Journal of Inequalities in Pure and Applied Mathematics ON THE EXTENDED HILBERT’S INTEGRAL INEQUALITY BICHENG YANG Department of Mathematics, Guangdong Institute of Education, Guangzhou, Guangdong 510303 People’s Republic of China. volume 5, issue 4, article 96, 2004. Received 21 May, 2003; accepted 21 October, 2004. Communicated by: S.S. Dragomir EMail: bcyang@pub.guangzhou.gd.cn Abstract Contents JJ J II I Home Page Go Back Close c 2000 Victoria University ISSN (electronic): 1443-5756 068-03 Quit Abstract This paper gives two distinct generalizations of the extended Hilbert’s integral inequality with the same best constant factor involving the β function. As applications, we consider some equivalent inequalities. 2000 Mathematics Subject Classification: 26D15. Key words: Hilbert’s inequality, weight function, β function, Hölder’s inequality. On the Extended Hilbert’S Integral Inequality Research support by the Science Foundation of Professor and Doctor of Guangdong Institute of Education. Bicheng Yang Contents Title Page 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Some Lemmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Main Results and Applications . . . . . . . . . . . . . . . . . . . . . . . . . References 3 6 9 Contents JJ J II I Go Back Close Quit Page 2 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au 1. Introduction R∞ R∞ If f, g ≥ 0, such that 0 < 0 f 2 (x)dx < ∞ and 0 < 0 g 2 (x)dx < ∞, then the famous Hilbert’s integral inequality is given by ∞ Z ∞ Z (1.1) 0 0 f (x)g(y) dxdy < π x+y Z ∞ 2 Z 2 f (x)dx 0 12 ∞ g (x)dx , 0 where the constant factor π is the best possible (see [2]). Inequality (1.1) had been generalized by Hardy-Riesz R ∞ [1] as: R∞ If p > 1, p1 + 1q = 1, 0 < 0 f p (x)dx < ∞ and 0 < 0 g q (x)dx < ∞, then Z ∞ Z (1.2) 0 0 ∞ f (x)g(y) dxdy x+y < sin π π p On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Z ∞ p p1 Z q g (x)dx f (x)dx 0 ∞ 1q Contents , 0 π where the constant factor sin(π/p) is the best possible. When p = q = 2, inequality (1.2) reduces to (1.1). We call (1.2) Hardy-Hilbert’s integral inequality, which is important in analysis and its applications (see [4]). In recent years, by introducing a parameter λ and the β function, Yang [7, 8] gave an extension of (1.2) as:R R∞ ∞ If λ > 2−min{p, q}, 0 < 0 x1−λ f p (x)dx < ∞ and 0 < 0 x1−λ g q (x)dx < JJ J II I Go Back Close Quit Page 3 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au ∞, then Z ∞ Z (1.3) 0 0 ∞ f (x)g(y) dxdy (x + y)λ Z ∞ p1 Z 1−λ p < kλ (p) x f (x)dx 0 ∞ x 1−λ q g (x)dx 1q , 0 p+λ−2 where the constant factor kλ (p) = B p+λ−2 , is the best possible (B(u, v) p p is the β function). Its equivalent inequality is (see [9, (2.12)]): p Z ∞ f (x) p (1.4) y dx dy < [kλ (p)] x1−λ f p (x)dx, λ (x + y) 0 0 0 h ip where the constant factor [kλ (p)]p = B p+λ−2 is the best possible. , p+λ−2 p p When λ = 1, inequality (1.3) reduces to (1.2), and (1.4) reduces to the equivalent form of (1.2) as: p p Z ∞ Z ∞ Z ∞ f (x) π (1.5) dx dy < f p (x)dx. π x + y 0 0 0 sin p Z ∞ (λ−1)(p−1) Z ∞ For p = q = 2, by (1.3), we have λ > 0, and Z ∞Z ∞ f (x)g(y) (1.6) dxdy (x + y)λ 0 0 Z ∞ 12 Z ∞ λ λ 1−λ 2 1−λ 2 <B , x f (x)dx x g (x)dx . 