RESEARCH Open Access Norm inequalities for the conjugate operator in two-weighted Lebesgue spaces Kyung Soo Rim * and Jaesung Lee * Correspondence: ksrim@sogang. ac.kr Department of Mathematics, Sogang University, Seoul 121-742, Korea Abstract In this article, first, we prove that weighted-norm inequalities for the M-harmonic conjugate operator on the unit sphere whenever the pair (u, v) of weights satisfies the A p -condition, and uds,vds are doubling measures, where ds is the rotation- invariant positive Borel measure on the unit sphere with total measure 1. Then, we drive cross-weighted norm inequalities between the Hardy-Littlewood maximal function and the sharp maximal function whenever (u, v) satisfies the A p -condition, and vds does a certain regular condition. 2000 MSC: primary 32A70; secondary 47G10. Keywords: two-weighted norm inequality, non-isotropic metric, maximal function, sharp maximal function, M-harmonic conjugate operator, Hilbert transform 1 Introduction Let B be the unit ball of ℂ n with norm |z|=〈z, z 〉 1/2 where 〈, 〉 is the Hermitian inner product, S be the unit sphere and s be the rotation-invariant probability measure on S. For z Î B, ξ Î S, we define the M -harmonic conjugate kernel K(z, ξ)by iK ( z, ξ ) =2C ( z, ξ ) − P ( z, ξ ) − 1 , where C (z, ξ ) = (1 - 〈z, ξ〉 ) -n is the Cauchy kernel and P(z, ξ)=(1-|z| 2 ) n /|1 - 〈z, ξ〉 | 2n is the invariant Poisson kernel [1]. For the kernels, C and P, refer to [2]. And for all f - A(B), the ball algebra, such that f (0) is real, the reproducing property of 2C(z, ξ) - 1 [2, Theorem 3.2.5] gives  S K(z, ξ )Re f (ξ)dσ (ξ)=−i  f (z) − Re f (z)  =Imf (z) . For n = 1, the definition of Kfis the same as the classical harmonic conjugate func- tion and so we can regard Kfas the Hilbert transform on the unit circle. The L p boundedness property of harmonic conjugate functions on the unit circle for 1 <p < ∞ was introduced by Riesz in 1924 [3, Theorem 2.3 of Chapte r 3]. Later, in 1973, Hunt et al. [4] proved that, for 1 <p < ∞, conjugate functions are bounded on weighted mea- sured Lebesgue space if and only if the weight satisfies A p -condition. Most recently, Lee and Rim [5] provided an analogue of that of [4] by proving that, for 1 <p < ∞,M- harmonic conjugate operator K is bounded on L p ( ω) if and only if the nonnegative weight ω satisfies the A p (S)-condition on S; i.e., the nonnegative weight ω satisfies Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 © 2011 Rim and Lee; licensee Sprin ger. This is a n Open Access article distributed under the terms o f the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. sup Q 1 σ (Q)  Q ωdσ  1 σ (Q)  Q ω −1/(p−1) dσ  p−1 := A ω p < ∞ , where Q = Q(ξ, δ)={h Î S : d(ξ, h) = |1 - 〈ξ, h〉| 1/2 <δ} is a non-isotropic ball of S. To define the A p (S)-condition for two weights, we let (u, v) be a pair of two non- neg ative integrab le functions on S. For p > 1, we say that (u, v) satisfies two-w eighted A p (S)-condition if sup Q 1 σ (Q)  Q udσ  1 σ (Q)  Q v −1/(p−1) dσ  p−1 := A p < ∞ , (1:1) where Q is a non-isotropic ball of S. For