∑j|Δj(A)|²_1/2 is in L^p. The operatorsΔjhere are as in Theorem 5.1.2. Both proofs are similar; we present the second, and we refer to Exercise 6.6.3 for the first.

Let A(x)be an atom that we assume is supported in a cube Q centered at the origin [otherwise apply the argument to the atom A(x−cQ), where cQis the center of Q]. We control the L^pquasinorm of

∑j|Δj(A)|²1/2

by estimating it over the cube Q^∗and over(Q^∗)^c, where Q^∗=2√

n Q. We have

Q^∗

∑

j

|Δj(A)|^2p₂ dx

¹_p

≤

Q^∗

∑

j

|Δj(A)|²dx ¹₂

|Q^∗|^p(2/1p) .

Using that the square function f →

∑j|Δj(f)|²¹₂

is L²bounded, we obtain

Q^∗

∑

j

|Δj(A)|^2p

2dx ¹_p

≤CnA

L²|Q^∗|^p(2/1p)

≤Cn(2√

n)^np−n2|Q|^12−1p|Q|^1p−12

=C_n.

(6.6.15)

To estimate the contribution of the square function outside Q^∗, we use the cancella- tion of the atoms. Let k= [ⁿ_p−n] +1. We have

Δj(A)(x) =

Q

A(y)Ψ₂−j(x−y)dy

=2^jn

QA(y)

Ψ(2^jx−2^jy)−

∑

|β|≤k−1

(∂^βΨ)(2^jx)(−2^jy)^β β!

dy

=2^jn

QA(y)

∑

|β|=k

(∂^βΨ)(2^jx−2^jθy)(−2^jy)^β β!

dy,

where 0≤θ ≤1. Taking absolute values, using the fact that∂^βΨ are Schwartz functions, and that|x−θy| ≥ |x|−|y| ≥¹₂|x|whenever y∈Q and x∈/Q^∗, we obtain the estimate

|Δj(A)(x)| ≤ 2^jn

Q|A(y)|

∑

|β|=k

CN

(1+2^{j 1}₂|x|)^N

|2^jy|^k β! dy

≤ CN,p,n2^j(k+n) (1+2^j|x|)^N

Q|A(y)|²dy ¹₂

Q|y|^2kdy ¹₂

≤ C_N,p,n2^j(k+n)

(1+2^j|x|)^N |Q|^12−1p|Q|^kn+12

= CN,p,n2^j(k+n)

(1+2^j|x|)^N |Q|^1+kn−1p for x∈(Q^∗)^c. For such x we now have

∑

j∈Z

|Δj(A)(x)|^{2 1}₂

≤CN,p,n|Q|^1+kn−1p

∑

j∈Z

2^{2 j(k+n)} (1+2^j|x|)^2N

¹₂

. (6.6.16) It is a simple fact that the series in (6.6.16) converges. Indeed, considering the cases 2^j≤1/|x|and 2^j>1/|x|we see that both terms in the second series in (6.6.16) con- tribute at most a fixed multiple of|x|^−2k−2n. It remains to estimate the L^pquasinorm of the square root of the second series in (6.6.16) raised over(Q^∗)^c. This is bounded by a constant multiple of

(Q^∗)^c

1

|x|^p(k+n)dx ¹

p ≤Cn,p

_∞

c|Q|¹ⁿr−p(k+n)+n−1dr ¹

p

,

for some constant c, and the latter is easily seen to be bounded above by a constant multiple of |Q|^−1−kn+1p. Here we use the fact that p(k+n)>n or, equivalently, k> ⁿ_p−n, which is certainly true, since k was chosen to be[ⁿ_p−n] +1. Combining this estimate with that in (6.6.15), we conclude the proof of the theorem.

We now know that L^q-atoms for H^pare indeed elements of H^p. The main result of this section is to obtain the converse (i.e., every element of H^pcan be decomposed as a sum of L²-atoms for H^p).

Applying the same idea as in Corollary 6.6.7 to H^p, we obtain the following result.

Theorem 6.6.10. Let 0<p≤1. Given a distribution f ∈H^p(Rⁿ), there exists a sequence of L²-atoms for H^p,{Aj}^∞_j=1, and a sequence of scalars{λj}^∞_j=1such that

∑

N j=1

λjAj→f in H^p.

Moreover, we have f

H^p ≈inf ^∞

∑

j=1

|λj|^p1_p

: f = lim

N→∞

∑

N j=1

λjAj,

Ajare L²-atoms for H^pand the limit is taken in H^p

.

(6.6.17)

Proof. Let Aj be L²-atoms for H^pand∑^∞j=1|λj|^p<∞. It follows from Theorem 6.6.9 that

∑

N j=1

λjAj^p

H^p ≤C_n,p^p

∑

N j=1

|λj|^p. Thus if the sequence∑^Nj=1λjAjconverges to f in H^p, then

f

H^p ≤Cn,p

∞ j=1

∑

|λj|^p1_p ,

which proves the direction≤in (6.6.17). The gist of the theorem is contained in the converse statement.