2 2 0 0 On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 4 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au We define (1.6) as the extended Hilbert’s integral inequality. Recently, Yang et al. [10] provided an extensive account of the above results and Yang [6] gave a reverse of (1.4) with the same best constant factor. The main objective of this paper is to build two distinct generalizations of (1.6), with the same best constant factor but different from (1.3). As applications, we consider some equivalent inequalities. For this, we need some lemmas. On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 5 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au 2. Some Lemmas We have the formula of the β function as (see [5]): Z ∞ tu−1 (2.1) B(u, v) = dt = B(v, u) (1 + t)u+v 0 (u, v > 0). Lemma 2.1 (see [3]). If p > 1, p1 + 1q = 1, ω(σ) > 0, f, g ≥ 0, f ∈ Lpω (E) and g ∈ Lqω (E) , then the weighted Hölder’s inequality is as follows: Z p1 Z 1q Z q p (2.2) ω(σ)f (σ)g(σ)dσ ≤ ω(σ)f (σ)dσ ω(σ)g (σ)dσ , E E E On the Extended Hilbert’S Integral Inequality where the equality holds if and only if there exists non-negative real numbers A and B, such that they are not all zero and Af p (σ) = Bg q (σ), a.e. in E. Bicheng Yang Lemma 2.2. If r > 1, and λ > 0, define the weight function ωλ (r, x) as Z ∞ 1 λ(1− r1 ) (2.3) ωλ (r, x) := x y (λ−r)/r dy. λ (x + y) 0 Then we have λ 1 (2.4) ωλ (r, x) = B ,λ 1 − . r r Proof. Setting y = xu in the integral of (2.3), we find Z ∞ (xu)(λ−r)/r λ(1− r1 ) ωλ (r, x) = x xdu xλ (1 + u)λ 0 Z ∞ λ 1 = u r −1 du. λ (1 + u) 0 By (2.1), we have (2.4) and the lemma is proved. Title Page Contents JJ J II I Go Back Close Quit Page 6 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au Note. It is obvious that for p > 1, p1 + (2.5) ωλ (p, x) = B Lemma 2.3. If p > 1, p1 + I1 := ∞ = 1 and λ > 0, one has λ λ , p q = ωλ (q, x). = 1 and 0 < ε < λ, one has ∞ λ−p−ε 1 p y x dxdy (x + y)λ 1 1 2 λ−ε λ ε p 1 > B , + − . ε p q p λ−ε Z (2.6) 1 q 1 q λ−q−ε q Z Proof. Setting x = yu in I1 , in view of (2.1), one has Z ∞ Z ∞ λ−p−ε 1 −1−ε I1 = y u p du dy λ 1 1/y (1 + u) Z ∞ Z ∞ λ−ε 1 −1 −1−ε p = y u du dy (1 + u)λ 1 0 "Z 1 # Z ∞ y λ−ε 1 − y −1−ε u p −1 du dy λ (1 + u) 1 0 Z ∞ Z 1 y λ−ε λ−ε λ ε 1 −1 > B , + − y u p −1 dudy. ε p q p 1 0 By calculating the above integral, one has (2.6). The lemma is proved. On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 7 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au Lemma 2.4. If p > 1, p1 + I2 := (2.7) = 1 and 0 < ε < λ(p − 1), one has ∞ λ+ε 1 y xλ− p −1 dxdy λ (x + y) 1 1 −2 1 λ ε λ ε λ ε > B − , + − − . ε q p p p q p Z ∞ 1 q −1 λ− λ+ε q Z Proof. Setting x = yu in I2 , in view of (2.1), one has Z ∞ Z ∞ 1 λ− λ+ε −1 −1−ε p I2 = y u du dy λ 1 1/y (1 + u) Z ∞ Z ∞ 1 λ− λ+ε −1 −1−ε = y u p du dy (1 + u)λ 1 0 # "Z 1 Z ∞ y λ+ε 1 uλ− p −1 du dy − y −1−ε λ (1 + u) 0 1 Z ∞ Z 1 y λ+ε λ ε λ ε 1 −1 − , + − y uλ− p −1 dudy. > B ε q p p p 1 0 On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back