Journal of Inequalities in Pure and Applied Mathematics ON HARDY-HILBERT’S INTEGRAL INEQUALITY WITH PARAMETERS volume 4, issue 5, article 94, 2003. LEPING HE, MINGZHE GAO AND WEIJIAN JIA Department of Mathematics and Computer Science, Normal College, Jishou University, Jishou Hunan, 416000 People’s Republic of China. EMail: lianheping@163.com EMail: mingzhegao@163.com EMail: jwj1959@163.com Received 01 August, 2003; accepted 28 August, 2003. Communicated by: L. Debnath Abstract Contents JJ J II I Home Page Go Back Close c 2000 Victoria University ISSN (electronic): 1443-5756 108-03 Quit Abstract In this paper, by means of a sharpening of Hölder’s inequality, Hardy-Hilbert’s integral inequality with parameters is improved. Some new inequalities are established. 2000 Mathematics Subject Classification: 26D15, 46C99 Key words: Hardy-Hilbert integral inequality, Hölder’s inequality, Weight function, Beta function. On Hardy-Hilbert’s Integral Inequality with Parameters The authors thank the referee for his help and patience in improving the paper. Leping He, Mingzhe Gao and Weijian Jia Contents Title Page 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Lemmas and their Proofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Main Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References 3 5 9 Contents JJ J II I Go Back Close Quit Page 2 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au 1. Introduction Let p > 1, p1 + +∞, then 1 q ∞ Z Z (1.1) 0 0 = 1, f, g > 0. If 0 < ∞ R∞ 0 f p (t)dt < +∞, 0 < R∞ 0 g q (t)dt < f (x)g(y) dxdy x+y π < sin (π/p) Z 0 ∞ p1 Z f (t) dt p ∞ 1q g (t) dt , q 0 π where the constant sin(π/p) is best possible. The inequality (1.1) is well known as Hardy-Hilbert’s integral inequality. In recent years, some improvements and extensions of Hilbert’s inequality and Hardy-Hilbert’s inequality have been given in [2] – [6], Yang [2] gave a generalization of (1.1) as follows: If λ > 2 − min{p, q}, α < T 6 ∞ then Z (1.2) T Z T f (x)g(y) dxdy λ α α (x + y − 2α) (Z " # ) p1 p+λ−2 T p t−α < kλ (p) − θλ (p) (t − α)1−λ f p (t)dt T − α α (Z " # ) 1q q+λ−2 T q t−α × kλ (p) − θλ (q) (t − α)1−λ g q (t)dt T − α α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close Quit Page 3 of 19 (T < ∞) J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au and Z ∞ (1.3) α Z ∞ f (x)g(y) dxdy (x + y − 2α)λ α Z ∞ p1 Z 1−λ p < kλ (p) (t − α) f (t)dt α ∞ 1q g (t)dt , 1−λ q (t − α) α where p+λ−2 q+λ−2 kλ (p) = B , , p q (2−λ)/r Z 1 1 1 θλ (r) = du λ u 0 (1 + u) On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia (r = p, q). Title Page The main purpose of this paper is to build a few new inequalities which include improvements of the inequalities (1.2) and (1.3), and extensions of corresponding results in [3] – [5]. Contents JJ J II I Go Back Close Quit Page 4 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au 2. Lemmas and their Proofs For convenience, we firstly introduce some notations: r s Z T (f , g ) = r s Z f (x)g (x) dx, kf kp = α p1 T p f (x) dx , kf k2 = kf k . α We next introduce a function defined by Sr (H, x) = H r/2 , x kHk−r/2 , r On Hardy-Hilbert’s Integral Inequality with Parameters where x is a parametric variable vector which is a variable unit vector. Under the general case, it is properly chosen such that the specific problems discussed are simplified. p/2 Clearly, Sr (H, x) = 0 when the vector x selected is orthogonal n o to H . Throughout this paper, the exponent m indicates m = min p1 , 1q , α < T 6 ∞. In order to verify our assertions, we need to