Using Theorem 6.6.3 (with L= [ⁿ_p−n]), we can write every element f in ˙Fp^0,2= H^pas a sum of the form f=∑_Q∈DsQaQ, where each aQis a smooth L-atom for the cube Q and s={sQ}Q∈Dis a sequence in ˙fp^0,2. We now use Theorem 6.6.5 to write the sequence s={sQ}Qas

s=

∑

∞ j=1

λjrj, i.e., as a sum of∞-atoms rjfor ˙fp^0,2, such that

^∞

∑

j=1

|λj|^p1

p ≤Cs_˙

f^0,2_p ≤Cf

H^p. (6.6.18)

Then we have

f =

∑

Q∈D

sQaQ=

∑

Q∈D

∑

∞ j=1

λjrj,QaQ=

∑

∞ j=1

λjAj, (6.6.19)

where we set

Aj=

∑

Q∈D

rj,QaQ (6.6.20)

and the series in (6.6.19) converges inS (Rⁿ). Next we show that each Ajis a fixed multiple of an L²-atom for H^p. Let us fix an index j. By the definition of the∞-atom for ˙fp^0,2, there exists a dyadic cube Q₀^j such that r_j,Q=0 for all dyadic cubes Q not contained in Q₀^j. Then the support of each aQthat appears in (6.6.20) is contained in 3Q, hence in 3Q₀^j. This implies that the function Ajis supported in 3Q₀^j. The same is true for the function g^0,2(rj)defined in (6.6.1). Using this fact, we have

Aj

L² ≈ Aj

F˙₂^0,2

≤ crj_˙

f₂^0,2

= cg^0,2(rj)

L²

≤ cg^0,2(rj)

L^∞|3Q₀^j|¹²

≤ c|3Q₀^j|^−1p+12.

Since the series (6.6.20) defining Ajconverges in L²and Ajis supported in some cube, this series also converges in L¹. It follows that the vanishing moment condi- tions of Ajare inherited from those of each aQ. We conclude that each Ajis a fixed multiple of an L²-atom for H^p.

Finally, we need to show that the series in (6.6.19) converges in H^p(Rⁿ). But

∑

^M

j=N

λjAj

H^p≤Cn,p

^M

j=N

∑

|λj|^p1_p

→0

as M,N→∞in view of the convergence of the series in (6.6.18). This implies that the series∑^∞j=1λjAjis Cauchy in H^p, and since it converges to f inS (Rⁿ), it must converge to f in H^p. Combining this fact with (6.6.18) yields the direction ≥ in

(6.6.17).

Remark 6.6.11. Property (c) in Definition 6.6.8 can be replaced by

x^γA(x)dx=0 for all multi-indicesγwith|γ| ≤L,

for any L≥[ⁿ_p−n], and the atomic decomposition of H^pholds unchanged. In fact, in the proof of Theorem 6.6.10 we may take L≥[ⁿ_p−n]instead of L= [ⁿ_p−n]and then apply Theorem 6.6.3 for this L. Observe that Theorem 6.6.3 was valid for all L≥[ⁿ_p−n].

This observation can be very useful in certain applications.

Exercises

6.6.1. (a) Prove that there exists a Schwartz functionΘ supported in the unit ball

|x| ≤1 such that_Rnx^γΘ(x)dx=0 for all multi-indicesγwith|γ| ≤N and such that

|Θ| ≥ ¹₂on the annulus¹₂≤ |ξ| ≤2.

(b) Prove there exists a Schwartz functionΨ whose Fourier transform is supported in the annulus ¹₂≤ |ξ| ≤2 and is at least c>0 in the smaller annulus³₅≤ |ξ| ≤⁵₃ such that we have

j∈Z

∑

Ψ(2^−jξ⁾Θ(2^−jξ^{) =}1

for allξ ∈Rⁿ\ {0}.

Hint: Part (a): Let θ be a real-valued Schwartz function supported in the ball

|x| ≤1 and such that θ(0) = 1. Then for some ε^>0 we have θ⁽ξ⁾≥ ¹₂ for all ξ satisfying |ξ|<2ε^<1. SetΘ ^{= (}−Δ^)N(θε). Part (b): Define the function Ψ⁽ξ^{) =}η⁽ξ⁾∑_j∈Zη(2^−jξ⁾Θ(2^−jξ⁾⁻¹for a suitableη.

6.6.2. Letα∈R, 0<p≤1, p≤q≤+∞.

(a) For all f,g inS (Rⁿ)show that f+g^p_˙

Fp^α,q ≤f^p_˙

Fp^α,q+g^p_˙

Fp^α,q.