p =1,theA 1 (S)-condition can be viewed as a limit case of the A p (S)-condition as p ↘ 1, which means that (u, v) satisfies the A 1 (S)-condition if sup Q 1 σ (Q)  Q udσ  esssup Q v −1  := A 1 < ∞ , where Q is a non-isotropic ball of S. In succession of classical weighted-norm inequalities, starting from Muck enhoupt ’ s result in 1975 [6], there have been extensive studies on two-weighted norm inequalities (for textbooks [7-10] and for related topics [11-17]). In [6], Muckenhoupt derives a necessary and sufficient condition on two-weighted norm inequalities for the Poisson integral operator. And then, Sawyer [18,19] obtained characterizations of tw o-weighted norm inequalities for the Hardy-Littlewood maxima l function and for the fractional and Poisson i ntegral operators, respectively. As a result on two- weighted A p (S)-condi- tion itself, Neugebauer [20] proved the existence of an inser ting pair of weights. Cruz- Uribe and Pérez [21] give a sufficient conditi on for Calderón-Zygmu nd operators to satisfy the weighted weak (p, p) inequality. More recently, Martell et al. [22] provide two-weighted norm inequalities for Calderón-Zygmund operators that are sharp for the Hilbert transform and for the Riesz transforms. Ding and Lin [23] consider the fractional integral operator and the maximal operator that contain a func tion homogeneous of degree zero as a part of kernels and the authors prove weighted (L p , L q )-boundedness for those operators for two weights. In [24], Muckenhoupt and Wheeden provided simple examples of a pair that satisfies two-wei ghted A p (ℝ)-condition but not two-weighted norm inequalities for the Hardy- Littlewood maximal operator and the Hilbert transform. In this article, we prove the converse of the main theorem of [5] by adding a doubling condition for a weight func- tion. And then by adding a suitable regularity condition on a weight function, we derive and prove a cross-weighted norm inequalities between the Hardy-Littlewood maximal function and the sharp maximal function. Throughout this article, Q denotes a non-isotropic ball of S induced b y the non-iso- tropic metric d on S which is defined by d(ξ, h) = |1 - 〈ξ, h〉| 1/2 . For notational simpli- city, we denote ʃ Q fds := f(Q)theintegraloff over Q,and 1 σ (Q)  Q fdσ := f Q the average of f over Q. Also, for a nonne gative integrable functio n u and a measurable subset E of S, we write u(E) for the integral of u over E. We write Q(δ) in place of Q(ξ, δ) whenever the center ξ has no meaning in a context. For a positive constant s, sQ(δ ) Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 2 of 14 means Q(sδ). We say that a weight v satisfies a doubling condition if there is a constant C independent of Q such that v(2Q) ≤ Cv(Q) for all Q. Theorem 1.1. Let 1<p < ∞ .If(u, v) satisfies two-weighted A p’ (S)-condition for some p’ <panduds,vds are doubling measures, then there exists a constant C which depends on u, v and p, such that for all function f,  S   Kf   p udσ ≤ C  S   f   p vdσ for all f ∈ L p (v) . (1:2) To prove the next theorem, we need a reg