By calculating the above integral, one has (2.7). The lemma is proved. Close Quit Page 8 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au 3. Main Results and Applications Theorem 3.1. If f, g ≥ 0, p > 1, p1 + 1q = 1, λ > 0, such that 0 < R∞ < ∞ and 0 < 0 xq−1−λ g q (x)dx < ∞, then Z (3.1) 0 ∞ Z R∞ 0 xp−1−λ f p (x)dx ∞ f (x)g(y) dxdy (x + y)λ 0 1q Z ∞ p1 Z ∞ λ λ q−1−λ q p−1−λ p x g (x)dx ; <B , x f (x)dx p q 0 0 On the Extended Hilbert’S Integral Inequality Bicheng Yang Z ∞ Z ∞ p f (x) dx dy (x + y)λ 0 0 p Z ∞ λ λ < B , xp−1−λ f p (x)dx, p q 0 h ip where the constant factors B λp , λq and B λp , λq are all the best possible. Inequality (3.2) is equivalent to (3.1). In particular, for λ = 1, one has the following two equivalent inequalities: y λ(p−1)−1 (3.2) Title Page Contents JJ J II I Go Back Close Z ∞ Z (3.3) 0 0 ∞ f (x)g(y) dxdy x+y Z ∞ p1 Z ∞ 1q π p−2 p q−2 q < x f (x)dx x g (x)dx ; 0 0 sin πp Quit Page 9 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au Z ∞ y (3.4) p−2 Z 0 ∞ 0 f (x) dx x+y p p dy < sin π π p Z ∞ xp−2 f p (x)dx. 0 Proof. By (2.2), one has Z ∞Z ∞ f (x)g(y) J1 := dxdy (x + y)λ 0 0 " # " # 1 1 Z ∞Z ∞ y q−λ pq 1 xp−λ pq = f (x) g(y) dxdy (x + y)λ y q−λ xp−λ 0 0 (Z "Z ) p1 p−λ 1q # ∞ ∞ x 1 ≤ dy f p (x)dx λ q−λ (x + y) y 0 0 ) 1q (Z "Z q−λ p1 # ∞ ∞ y 1 (3.5) dx g q (y)dy . × λ p−λ (x + y) x 0 0 If (3.5) takes the form of an equality, then by Lemma 2.1, there exist real numbers A and B, such that they are not all zero, and 1 A (x + y)λ xp−λ y q−λ =B 1 q Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit f p (x) 1 (x + y)λ On the Extended Hilbert’S Integral Inequality Page 10 of 19 p1 q−λ y xp−λ g q (y), a.e. in (0, ∞) × (0, ∞). J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au Hence we find Axp−λ f p (x) = By q−λ g q (y), a.e. in (0, ∞) × (0, ∞). It follows that there exists a constant C, such that Axp−λ f p (x) = C, By a.e. in (0, ∞); q−λ q a.e. in (0, ∞). g (y) = C, Without loss of generality, suppose that A 6= 0. One has xp−λ−1 f p (x) = C , Ax R∞ On the Extended Hilbert’S Integral Inequality a.e. in (0, ∞), Bicheng Yang which contradicts the fact that 0 < 0 xp−1−λ f p (x)dx < ∞. Hence, (3.5) takes the form of strict inequality, and by (2.3), we may rewrite (3.5) as Z (3.6) J1 < ∞ ωλ (q, x)xp−1−λ f p (x)dx Contents p1 JJ J 0 ∞ Z × ωλ (p, y)y q−1−λ q g (y)dy 0 ∼ ∼ Go Back Page 11 of 19 ∼ f (x) = g(y) = 0, ∼ . Quit For 0 < ε < λ, setting f (x) and g(y) as: f (x) = x 1q II I Close Hence by (2.5), one has (3.1). ∼ Title Page λ−p−ε p ∼ x, y ∈ (0, 1); , g(y) = y λ−q−ε q , x, y ∈ [1, ∞), J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au then we find Z (3.7) ∞ x p−1−λ p1 Z ∼p f (x)dx 0 ∞ x q−1−λ ∼q 1q g (x)dx 0 1 = . ε If there exists λ > 0, such that the constant factor in (3.1) is not the best posλ λ sible, then there exists a positive number K ( with K < B p , q ), such that λ λ (3.1) is still valid if one replaces B p , q by K. In particular, one has Z ∞ Z ∞ ∼ 0 On the Extended Hilbert’S Integral Inequality ∼ f (x)g(y) εI1 = ε dxdy (x + y)λ 0 0 p1 Z Z ∞ ∼p p−1−λ x f (x)dx < εK Bicheng Yang ∞ ∼q xq−1−λ g (x)dx 1q . 