Journal of Inequalities in Pure and Applied Mathematics CONVOLUTION OPERATORS WITH HOMOGENEOUS SINGULAR MEASURES ON R3 OF POLYNOMIAL TYPE. THE REMAINDER CASE. volume 7, issue 3, article 89, 2006. MARTA URCIUOLO Famaf-Ciem, Universidad Nacional de Córdoba-Conicet. Medina Allende s/n Ciudad Universitaria 5000, Córdoba, Argentina. Received 22 December, 2005; accepted 23 September, 2006. Communicated by: A. Fiorenza EMail: urciuolo@mate.uncor.edu Abstract Contents JJ J II I Home Page Go Back Close c 2000 Victoria University ISSN (electronic): 1443-5756 374-05 Quit Abstract Let ϕ (y1 , y2 ) = y2l P (y1 , y2 ) where P is a polynomial function of degree l such 3 that R P (1, 0) 6= 0. Let µδ be the Borel measure on R defined by µδ (E) = Vδ χE (x, ϕ (x)) dx where Vδ = x = (x1 , x2 ) ∈ R2 : |x1 | ≤ 1, and |x1 | ≤ δ |x2 | and let Tµδ be the convolution operator with the measure µδ . In this paper we explicitely describe the type set 1 1 Eµδ := , ∈ [0, 1] × [0, 1] : kTµδ kp,q < ∞ , p q Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo for δ small enough. Title Page 2000 Mathematics Subject Classification: 42B20, 26B10. Key words: Convolution operators, Singular measures. Contents JJ J Partially supported by Conicet, Agencia Córdoba Ciencia, Agencia Nacional de Promoción Científica y Tecnológica y Secyt-UNC. The author is deeply indebted to Prof. F. Ricci for his useful suggestions. II I Go Back Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 The Main Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References Close 3 5 Quit Page 2 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au 1. Introduction Let ϕ : R2 → R be a homogeneous polynomial function of degree m ≥ 2 and let D = {y ∈ R2 : |y| ≤ 1} . Let µ be the Borel measure on R3 given by Z (1.1) µ (E) = χE (y, ϕ (y)) dy D and let Tµ be the operator for f ∈ S (R3 ) , by Tµ f = µ ∗ f. Let Eµ defined, be the set of the pairs p1 , 1q ∈ [0, 1] × [0, 1] such that there exists a positive constant c satisfying kT f kq ≤ c kf kp for all f ∈ S (R3 ) , where the Lp spaces 1 1 3 are taken with respect to the Lebesgue measure on R . For p , q ∈ Eµ , T can be extended to a bounded operator, still denoted by T, from Lp (R3 ) into Lq (R3 ) . Let ϕ = ϕe11 ...ϕenn be a decomposition of ϕ in irreducible factors with ϕi - ϕj for i 6= j. In [3] we could give a complete description of the set Eµ under the assumption that ei 6= m2 for each ϕi of degree 1. If det ϕ00 (y) is not identically zero and if it vanishes somewhere on R2 − {0}, the set of the points y where det ϕ00 (y) vanishes is a finite union of lines L1 , ..., Lk through the origin. So, after a possibly linear change of variables, we localized the problem to the x axes and we studied the type set corresponding to measures µδ defined by Z µδ (E) = χE (y, ϕ (y)) dy, Vδ where Vδ = D ∩ {(y1 , y2 ) ∈ R2 : |y2 | ≤ δ |y1 |} and δ is small enough such that det ϕ00 (y) only vanishes, on Vδ , along the x axes. The only case left was the one Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back Close Quit Page 3 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au corresponding to functions ϕ of the form ϕ (y1 , y2 ) = y2l P (y1 , y2 ) with l = m2 , P being a homogeneous polynomial function of degree