ularity condit ion for v such that for 1 ≤ p < ∞, we assume that for a measurable set E ⊂ Q and for s(E) ≤ θs(Q)with0≤ θ ≤ 1, we get v(E) ≤  1 − (1 − θ ) p  v(Q) . (1:3) Let f Î L 1 (S) and let 1 <p < ∞. The (Hardy-Littlewood) maximal and the sharp maxi- mal functions Mf,f #p , resp. on S are defined by Mf(ξ)=sup Q 1 σ (Q)  Q   f   dσ , f #p (ξ)=sup Q  1 σ (Q)  Q   f − f Q   p dσ  1/p , where each supremum is taken over all balls Q containing ξ. From the definition, the sharp maximal f unction f ↦ f #p is an analogue of the maximal function Mf,which satisfies f # 1 (ξ) ≤ 2Mf(ξ). Theorem 1.2. Let 1<p < ∞. If (u, v) satisfies two-weighted A p (S)-conditi on and vds does (1.3), then there exists a constant C which depends on u, v and p, such that for all function f,  S (Mf) p udσ ≤ C   S  f #1  p vdσ +  S   f   p vdσ  . Remark. On the unit circle, we derive a sufficient condition for weighted-norm inequalities for the Hilbert transform for two weights. The proofs of Theorem 1.1 will be given in Section 3. We start Section 2 by deriving some preliminary properties of (u, v) which satisfies the A p (S)-condition. In Section 4, we prove Theorem 1.2. 2 Two-weight on the unit sphere Lemma 2.1. If (u, v) satisfies two-weighted A p (S)-condition, then for every function f ≥ 0 and for every ball Q, (f Q ) p u(Q) ≤ A p  Q f p vdσ . Proof.Ifp = 1 and (u, v) satisfies two-weighted A 1 (S)-condition, we get, for every ball Q and every f ≥ 0, Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 3 of 14 f Q u(Q)=f (Q)u Q ≤ A 1 f (Q) 1 ess sup Q v −1 ≤ A 1  Q fv dσ , since 1/ ess sup Q v −1 =essinf Q v ≤ (ξ ) for all ξ Î Q. If 1 <p < ∞ and (u, v) satisfi es two-weighted A p (S)-condition, we have, for ev ery ball Q and every f ≥ 0, using Holder’s inequality with p and its conjugate exponent p/(p - 1), f Q = 1 σ (Q)  Q fv 1/p v −1/p dσ ≤  1 σ (Q)  Q f p vdσ  1/p  1 σ (Q)  Q v −1/(p−1) dσ  (p−1)/ p Hence,  f Q  p u(Q)= u(Q) σ (Q)  1 σ (Q)  Q v −1/(p−1) dσ  p−1  Q f p vd σ ≤ A p  Q f p vdσ . Therefore, the proof is complete. Corollary 2.2. If (u, v) satisfies two-weighted A p (S)-condition, then  σ (E) σ ( Q )  p u(Q) ≤ A p v(E) , where E is a measurable subset of Q. Proof. Applying Lemma 2.1 with f replaced by c E proves the conclusion. 3 Proo f of Theorem 1.1 In thi s section, we will prove the first main theorem. First, we derive the inequality between two sharp maximal functions of Kfand f. Lemma 3.1. Let f Î L 1 (S). Then, for q >p >1,there is a constant C p,q such that (Kf) #p (ξ) ≤ C p,q f #q (ξ) for almost every ξ. Proof. It suffices to show that for r ≥ 1 and q > 1, there is a constant C rq such that (K f) #r (ξ) ≤ C rq f #rq (ξ) for almost every ξ, i.e., for Q = Q(ξ Q , δ ) a ball of S, we prove that there are constants l = l(Q, f) and C rq such that  1 σ (Q)  Q   Kf(η) − λ   r dσ  1 / r ≤ C r,q f # q (ξ Q ) . (3:1) Fix Q = Q(ξ Q , δ) and write f (η)=  f (η) − f Q  χ 2Q (η)+  f (η) − f Q  χ S\2Q (η)+f Q := f 1 (η)+f 2 (η)+f Q . Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 4 of 14 Then, Kf= Kf 1 + Kf 2 , since Kf Q =0. For each z Î B, put g(z)=  S  2C(z, ξ ) − 1  f 2 (ξ)dσ (ξ) . Then, g is continuous on B ∪ Q By setting l =-ig(ξ Q ) in (3.1), we shall drive the conclusion. By Minkowski’s inequality, we split the integral in (3.1) into two parts,  1 σ (Q)  Q   Kf(η)+ig(ξ Q )   r dσ (η)  1 / r ≤  1 σ (Q)  Q   Kf 1   r dσ  1/r +  1 σ (Q)  Q   Kf 2 + ig(ξ Q )   r dσ  1/r := I 1 + I 2 . (3:2) We estimate I 1 . By Holder’s inequality, it is estimated as I 1 ≤  1 σ (Q)  Q   Kf 1   rq dσ  1 / rq ≤  1 σ (Q)  S   Kf 1   rq dσ  1/rq ≤ C rq σ ( Q ) 1/rq   f 1   L rq , since K is bounded on L rq (S)(rq > 1). By replacing f 1 by f - f Q , we get   f 1   L rq =   2Q   f − f Q   rq dσ  1/rq ≤   2 Q   f − f 2Q   rq dσ  1/rq + σ(2Q) 1/rq   f 2Q − f Q   . Thus, by applying Hölder’s inequality in the last term of the above, σ (2Q) 1/rq   f 2Q − f Q   ≤ σ (2Q) 1 / rq σ (Q)  Q   f − f 2Q   dσ ≤ σ (2Q) 1/rq σ (Q) 1−1/rq σ (Q)   2Q   f − f 2Q   rq dσ  1/r q = R 1/rq 2   2 Q   f − f 2Q   rq dσ  1/rq (by(4.2)). Hence, I 1 ≤ C rq  1+R 1/rq 2  f # rq (ξ Q ) . (3:3) Now, we estimate I 2 . Since f 2 ≡ 0on2Q, the invariant Poisson integral of f 2 vanishes on Q, i.e., lim t  1  S P( tη, ξ )f 2 (η) dσ (η)= 0 whenever ξ Î Q. Thus, for almost all ξ Î Q, iK f 2 (ξ)=  S \ 2Q  2C(ξ, η) − 1  f 2 (η) dσ (η)=g(ξ ) Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 5 of 14 and then, by Minkowski’s inequality for integrals, I 2 =  1 σ (Q)  Q   iK f 2 − g(ξ Q )   r dσ  1/r ≤  S \ 2Q 2   f 2 (η)    1 σ (Q)  Q   C(ξ, η) − C(ξ Q , η)   r dσ (ξ)  1/r dσ (η) . By Lemma 6.1.1 of [2], we get an upper bound such that I 2 ≤ Cδ  S\2Q   f 2 (η)     1 −  η, ξ Q    n+1/2 dσ (η) , (3:4) where C is an absolute constant. Write S\2Q =  ∞ k =1 2 k+1 Q\2 k Q . Then, the integral of (3.4) is equal to ∞  k=1  2 k+1 Q\2 k Q   f (η) − f Q     1 −  η, ξ Q    n+1/2 dσ (η) ≤ ∞  k=1 1 2 (2n+1)kδ 2n+1  2 k+1 Q\2 k Q   f − f Q   dσ ≤ ∞  k=1 1 2 (2n+1)kδ 2n+1 ⎛ ⎝  2 k+1 Q   f − f 2 k+1 Q   dσ + k  j=0  2 k+1 Q   f 2 j+1 Q − f 2 j Q   dσ ⎞ ⎠ . By Hölder’s inequality, by (4.3),  2 k+1 Q   f − f 2 k+1 Q   dσ ≤ R 2 k+1 δ  1 σ  2 k+1 Q   2 k+1 Q   f − f 2 k+1 Q   rq dσ  1 / r q ≤ R 2 k+1 δ f # rq (ξ Q ), (3:5) Similarly, for each j,  2 k+1 Q   f 2 j+1 Q − f 2 j Q   dσ ≤ σ  2 k +1 Q  σ (2 j Q)  2 j Q   f − f 2 j+1 Q   dσ ≤ R 2 k−j+1  2 j+1 Q   f − f 2 j+1 Q   dσ (by (4.2)) ≤ R 2 k−j+1 R 2 j+1 δ f # rq (ξ Q )  from (3.5) with k = j  = R 1 R 2 k+2 δ f # rq (ξ Q ). Thus, k  j =0  2 k+1 Q   f 2 j+1 Q − f 2 j Q   dσ ≤ ( k +1 ) R 1 R 2 k+2 δ f # rq (ξ Q ) . (3:6) Since R s increases as s ↗ ∞ and R 1 > 1, by adding (3.5) to (3.6), we have the upper bound as (k +2)R 1 R 2 k+2 δ f # rq (ξ Q ) . Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 6 of 14 Eventually, the identity of R 2 k+2 δ = R 1 2 2n (k +2 ) δ 2 n yields that I 2 ≤ 2 4n CR 2 1 ∞  k =1 k +2 2 k f # rq (ξ Q ) , (3:7) and therefore, combining (3.3) and (3.7), we complete the proof. The main t heorem depends on Marcinkiewicz interpolation theorem betw een two abstract Lebesgue spaces, which is as follows. Prop osition 3.2. Suppose (X, μ) and (Y, ν) are measure spaces; p 0 , p 1 , q 0 , q 1 are