0 Hence by (2.6) and (3.7), one has 2 λ−ε λ ε p , + −ε < K, B p q p λ−ε λ λ and then B p , q ≤ K (ε → 0+ ). This contradicts the fact that K < B λp , λq . It follows that the constant factor in (3.1) is the best possible. R∞ Since 0 < 0 xp−1−λ f p (x)dx < ∞, there exists T0 > 0, such that for any RT T > T0 , one has 0 < 0 xp−1−λ f p (x)dx < ∞. We set Z T p−1 f (x) λ(p−1)−1 g(y, T ) := y dx , λ 0 (x + y) Title Page Contents JJ J II I Go Back Close Quit Page 12 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au and use (3.1) to obtain Z T 0< y q−1−λ g q (y, T )dy 0 Z T p Z T f (x) λ(p−1)−1 = y dx dy λ 0 0 (x + y) Z TZ T f (x)g(y, T ) = dxdy (x + y)λ 0 0 1q Z T p1 Z T λ λ (3.8) < B , xp−1−λ f p (x)dx y q−1−λ g q (y, T )dy . p q 0 0 Hence we find Bicheng Yang Title Page T Z y q−1−λ g q (y, T )dy 0< 1− 1q 0 Z = T y λ(p−1)−1 <B λ λ , p q Z p p1 f (x) dx dy λ 0 (x + y) p1 xp−1−λ f p (x)dx . Z 0 (3.9) On the Extended Hilbert’S Integral Inequality T T Contents JJ J II I Go Back Close 0 R∞ It follows that 0 < 0 y q−1−λ g q (y, ∞)dy < ∞. Hence (3.8) and (3.9) are strict inequalities as T → ∞. Thus inequality (3.2) holds. Quit Page 13 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au On the other hand, if (3.2) is valid, by Hölder’s inequality (2.2), one has Z ∞Z ∞ f (x)g(y) dxdy (x + y)λ 0 0 h Z ∞ Z ∞ i λ+1−q f (x) − λ+1−q q = y y q dx g(y) dy (x + y)λ 0 0 p p1 Z ∞ Z ∞ f (x) λ(p−1)−1 ≤ y dx dy (x + y)λ 0 0 Z ∞ 1q q−1−λ q (3.10) × y g (y)dy . 0 Hence by (3.2), one has (3.1). It follows that (3.2) is equivalent to (3.1). If the constant factor in (3.2) is not the best possible, one can get a contradiction that the constant factor in (3.1) is not the best possible by using (3.10). The theorem is thus proved. Theorem 3.2. If f, g ≥ 0, p > 1, Z 1 p + 1 q = 1, λ > 0, such that ∞ x(p−1)(1−λ) f p (x)dx < ∞ 0< Z 0< Bicheng Yang Title Page Contents JJ J II I Go Back Close 0 and On the Extended Hilbert’S Integral Inequality Quit ∞ x (q−1)(1−λ) q g (x)dx < ∞, Page 14 of 19 0 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au then Z (3.11) 0 ∞ Z ∞ f (x)g(y) dxdy (x + y)λ 0 Z ∞ p1 λ λ <B , x(p−1)(1−λ) f p (x)dx p q 0 Z ∞ 1q (q−1)(1−λ) q × x g (x)dx ; 0 Z ∞ y λ−1 Z ∞ f (x) dx (x + y)λ p dy p Z ∞ λ λ < B , x(p−1)(1−λ) f p (x)dx, p q 0 h ip where the constant factors B λp , λq and B λp , λq are the best possible. Inequality (3.12) is equivalent to (3.11). In particular, for λ = p > 1, one has the following two equivalent inequalities: Z ∞Z ∞ f (x)g(y) (3.13) dxdy (x + y)p 0 0 Z ∞ p p1 Z ∞ q 1q f (x) 1 g (x) < dx dx p−1 x x(p−1)2 0 0 and Z ∞ p p Z ∞ p Z ∞ f (x) 1 f (x) p−1 (3.14) y dx dy < dx. (x + y)p p−1 x(p−1)2 0 0 0 (3.12) 0 0 On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 15 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au Proof. By (2.2), one has Z ∞Z ∞ f (x)g(y) J1 = dxdy (x + y)λ 0 0 " ! # Z ∞Z ∞ 2 1 x(q−λ)/q = f (x) (x + y)λ y (p−λ)/p2 0 0 ! # " 2 y (p−λ)/p g(y) dxdy × x(q−λ)/q2 ! # ) p1 (Z "Z 2 ∞ ∞ x(q−λ)p/q 1 ≤ dy f p (x)dx λ (p−λ)/p (x + y) y 0 0 (Z "Z ! # ) 1q 2 ∞ ∞ 1 y (p−λ)q/p (3.15) . × dx g q (y)dy λ (q−λ)/q (x + y) x 0 0 On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J Following the same manner as (3.6), one has Z (3.16) J1 < ∞ ωλ (p, x)x (p−1)(1−λ) p p1 II I Go Back f (x)dx 0 Close Z × 0 ∞ ωλ (q, x)x(q−1)(1−λ) g q (x)dx 1q . Quit Page 16 of 19 Hence by (2.5), one has (3.11). J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au ∼ ∼ For 0 < ε < λ(p − 1), setting f (x) and g(y) as: ∼ ∼ x, y ∈ (0, 1); f (x) = g(y) = 0, ∼ f (x) = xλ−1− λ+ε p ∼ , g(y) = y λ−1− λ+ε q , x, y ∈ [1, ∞), by Lemma 2.4 and the same way of Theorem 3.1, we can show that the constant factor in (3.11) is the best possible. In a similar fashion to Theorem 3.1, we can show that (3.12) is valid, which is equivalent to (3.11). By the equivalence of (3.11) and (3.12), we may conclude that the constant factor in (3.12) is the best possible. The theorem is proved. Remark 3.1. (i) For p = q = 2, both inequalities (3.1) and (3.11) reduce to (1.6). Inequalities (3.1) and (3.11)are distinct generalizations of (1.6) λ λ with the same best constant factor B p , q , but different from (1.3). (ii) Since inequalities (3.3) and (1.2) are different, we may conclude that inequality (3.1) is not a generalization of (1.3). (iii) Since all the given inequalities are with the best constant factors, we have obtained some new results. On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 17 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au References [1] G.H. HARDY, Note on a theorem of Hilbert concerning series of positive terms, Proc. Math. Soc., 23(2) (1925), Records of Proc. XLV-XLVI. [2] G.H. HARDY, J.E. LITTLEWOOD AND G. POLYA, Inequalities, Cambridge University Press, Cambridge, UK, 1952. [3] JICHANG KUANG, Applied Inequalities, Shangdong Science and Technology Press, Jinan, China, 2003. [4] D.S. MITRINOVIĆ, J.E. PEČARIĆ AND A.M. FINK, Inequalities Involving Functions and their Integrals and Derivatives, Kluwer Academic Publishers, Boston, 1991. [5] ZHUXI WANG AND DUNRIN GUO , An Introduction to Special Functions, Science Press, Beijing, 1979. [6] BICHENG YANG, A reverse of Hardy-Hilbert’s integral inequality, Journal of Jilin University (Science Edition), 42(4) (2004), 489–493. [7] BICHENG YANG, On a general Hardy-Hilbert’s inequality with a best value, Chinese Annals of Math., 21A(4) (2000), 401–408. On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close [8] BICHENG YANG, On Hardy-Hilbert’s integral inequality, J. Math. Anal. Appl., 261 (2001), 295–306. [9] BICHENG YANG AND L. DEBNATH, On the extended Hardy-Hilbert’s inequality, J. Math. Anal. Appl., 272 (2002), 187–199. Quit Page 18 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au [10] BICHENG YANG AND Th.M. RASSIAS, On the way of weight coefficient and research for the Hilbert-type inequality, Math. Inequal. and Applics., 6(4) (2003), 625–658. On the Extended Hilbert’S Integral Inequality Bicheng Yang Title Page Contents JJ J II I Go Back Close Quit Page 19 of 19 J. Ineq. Pure and Appl. Math. 5(4) Art. 96, 2004 http://jipam.vu.edu.au