l such that P (1, 0) 6= 0. In this paper we characterize Eµδ in this remainder case. Lp improving properties of convolution operators with singular measures supported on hypersurfaces in Rn have been widely studied in [2], [5], [6]. In particular, in [5], the type set was studied under our actual hypothesis, but the endpoint problem was left open there. Our proof of the main result involves a biparametric family of dilations and will be based on a suitable adaptation of arguments due to M. Christ, developed in [1], where the author studied the type set associated to the two dimensional measure supported on the parabola. Also, oscillatory integral estimates are involved. A very careful study of this kind of estimate can be found in [4] where the authors study the boundedness of maximal operators associated to mixed homogeneous hypersurfaces. Throughout this paper c will denote a positive constant, not the same at each occurrence. Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back Close Quit Page 4 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au 2. The Main Result We assume ϕ (y1 , y2 ) = y2l P (y1 , y2 ) , where l = m2 and P is a homogeneous polynomial function of degree l such that P (1, 0) 6= 0. We take δ1 > 0 such that, for y ∈ Vδ1 such that y2 6= 0, det ϕ00 (y) 6= 0. Moreover, since P (1, 0) 6= 0 we can assume that P (y) 6= 0 and P1 (y) 6= 0 for all y ∈ Vδ1 . Now, if maxVδ1 |P2 (y1 , y2 )| = 6 0, we choose δ < min l minVδ |P (y1 ,y2 )| 1 2 maxVδ |P2 (y1 ,y2 )| , δ1 . In the 1 other case we take δ = δ1 . The main result we prove is the following. Theorem 2.1. Let ϕ (y1 , y2 ) = y2l P (y1 , y2 ) where l = m2 and P is a homogeneous polynomial function of degree l such that P (1, 0) 6= 0 and y2 - P (y1 , y2 ) . Let Vδ be defined as above and let EVδ be the corresponding type set. Then EVδ is the closed polygonal region with vertices (0, 0) , (1, 1) , 2l+1 , 2l−1 and 2l+2 2l+2 3 , 1 . 2l+2 2l+2 Standard arguments (see, for example Lemma 2 and Lemma 3 in [3]) imply the following result. 1 Lemma 2.2. If p1 , 1q ∈ Eµδ then 1q ≤ p1 , 1q ≥ p3 − 2 and 1q ≥ p1 − l+1 . So, since kTµδ k1,1 < ∞, by duality arguments it only remains to prove that (2.1) kTµδ k 2l+2 , 2l+2 2l+1 2l−1 < ∞. δ δ ∞ 2 We set Q0 = 41 , 2 × 64 , 8 . We take a truncation 1 function δ δ θ ∈ C (R ), θ (y1 , y2 ) ≥ 0, supp θ ⊂ Q0 and θ (y1 , y2 ) = 1 on 2 , 1 × 32 , 16 . We define, Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back Close Quit Page 5 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au for ε, γ > 0, the biparametric family of dilations on R2 and R3 given by (ε, γ) ◦ l l (y1 , y2 ) = (εy1 , γy2 ) and (ε, γ) ◦ (y1 , y2 , y3 ) = εy1 , γy2 , ε γ y3 repectively. Also, for j, k ≥ 0, we set Qj,k = 2−j , 2−k ◦ Q0 . For f ∈ S (R3 ) , we define (2.2) Tj,k f (x1 , x2 , x3 ) Z = f (x1 − y1 , x2 − y2 , x3 − ϕ (y1 , y2 )) θ 2j y1 , 2k y2 dy1 dy2 so for f ≥ 0, Tµ δ f ≤ c (2.3) 8 X Tj,k f. Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case 0≤j≤k Marta Urciuolo To study P Tj,k f, we will adapt the argument developed by M. Christ (see 0≤j≤k [1]) to the setting of biparametric dilations. First of all, we prove the following Title Page Proposition 2.3. There exists a positive constant c > 0 such that for 0 ≤ j ≤ k, Contents kTj,k k 2l+2 , 2l+2 ≤ c. 2l+1 2l−1 Proof. Tj,k f (x1 , x2 , x3 ) Z = f (x1 − y1 , x2 − y2 , x3 − ϕ (y1 , y2 )) θ 2j y1 , 2k y2 dy1 dy2 Z −(j+k) =2 f x1 − 2−j y1 , x2 − 2−k y2 , x3 − ϕ 2−j y1 , 2−k y2 θ (y1 , y2 ) dy1 dy2 = 2−(j+k) T (j−k) fj,k 2j x1 , 2k x2 , 2(j+k)l x3 , JJ J II I Go Back Close Quit Page 6 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au where we denote T (j) Z f (x1 , x2 , x3 ) = f x1 − y1 , x2 − y2 , x3 − y2l P y1 , 2j y2 θ (y1 , y2 ) dy1 dy2 and fj,k (x1 , x2 , x3 ) = f 2−j , 2−k ◦ (x1 , x2 , x3 ) . So (2.4) kTj,k f (x1 , x2 , x3 )kq = 2(j+k)( 1+l − 1+l −1 p q ) T (j−k) p,q Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case kf kp . Now, det y2l P y1 , 2j−k y2 00 Marta Urciuolo = 2(2−2l)(j−k) det (ϕ)00 y1 , 2j−k y2 . so in the proof of Lemma 4 in [3] we obtain that there exists c > 0 such that as T (j−k) 2l+2 2l+2 ≤ c for 0 ≤ j ≤ k, and the proposition follows. , 2l+1 2l−1 We take 0 ≤ j ≤ k, and denote by µj,k and µ(j) the measures associated to Tj,k and T (j) respectively. For ξ = (ξ1, ξ2 , ξ3 ) , Z l j−k \ (j−k) µ (ξ) = e−i(ξ1 y1 +ξ2 y2 +ξ3 y2 P (y1 ,2 y2 )) θ (y1 , y2 ) dy1 dy2 . Title Page Contents JJ J II I Go Back Close (j−k) If for some ξ on the unit sphere, Ωξ (y1 , y2 ) = ξ1 y1 +ξ2 y2 +ξ3 y2l P y1 , 2j−k y2 has a critical point belonging to the supp θ, then ξ1 + ξ3 y2l P1 y1 , 2j−k y2 = 0 Quit Page 7 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au and ξ2 + ξ3 2j−k y2l P2 y1 , 2j−k y2 + ly2l−1 P y1 , 2j−k y2 = 0, but then, since P1 (y) 6= 0 for y ∈ Vδ1 , from the first equation we obtain that there exist constants a, b ∈ Z with a < b such 2a |ξ3 | ≤ |ξ1 | ≤ 2b |ξ3 | , and, from the second one and the choice of δ we obtain constants c, d ∈ Z2 with c < d such that 2c |ξ3 | ≤ |ξ2 | ≤ 2d |ξ3 |. So ξ belongs to the cone C0 = ξ ∈ R3 : 2a |ξ3 | < |ξ1 | < 2b |ξ3 | , 2c |ξ3 | < |ξ2 | < 2d |ξ3 | . Lemma 2.4. Suppose C0 is as above. Then the family of cones 2j , 2k ◦ C0 j,k∈Z has finite overlapping (i.e., # (j, k) ∈ Z2 : C0 ∩ 2j , 2k ◦ C0 6= ∅ < ∞). Proof. We suppose ξ ∈ C0 and 2j , 2k ◦ ξ ∈ C0 , then 2a |ξ3 | < |ξ1 | < 2b |ξ3 | , 2c |ξ3 | < |ξ2 | < 2d |ξ3 | Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents and (j+k)l+a 2 j (j+k)l+b |ξ3 | < 2 |ξ1 | < 2 |ξ3 | , 2(j+k)l+c |ξ3 | < 2k |ξ2 | < 2(j+k)l+d |ξ3 | so 2j |ξ1 | < 2(j+k)l+b |ξ3 | < 2(j+k)l+b−a |ξ1 | and 2b |ξ3 | > |ξ1 | > 2−j 2(j+k)l+a |ξ3 | , JJ J II I Go Back Close Quit Page 8 of 15 so a − b − kl < j (l − 1) < b − a − kl, J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au analogously we obtain c − d − jl < k (l − 1) < d − c − jl, thus (c − d) (l − 1) + (a − b) l (d − c) (l − 1) + (b − a) l <k< 2 2 l − (l − 1) l2 − (l − 1)2 and so a−b (d − c) (l − 1) + (b − a) l −l l−1 l2 − (l − 1)2 (l − 1) (b − a) (d − c) (l − 1) + (b − a) l +l . <j< l−1 l2 − (l − 1)2 (l − 1) Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents We define m0 (ξ) = n (ξ1 , ξ3 ) r (ξ2 , ξ3 ) where n and r belong to C ∞ (R2 − {0}) , are homogeneous of degree zero with respect to the isotropic dilations, supp n ⊂ (ξ1 , ξ3 ) : 2a−1 |ξ3 | < |ξ1 | < 2b+1 |ξ3 | n ≥ 0 and n ≡ 1 on (ξ1 , ξ3 ) : 2a |ξ3 | < |ξ1 | < 2b |ξ3 | , supp r ⊂ (ξ2 , ξ3 ) : 2c−1 |ξ3 | < |ξ2 | < 2d+1 |ξ3 | , JJ J II I Go Back