ele- ments of [1, ∞] such that p 0 ≤ q 0 , p 1 ≤ q 1 and q 0 ≠ q 1 and 1 p = 1+t p 0 + t p 1 , 1 q = 1 − t q 0 + t q 1 (0 < t < 1) . If T is a sublinear map from L p 0 ( μ ) + L p 1 ( μ ) to the space of measurable functions on Y which is of weak-types (p 0 ,q 0 ) and (p 1 ,q 1 ), then T is of type (p, q). Now, we prove the main theorem. Proof of Theorem 1.1. Under the assumption of the main theorem, we will prove that (1.2) holds. We fix p > 1 and let f Î L p (v). By Theorem 1.2, there is a constant C p such that  S   Kf   p udσ ≤  S   M u (Kf)   p udσ ≤ C p  S    (Kf) # 1    p udσ ≤ C p  S    f # q    p udσ  by Lemma 3.1 with q > 1, p/q > 1  ≤ 2 p C p  S  M   f   q  p/q udσ (by the triangle inequality). (3:8) where M u is the maximal operator with respect to uds, the second inequality follows from the doubling condition of uds. Without loss of generality, we assume f ≥ 0. By Holder’s inequality and by (1. 1), we have 1 σ (Q)  Q dσ ≤  1 σ (Q)  Q f p/q vdσ  q/p  1 σ (Q)  Q v −1/(p/q−1) dσ  1−q/ p ≤ A q/p p/q  1 σ (Q)  Q f p/q vdσ  q/p  σ (Q) v(Q)  q/p for all Q. Thus, if f Q >l, then u (Q) ≤ A p/q λ p/q  Q f p/q vdσ for all Q . (3:9) Let E be an arbitrary compact subset of {ξ Î S: Mf(ξ)>l}. Since vd s is a doubl ing measure, from (3.9), there exists a constant C p,q such that u (E) ≤ C p,q λ p/q  S f p/q vdσ . Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 7 of 14 Thus, Mfis of weak-type (L p/q (vds),L p/q (uds)). Moreover,   Mf   L ∞ (udσ ) ≤   Mf   L ∞ (since udσ  dσ) ≤   f   L ∞ =   f   L ∞ ( udσ ) (since udσ  dσ, v > 0 a.e. by (1.1)) . Now, by Proposition 3.2, Mfis of type (L r (vds), L r (uds)) for f >p/q. Hence, the last integral of (3.8) is bounded by some constant times  S   f   qr vdσ (for all r > p/q) . Since q is arbitrary so that p/q > 0, we can replace qr by p with p > 1. Therefore, the proof is completed. 4 Proo f of Theorem 1.2 Theorem 1.2 can be regarded as cross-weighted norm inequalities for the Hardy-Little- wood maximal fun ction and the sharp maximal function on the unit sphere. For a sin- gle A p -weight in ℝ n , refer to Theorem 2.20 of [8]. From Proposition 5.1.4 of [2], we conclude that when n >1,  2 (n/2 + 1) 2 n−2  ( n +1 ) s 2n ≤ σ (Q(sδ)) σ ( Q ( δ )) ≤ 2 n−2 (n +1)  2 ( n/2+1 ) s 2n , and when n =1, 2 π s 2 ≤ σ (Q(sδ)) σ ( Q ( δ )) ≤ π 2 s 2 for any s > 0. Throughout the article, several kinds of constants will appear. To avoid confusion, we define the maximum ratio between sizes of two balls by R s : R s,n =max  2 n−2 (n +1)  2 ( n/2 + 1 ) , π 2  s 2n , (4:1) and thus, for every s > 0, for every δ >0, σ ( sQ ( δ )) ≤ R s σ ( Q ( δ )). (4:2) Putting δ = 1 in (4.2), we get σ ( Q ( s )) ≤ R s . (4:3) To prove Theorem 1.2, we need some lemmas. The next result is a covering lemma on the unit sphere, related to the maximal function. Let f Î L 1 (S)andlet t >   f   L 1 ( S ) . We may assume   f   L 1 ( S ) = 0 . Since {Mf>t} is open, take a ball Q ⊂ {Mf>t} wit h cen- ter at each point of {Mf>t}. For such a ball Q, σ (Q) ≤ 1 t  Q   f   dσ . (4:4) Thus, to each ξ Î {Mf>t} corresponds a largest radius δ such th at the ball Q = Q(ξ, δ) ⊂ {Mf>t} satisfies (4.4). Hence, we conclude the following simple covering lemma. Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 8 of 14 Lemma 4.1 (Covering l emma on S). Let f Î L 1 (S) be non-trivial. Then, for t >   f   L 1 ( S ) , there is a collection of balls {Q t,j } such that (i)  ξ ∈ S : Mf (ξ ) > t  ⊂  j Q t,j , (ii) σ (Q t,j ) ≤ t −1   f   (Q t,j ) , where each Q t,j has the maximal radius of all the balls that satisfy (ii) in the sense that if Q is a ball that contains some Q t,j as its proper subset, then s(Q)>t -1 ʃ Q |f| ds holds. Now, we are ready to prove Theorem 1.2. Proof of Theorem 1.2.Fix1<p < ∞. We may assume f # 1 Î L p (v) and f Î L p (v), other- wise, Theorem 1.2 holds clearly. Since v satisfies the doubling condition, we have ||M f|| Lp(v) ≤ C||f # 1 || Lp(v) . Combining this with f # 1 Î L p (v), we have ||Mf|| Lp(v) < ∞. Suppose that f is non-trivial and we may assume that f ≥ 0. Let t > max  2, 2R 2 2 , R 3    f   L 1 ( S ) . For ε >0,E t be a compact subset of {Mf>t} such t hat u({Mf>t}) <u(E t )+e - t ε. Indeed, since u is integrable, uds is a regular Borel measure absolutely continuous with respect to s. Suppose {Q t,j } is a collection of balls having the properties (i) and (ii) of Lemma 4.1. Since {Q t,j } is a cover of a compact set E t , there is a finite subcollect ion of {Q t,j }, wh ich covers E t . By Lemma 5.2.3 of [2], there are pairwise disjoint balls, Q t, j 1 , Q t, j 2 , , Q t, j  of the previous subcollection such that E t ⊂   k=1 3Q t,j k , σ (E t ) ≤ R 3   k =1 σ (Q t,j k ) , where ℓ may depend on t. To avoid the abuse of subindices, we rewrite Q t, j k as Q t,j . Let us note that from the maximality of Q t,j , t > 1 σ (2Q t.j )  2Q t, j fdσ ≥ σ (Q t.j ) σ (2Q t.j ) t ≥ R −1 2 t . (4:5) Fix Q 0 =2Q kt, j 0 , where  -1 =2R 2 . (Here,  < 1/2, since R 2 > 1.) Let l > 0 that will be chosen late r. From the definition of the sharp maximal function, there are two possibi- lities: either Q 0 ⊂{ f # 1 >λt} or Q 0 ⊂{ f # 1 >λt} . (4:6) In the first case, since Q t,j ’s are pairwise disjoint,   j:Q t,j ⊂Q 0 ⊂{f # 1 >λt}  v(Q t,j ) ≤ v({f # 1 >λt}) , Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 9 of 14 and also,  Q 0 Q 0 ⊂{f # 1 >λt}  {j:Q t,j ⊂Q 0 } v(Q t,j ) ≤ v({f # 1 >λt}) . (4:7) In the second case, 1 σ (Q 0 )  Q 0 |f − f Q 0 |dσ ≤ λt . (4:8) Since 2 −1 t >   f   L 1 ( S ) , by (4.5), taking f Q 0 ≤ R 2 kt =2 −1 t into account, we have   j:Q t,j ⊂Q 0 ⊂{f # 1 >λt}  (t − t/2)σ (Q t,j ) ≤   j:Q t,j ⊂Q 0 ⊂{f # 1 >λt}   Q t,j f − f Q 0 dσ ≤   j:Q t,j ⊂Q 0 ⊂{f # 1 >λt}   Q t,j |f − f Q 0 |d σ ≤  Q 0 |f − f Q 0 |dσ ≤ λtσ ( Q 0 )( by ( 4.8 )) . Thus,   j:Q t,j ⊂Q 0 ⊂{f # 1 >λt}  σ (Q t,j ) ≤ 2λσ (Q 0 ) . (4:9) In (4.9), take a small l > 0 such that 2 λ < 1 . (4:10) (Note that the condition (4.10) enables us to use (1.3).) Thus, (4.9) can be written as   j:Q t.j ⊂Q 0 ⊂{f # 1 >λt}  v(Q t,j ) ≤  1 − (1 − 2λ) p  v(2Q κt,j 0 ) . Adding up all possible Q 0 ’s in the second case of (4.6), we get  Q 0 Q 0 ⊂{f # 1 >λt}  {j:Q t,j ⊂Q 0 } v(Q t,j ) ≤  1 − (1 − 2λ) p   k v(2Q κt,k ) . (4:11) Since  Mf > t  ⊂  Mf > R − 1 2 t  and σ  2Q t,j  ≤ R 2 t −1  2Q t,j fd σ holds (4.5), we can construct the collection of balls  Q R −1 2 t,j  which covers  Mf > R − 1 2 t  with maximal radius just the same way as Lemma 4.1, so that 2Q t,j is contained in  Q R −1 2 t,i  for some i. Recall that R − 1 2 kt =2 −1 R − 2 2 t >   f   L 1 ( S ) , hence, (4.11) turns into  Q 0 Q 0 ⊂{f # 1 >λt}  {j:Q t,j ⊂Q 0 } v(Q t,j ) ≤  1 − (1 − 2λ) p   k v(Q R −1 2 κt,k ) . (4:12) Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 10 of 14 [...]... sufficient condition for two-weighted norm inequalities for K In proving cross-weighted norm inequalities between the Hardy-Littlewood maximal function and the sharp maximal function on the unit sphere, and JL carried out the study about the covering lemma on the sphere All authors read and approved the final manuscript Competing interests The author Kyung Soo Rim was supported in part by a National... 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Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 Page 12 of 14 Hence, |Mf |p u dσ ≤ Np Ap f S p Lp (v) p +Ap Cp R3 1 |f # |p v dσ + ep ε S The first and the last integrals are independent of ε Letting ε ↘ 0, therefore, the proof is complete after accepting Lemma 4.3 Lemma 4.2 Let av be defined in (4.14) Then, for every q... maximal operators and fractional integrals on nonhomogeneous spaces Indiana Univ Math J 50, 1241–1280 (2001) 14 Pérez, C: Two weighted norm inequalities for Riesz potentials and uniform Lp-weighted Sobolev inequalities Indiana Univ Math J 39, 31–44 (1990) doi:10.1512/iumj.1990.39.39004 15 Pérez, C: Two weighted inequalities for potential and fractional type maximal operators Indiana Univ Math J 43, 663–683... two-weight norm inequality for maximal operators Studia Math 75, 1–11 (1982) 19 Sawyer, ET: A characterization of two weight norm inequalities for fractional and Poisson integrals Trans Amer Math Soc 308, 533–545 (1988) doi:10.1090/S0002-9947-1988-0930072-6 20 Neugebauer, CJ: Inserting Ap-weights Proc Amer Math Soc 87, 644–648 (1983) 21 Cruz-Uribe, D, Pérez, C: Sharp two-weight, weak-type norm inequalities for. .. doi:10.1512/iumj.1994.43.43028 Rim and Lee Journal of Inequalities and Applications 2011, 2011:117 http://www.journalofinequalitiesandapplications.com/content/2011/1/117 16 Li, W: Two-weight norm inequalities for commutators of potential type integral operators J Math Anal Appl 322, 1215–1223 (2006) doi:10.1016/j.jmaa.2005.08.051 17 Zani, SL: Two-weight norm inequalities for maximal functions on homogeneous spaces... 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Ding and Lin [23] consider the fractional integral operator. not two-weighted norm inequalities for the Hardy- Littlewood maximal operator and the Hilbert transform. In this article, we prove the converse of the main theorem of [5] by adding a doubling. sufficient condition for two-weighted norm inequalities for K. In proving cross-weighted norm inequalities between the Hardy-Littlewood maximal function and the sharp maximal function on the unit sphere,