build the following lemmas. Leping He, Mingzhe Gao and Weijian Jia Lemma 2.1. Let f (x) , g (x) > 0, and 0 < kgkq < +∞, then (2.1) 1 p + 1 q = 1 and p > 1. If 0 < kf kp < +∞ where R = (Sp (f, h) − Sq (g, h)) , khk = 1, f linearly independent. Contents JJ J II I Go Back Close (f, g) < kf kp kgkq (1 − R)m , 2 Title Page p/2 (x), g Quit q/2 (x)and h(x) are Page 5 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au Proof. First of all, we discuss the case of p 6= q. Without loss of generality, p suppose that p > q > 1, since p1 + 1q = 1, we have p > 2. Let R = p2 , Q = p−2 . 1 1 Then R + Q = 1. By Hölder’s inequality we obtain, Z T (2.2) (f, g) = f (x)g (x) dx a Z T f · g q/p g 1−(q/p) dx = a Z T f ·g 6 q/p R R1 Z dx g a = f p/2 , g T 1−(q/p) Q Q1 dx a 2 q/2 p q (1− 2 ) kgkq p Leping He, Mingzhe Gao and Weijian Jia . And the equality in (2.2) holds if and only if f p/2 and g q/2 are linearly dependent. In fact, the equality in (2.2) holds if and only if, there exists a c1 such that R Q f · g q/p = c1 g 1−(q/p) . It is easy to deduce that f p/2 = c1 g q/2 . In our previous paper [3], with the help of the positive definiteness of the Gram matrix, we established an important inequality of the form (α, β)2 6 kαk2 kβk2 − (kαk x − kβk y)2 = kαk2 kβk2 (1 − γ) 2 y x where γ = kαk − kβk , x = (β, γ) , y = (α, γ) with kγk = 1 and xy > 0. The equality in (2.3) holds if and only if α and β are linearly dependent; or the vector γ is a linear combination of α and β, and xy = 0 but x 6= y. If α, β and γ in (2.3) are replaced by f p/2 , g q/2 and h respectively, then we get 2 (2.4) f p/2 , g q/2 6 kf kpp kgkqq (1 − R) , (2.3) On Hardy-Hilbert’s Integral Inequality with Parameters Title Page Contents JJ J II I Go Back Close Quit Page 6 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au where R = (Sp (f, h) − Sq (g, h))2 with ||h|| = 1. The equality in (2.4) holds if and only if f p/2 and g q/2 are linearly dependent, or h is a linear combination of f p/2 and g q/2 , and f p/2 , h g q/2 , h = 0, but f p/2 , h 6= g q/2 , h . Since f p/2 and g q/2 are linearly independent, it is impossible to have equality in (2.4). Substituting (2.4) into (2.2), we obtain after simplifications 1 (f, g) < kf kp kgkq (1 − R) p . (2.5) Provided that h(x) is properly chosen, then R 6= 0 is achieved. (The choice of h(x) is quite flexible, as long as condition khk = 1 is satisfied, on which we can refer to [3, 4], etc.). Noticing the symmetry of p and q, the inequality (2.1) follows from (2.5). Next, we discuss the case of p = q. According to the hypothesis: when f, g and h are linearly independent, we immediately obtain from (2.3) the following result: 1 (f, g) < kf k kgk (1 − r̄) 2 , 2 (f,h) (g,h) where r̄ = kf k − kgk , and khk = 1. Thus the lemma is proved. On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J Lemma 2.2. Let p > 1, p1 + 1q = 1, λ > 2 − min{p, q}, α < T < ∞. Define the weight function ωλ as follows: Z (2.6) T ωλ (α, T, r, x) = α 1 (x + y − 2α)λ x−α y−α Go Back Close 2−λ r dy Setting ωλ (α, ∞, r, x) = limT →∞ ωλ (α, T, r, x) and kλ (p) = B II I Quit x ∈ (α, T ]. Page 7 of 19 p+λ−2 q+λ−2 , q p , J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au Z θλ (r) = (2.7) 0 1 1 (1 + u)λ (2−λ)(1−1/r) 1 du, u (r = p, q), then we have ωλ (α, ∞, r, x) = kλ (p)(x − α)1−λ , (2.8) x ∈ (α, ∞) and On Hardy-Hilbert’s Integral Inequality with Parameters (2.9) ωλ (α, T, r, x) < kλ (p) − θ(r) x−α T −α 1+(λ−2)(1−1/r) Leping He, Mingzhe Gao and Weijian Jia ! (x − α)1−λ , x ∈ (α, T ), where B(m, n) is the beta function. The