Close Quit Page 9 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au r ≥ 0 and r ≡ 1 on (ξ2 , ξ3 ) : 2c |ξ3 | < |ξ2 | < 2d |ξ3 | , so m0 is homogeneous of degree zero with respect to the isotropic dilations, it belongs to C ∞ on each octant of R3 , m0 ≥ 0, m0 ≡ 1 on C0 and f0 supp m0 ⊂ C = ξ ∈ R3 : 2a−1 |ξ3 | < |ξ1 | < 2b+1 |ξ3 | , 2c−1 |ξ3 | < |ξ2 | < 2d+1 |ξ3 | . For (j, k) ∈ Z2 , we define mj,k (ξ) = m0 2−j , 2−k ◦ ξ and Qj,k the operator with multiplier mj,k . If ξ belongs to an open octant of R3 then ξ belongs to |ξ1 | 2| 2j , 2k ◦ C0 for some (j, k) ∈ Z2 (indeed 2−k ∼ |ξ and 2−j ∼ |ξ ) and from |ξ3 | 3| the previous lemma, it belongs to a finite number of them (independent of ξ). P So mj,k (ξ) ≤ c. Now it is easy to check that, for 1 < p < ∞, there Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo (j,k)∈Z2 exists Ap > 0 such that for f ∈ L2 ∩ Lp and any choice of εj,k = ±1, X ≤ Ap kf k . (2.5) ε Q f j,k j,k p (j,k)∈Z2 p Indeed, we now show that m (ξ) = Title Page Contents JJ J II I Go Back X εj,k mj,k (ξ) (j,k)∈Z2 satisfies the hypothesis of the Marcinkiewicz Theorem, as stated in Theorem 6’ in [7]. Close Quit Page 10 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au We have just observed that |m (ξ)| ≤ X mj,k (ξ) ≤ c. (j,k)∈Z2 Now we want to estimate ∂ξ∂1 m (ξ) . We recall that ∂ξ∂1 m0 is homogeneous of degree −1. We pick ξ in an open octant. In a small neighborhood of ξ only finitely many (j, k) ∈ Z2 (independent of ξ) are involved. For each one of them, ∂ ∂ mj,k (ξ) = 2−j m0 2−j ξ1 , 2−k ξ2 , 2−(j+k)l ξ3 ∂ξ1 ∂ξ1 −1 −1 −j −j ≤ c2 2 ξ1 , 2−k ξ2 , 2−(j+k)l ξ3 ≤ c2−j 2−j ξ1 , so Z 2s+1 sup ξ2 ,ξ3 2s ∂ ∂ξ1 m (ξ) dξ1 ≤ c, and in a similar way (using the homogeneity of the derivatives of mj,k ) we obtain that for each 0 < k ≤ 3, Z ∂k m (ξ) dξ1 ≤ c, sup ξk+1 ,...,ξ3 ρ ∂ξ1 ...∂ξk k as ρ ranges over dyadic rectangles of R and that this inequality holds for every one of the six pemutations of the variables ξ1 , ξ2 , ξ3 . We now define h (ξ) ∈ C ∞ (R3 ) , h ≥ 0, h ≡ 1 on the unit ball of R3 , hj,k (ξ) = h 2−j , 2−k ◦ ξ and Rj,k the operators with multipliers hj,k . Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back Close Quit Page 11 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au Lemma 2.5. There exists a constant C > 0, independent of K, such that X Tj,k Rj,k ≤ C. 0≤j≤k≤K 2l+2 , 2l+2 2l+1 2l−1 Proof. Let Kj,k be the kernel of Tj,k Rj,k . A computation shows that, Kj,k (x) = 2(j+k)l µ(j−k) ∗ hˆ∨ 2j , 2k ◦ x . Thus X X |Kj,k (ξ)| ≤ 0≤j≤k≤K 2(j+k)l G(j,k) 2j , 2k ◦ ξ 0≤j≤k ∧ ∧ with G(j,k) defined by G(j,k) = µ(j−k) h. Since j − k ≤ 0, as in Lemma 7 ∧ in [3] we obtain that G(j,k) ∈ S (R3 ) with each seminorm bounded on j, k, it follows that the same holds for G(j,k) . Now X 2(j+k)l G(j,k) 2j , 2k ◦ ξ 0≤j≤k X ≤ 2ja+ka+ha G(j,k,h) 2j ξ1 , 2k ξ2 , 2h ξ3 j,k,h≥0 l , l+1 (j,k,h) (j,k) Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back (j,k,h) with a = G =G for h = l (j + k) and G = 0 otherwise. It is well known that from the uniform boundedness properties of G(j,k,h) it follows that X c 2ja+ka+ha G(j,k,h) 2j ξ1 , 2k ξ2 , 2h ξ3 ≤ a a a, |ξ 1 | |ξ2 | |ξ3 | j,k,h≥0 Close Quit Page 12 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au so X |Kj,k (ξ)| ≤ 0≤j≤k≤K