(b) For all sequences{sQ}Q∈Dand{tQ}Q∈Dshow that {sQ}Q+{tQ}Q^p

˙f^αp^,q≤{sQ}Q^p_˙

f^αp^,q+{tQ}Q^p_˙

f^αp^,q.

Hint: Use|a+b|^p≤ |a|^p+|b|^pand apply Minkowski’s inequality on L^q/p(or on ^q/p).

6.6.3. LetΦ be a smooth function supported in the unit ball of Rⁿ. Use the same idea as in Theorem 6.6.9 to show directly (without appealing to any other theorem) that the smooth maximal function M(·,Φ^{)of an L2}-atom for H^plies in L^pwhen p<1. Recall that M(f,Φ^{) =}sup_t>0|Φt∗f|.

6.6.4. Extend Theorem 6.6.9 to the case 1<q≤∞. Precisely, prove that there is a constant Cn,p,qsuch that every L^q-atom A for H^psatisfies

A

H^p≤Cn,p,q.

Hint: If 1<q <2, use the boundedness of the square function on L^q, and for 2≤q≤∞, its boundedness on L².

6.6.5. Show that the space H_F^pof all finite linear combinations of L²-atoms for H^p is dense in H^p.

Hint: Use Theorem 6.6.10.

6.6.6. Show that for allμ^,j∈Z, all N,b>0 satisfying N>n/b and b<1, all scalars sQ(indexed by dyadic cubes Q with centers cQ), and all x∈Rⁿwe have

Q∈D

∑

(Q)=2^−μ

|sQ|

(1+2^min(j,μ)|x−cQ|)^N

≤c(n,N,b)2^{max(μ−j,0)nb} M

∑

Q∈D

(Q)=2^−μ

|sQ|^bχQ

(x)

!¹

b

,

where M is the Hardy–Littlewood maximal operator and c(n,N,b)is a constant.

Hint: DefineF0=

Q∈D:(Q) =2^−μ,|cQ−x|2^min(j,μ)≤1

and for k≥1 de- fineFk=

Q∈D: (Q) =2^−μ,2^k−1<|cQ−x|2^min(j,μ)≤2^k

. Break up the sum on the left as a sum over the familiesFkand use that∑Q∈Fk|s_Q| ≤

∑Q∈Fk|s_Q|^b1/b

and the fact that ⁽_Q∈F

kQ ≤cn2^{−min(j,μ)n+kn}.

6.6.7. Let A be an L²-atom for H^p(Rⁿ)for some 0<p<1. Show that there is a constant C such that for all multi-indicesαwith|α| ≤k= [ⁿ_p−n]we have

ξsup∈Rⁿ|ξ|^|α|−k−1 (∂^αA)( ξ⁾ ≤CA^{−22p−p(k+1n +12)−1}

L²(Rⁿ) .

[Hint: Subtract the Taylor polynomial of degree k− |α|at 0 of the function x→ e^−2πix·ξ.

6.6.8. Let A be an L²-atom for H^p(Rⁿ)for some 0<p<1. Show that for all multi- indicesα and all 1≤r≤∞there is a constant C such that

|∂^αA|²

L^r (Rⁿ)≤CA^{−22p−p(2|nα|+1r)+2}

L²(Rⁿ) .

Hint: In the case r=1 use the L¹→L^∞boundedness of the Fourier transform and in the case r=∞use Plancherel’s theorem. For general r use interpolation.

6.6.9. Let f be in H^p(Rⁿ)for some 0<p≤1. Then the Fourier transform of f , originally defined as a tempered distribution, is a continuous function that satisfies

|f(ξ⁾| ≤Cn,pf

H^p(Rⁿ)|ξ|^np−n for some constant Cn,pindependent of f .

Hint: If f is an L²-atom for H^p, combine the estimates of Exercises 6.6.7 and 6.6.8 withα⁼0 (and r=1). In general, apply Theorem 6.6.10.

6.6.10. Let A be an L^∞-atom for H^p(Rⁿ)for some 0<p<1 and letα^{= n}_p−n.

Show that there is a constant Cn,psuch that for all g in ˙Λα(Rⁿ)we have _RnA(x)g(x)dx

≤Cn,pg_Λ˙

α(Rⁿ).

Hint: Suppose that A is supported in a cube Q of side length 2^−ν and center cQ. Write the previous integrand as∑jΔj(A)Δj(g)for a suitable Littlewood–Paley op- eratorΔjand apply the result of Appendix K.2 to obtain the estimate

Δj(A)(x) ≤CN|Q|^−1p+1 2^min(j,ν)n2^−|j−ν|D 1+2^min(j,ν)|x−cQ|_N,

where D= [α^{] +}1 whenν≥j and D=0 whenν^<j. Use Theorem 6.3.6.