proof of this lemma is given in the paper [2]; it is omitted here. Title Page Contents JJ J II I Go Back Close Quit Page 8 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au 3. Main Results In order to state it conveniently, we need again to define the functions and introduce some notations 2−λ 2−λ f (x) x − α pq g(y) y − α pq , G= , F = (x + y − 2α)λ/p y − α (x + y − 2α)λ/q x − α Z T Z T F p/2 hT dxdy Sp (F, hT ) = α Z T α Z G α q/2 Z T T Z α T Sq (G, hT ) = T Z F p dxdy , α Z T hT dxdy α − 21 q − 21 G dxdy α , Leping He, Mingzhe Gao and Weijian Jia α where hT = hT (x, y) is a unit vector with two variants, namely Z T Z T 12 2 = 1, α < T 6 ∞, ||hT || = hT dxdy α On Hardy-Hilbert’s Integral Inequality with Parameters α and F p/2 , Gq/2 , hT are linearly independent. Theorem 3.1. Let p > 1, p1 + 1q = 1, λ > 2 − min {p, q}, α < T 6 ∞, f (t), g(t) > 0. If Z ∞ 0< (t − α)1−λ f p (t) dt < +∞ and Zα∞ 0< (t − α)1−λ g q (t) dt < +∞, Title Page Contents JJ J II I Go Back Close Quit Page 9 of 19 α then J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au (i) For T < ∞, we have Z TZ T f (x)g(y) (3.1) dxdy λ α α (x + y − 2α) (Z ) p1 (p+λ−2)/p ! T t−α < kλ (p) − θλ (p) (t − α)1−λ f p (t)dt T −α α ) 1q (Z (q+λ+2)/q ! T t−α (t − α)1−λ g q (t)dt (1−RT )m , × kλ (p) − θλ (q) T − α α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia where p+λ−2 q+λ−2 kλ (p) = B , , p q 2−λ Z 1 1 1 r θλ (r) = du (r = p, q). λ u 0 (1 + u) (ii) For T = ∞, we have Z ∞Z ∞ f (x)g(y) dxdy (3.2) (x + y − 2α)λ α α Z ∞ p1 1−λ p < kλ (p) (t − α) f (t)dt α Z ∞ × α 1q 1−λ q (t − α) g (t)dt (1 − R∞ )m , Title Page Contents JJ J II I Go Back Close Quit Page 10 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au where RT = (Sp (F, hT ) − Sq (G, hT ))2 , (3.3) 1 14 a−x 2 e x − a 2 , T = ∞; 1 π (x + y − 2a) 2 y − a hT (x, y) = T −α T −α T −α e(1− 2(x−α) − 2(y−α) ) , T < ∞. (x − α)(y − α) Proof. By Lemma 2.1, we get Z TZ T f (x)g(y) (3.4) dxdy λ α α (x + y − 2α) Z TZ T = F Gdxdy α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page α T Z Z 1 p T Z p T Z T G dxdy F dxdy 6 α Z T = α α q 1 q Contents (1 − RT )m α p1 p ωλ (α, β, q, t)f (t) dt α Z T × 1q q ωλ (α, β, p, t)g (t) dt (1 − RT )m , α where ωλ (α, T, r, t) (r = p, q) is the function defined by (2.6) . Now notice that θλ (p) = θλ (q), θλ (q) = θλ (p) and substituting (2.9) and (2.8) into (3.4) respectively, the inequalities (3.1) and (3.2) follow. JJ J II I Go Back Close Quit Page 11 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au It remains to discuss the expression of RT . We may choose the function hT indicated by (3.3). When T = ∞, setting s = x − α, t = y − α, then Z ∞ Z ∞ h2∞ (x, y)dxdy ||h∞ || = α 12 α Z ∞ Z ∞ s 12 12 2 1 dt = 1. = e−2s ds π 0 s+t t 0 When T < ∞, setting ξ = T Z Z α η= T khT k = Z T −α , x−α h2T dxdy T −α , y−α On Hardy-Hilbert’s Integral Inequality with Parameters then we have Leping He, Mingzhe Gao and Weijian Jia 12 α T T −α −α T − α (1− Tx−α T − α (1− Ty−α ) dy ) dx · e e 2 2 (x − α) (y − α) α α Z ∞ 21 Z ∞ = e1−ξ dξ · e1−η dη = 1. Z 12 = 1 Title Page Contents JJ J II I 1 According to Lemma 2.1 and the given hT , we have RT = (Sp (F, hT ) − Sq (G, hT ))2 . It is obvious that F p/2 , Gq/2 and hT are linearly independent, so it is impossible for equality to hold in (3.4). Thus the proof of theorem is completed. Remark 3.1. Clearly, the inequalities (3.1) and (3.2) are the improvements of (1.2) and (1.3) respectively . Go Back Close Quit Page 12 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au Owing to p, q > 1, when λ = 1, 2, 3, the condition λ > 2 − min(p, q) is satisfied, then we have γ1 Z 1 Z 1 1 1 1 θ1 (r) = du > du = ln 2, u 0 1+u 0 1+u 1 1 π k1 (p) = B , = , q p sin(π/p) Z 1 1 1 θ2 (r) = 2 du = , 2 0 (1 + u) p+2−2 q+2−2 k2 (p) = B , = B(1, 1) = 1, p q − γ1 Z 1 Z 1 1 1 u 1 θ3 (r) = du > du = , 3 3 u 8 0 (1 + u) 0 (1 + u) 1 1 (p − 1)π 1 k3 (p) = B , = 2 . 