so P c |ξ1 | l l+1 l l , |ξ2 | l+1 |ξ3 | l+1 Tj,k Rj,k convolves Lp (R3 ) into Lq (R3 ) for 0≤j≤k≤K 1 q 1 = p1 − l+1 with bounds independent of K. Lemma 2.6. There exists a constant C > 0, independent of K, such that X ≤ C. T (I − P ) (I − Q ) j,k j,k j,k 1≤j≤k≤K 2l+2 , 2l+2 2l+1 2l−1 Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Proof. The kernel Hj,k of X Tj,k (I − Pj,k ) (I − Qj,k ) Contents 1≤j≤k≤K satisfies X 1≤j≤k≤K |Hj,k (ξ)| ≤ Title Page X 2(j+k)l g (j,k) 2j , 2k ◦ ξ 0≤j≤k ∧ ∧ with g (j,k) defined by g (j,k) = µ(j−k) (1 − h) (1 − m0 ) . ∧ Observe that, from Lemma 7 in [3], we have µ(j−k) (1 − h) (1 − m0 ) ∈ S(R3 ) with each seminorm bounded on j, k. From this fact the proof follows as in the previous lemma. JJ J II I Go Back Close Quit Page 13 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au Proof of the theorem. We have just observed that it is enough to prove (2.1). Since we can suppose f ≥ 0, by (2.3), we need only check that there exists C > 0, independent of K such that X Tj,k ≤ C, 0≤j≤k≤K 2l+2 , 2l+2 2l+1 2l−1 where by (2.2). For a constant c0 > 0, we define Q0j,k = P Tj,k are defined Qi,k . So Q0j,k have the same properties as Qj,k and Q0j,k ◦ Qj,k = Qj,k |i−j|≤c0 thus we have that (2.5) holds for Q0j,k . Then, for 1 < p < ∞ and X p 2 0 F = {fj,k }j,k≥0 ∈ L l , Qj,k fj,k ≤ cp kF kLp (l2 ) . j,k≥0 Title Page Contents 0≤j≤k≤K = Marta Urciuolo p We decompose X Tj,k f X Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case X Tj,k (I − Pj,k ) I − Q0j,k f + 0≤j≤k≤K Tj,k Pj,k f 0≤j≤k≤K + X Tj,k Q0j,k (I − Pj,k ) f. JJ J II I Go Back Close 0≤j≤k≤K Now, proceeding as in [1], the theorem follows from Proposition 2.3, Lemmas 2.5 and 2.6 and the remarks in [8, p. 85] concerning the multiparameter maximal function. Quit Page 14 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au References [1] M. CHRIST, Endpoint bounds for singular fractional integral operators, UCLA, preprint, (1988). [2] E. FERREYRA, T. GODOY AND M. URCIUOLO, Endpoint bounds for convolution operators with singular measures, Coll. Math., 76(1) (1998), 35–47. [3] E. FERREYRA, T. GODOY AND M. URCIUOLO, The type set for homogeneous singular measures on R3 of polynomial type, to appear in Coll. Math. [4] I. IKROMOV, M. KEMPE AND D. MÜLLER, Damped oscillatory integrals and boundedness of maximal operators associated to mixed homogeneous hypersurfaces, Duke Math. J., 126(3) (2005), 471–490. [5] A. IOSEVICH AND E. SAWYER, Sharp Lp −Lq estimates for a class of averaging operators, Annales de l’institut Fourier, 46(5) (1996), 1359–1384. [6] F. RICCI AND E.M. STEIN, Harmonic analysis on nilpotent groups and singular integrals. III. Fractional integration along manifolds, J. Funct. Anal., 86 (1989), 360–389. [7] E.M. STEIN, Singular Integrals and Differentiability Properties of Functions, Princeton University Press (1970). [8] E.M. STEIN, Harmonic Analysis. Real Variable Methods, Orthogonality and Oscillatory Integrals, Princeton University Press (1993). Convolution Operators with Homogeneous Singular Measures on R3 of Polynomial Type. The Remainder Case Marta Urciuolo Title Page Contents JJ J II I Go Back Close Quit Page 15 of 15 J. Ineq. Pure and Appl. Math. 7(3) Art. 89, 2006 http://jipam.vu.edu.au