2pq q p 2p sin(π/p) By Theorem 3.1, some corollaries are established as follows: Corollary 3.2. If p > 1, p1 + 1q = 1, λ = 1, α < T 6 ∞ and f (t), g(t) > 0, RT RT 0 < α f p (t)dt < +∞ and 0 < α g q (t)dt < +∞ , then we have Z TZ T f (x)g(y) (3.5) dxdy α α x + y − 2α (Z ! ) p1 1q T π t−α < − · ln 2 f p (t)dt sin(π/p) T −α α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close Quit Page 13 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au (Z T × α π − sin(π/p) t−α T −α p1 ) 1q ! · ln 2 · g q (t)dt (1 − r1 )m , for T < ∞, and Z ∞ Z ∞ (3.6) α α f (x)g(y) dxdy x + y − 2α Z ∞ p1 Z ∞ 1q π p q < f (t)dt g (t)dt (1 − r1 )m . sin(π/p) α α Remark 3.2. When α = 0 and p = q = 2, the inequality (3.6) is reduced to a result which is equivalent to inequality (3.1) in [3] after simple computations. As a result, the inequalities (3.1), (3.2) and (3.5) – (3.6) are all extensions of (3.1) in [3]. Corollary 3.3. Let p > 1, 1 p + 1 q = 1, α < T 6 ∞ and f (t), g(t) > 0. If Z T 1 f p (t) dt < +∞ t−α 0< α and Z On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close T 0< α 1 q g (t) dt < +∞, t−α Quit Page 14 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au then we obtain Z (3.7) T T Z f (x)g(y) 2 dxdy α α (x + y − 2α) Z T p1 t−α 1 p < 1− · f (t)dt 2 (T − α) t − α α 1q Z T 1 t−α × 1− · g q (t)dt (1 − r2 )m , 2 (T − α) t − α α for T < ∞, and Z ∞ Z ∞ (3.8) α α f (x)g(y) dxdy (x + y − 2α)2 Z ∞ p1 Z ∞ 1q 1 1 q p < f (t)dt g (t)dt (1 − r2 )m . t−α t−α α α Corollary 3.4. If p > 1, 1 p + 1 q = 1, λ = 3, α < T 6 ∞ and f (t), g(t) > 0, Z T 1 f p (t)dt < +∞, 2 (t − α) 1 g q (t)dt < +∞, (t − α)2 0< α T Z 0< α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close Quit Page 15 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au then we get T Z T Z (3.9) α α (Z f (x)g(y) dxdy (x + y − 2α)3 T (p − 1)π 1 − 2p2 sin(π/p) 8 < α (Z T (p − 1)π 1 − 2p2 sin(π/p) 8 × α t−α T −α t−α T −α 1+ p1 ! 1+ 1q ! ) p1 1 f p (t)dt (t − α)2 ) 1q 1 g q (t)dt (t − α)2 (1 − r3 )m On Hardy-Hilbert’s Integral Inequality with Parameters T < ∞, Leping He, Mingzhe Gao and Weijian Jia and Z ∞ Z ∞ f (x)g(y) (3.10) dxdy (x + y − 2α)3 α α Z ∞ p1 (p − 1)π 1 p f (t)dt < 2 2p sin(π/p) (t − α)2 α 1q Z ∞ 1 q × g (t)dt (1 − r3 )m . 2 (t − α) α Since kλ (2) = B λ2 , λ2 , θλ (2) = 12 B λ2 , λ2 , and λ > 2 − min(2, 2) = 0, we also have Title Page Contents JJ J II I Go Back Close Quit Page 16 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au Corollary 3.5. If p = q = 2, λ > 0, α < T 6 ∞ and f (t), g(t) > 0, Z T 0< (t − α)1−λ f 2 (t) dt < +∞, α Z T 0< (t − α)1−λ g 2 (t) dt < +∞, α then we have Z TZ (3.11) T f (x)g(y) dxdy (x + y − 2α)λ ) 12 (Z T " λ/2 # 1 t−α λ λ 1− (t − α)1−λ f 2 (t)dt <B , 2 2 2 T −α α (Z " ) 12 λ/2 # T 1 t−α 1− (1 − R)m , × (t − α)1−λ g 2 (t)dt 2 T −α α α α for T < ∞ and Z ∞ Z ∞ (3.12) α α f (x)g(y) dxdy (x + y − 2α)λ Z ∞ 12 λ λ 1−λ 2 , <B (t − α) f (t)dt 2 2 α Z ∞ 12 1−λ 2 e m. × (t − α) g (t)dt (1 − R) α On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close Quit Page 17 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au Remark 3.3. The inequalities (3.11), (3.12) are new generalizations of (20) in [4] and improvements of the inequalities (4) and (12) in [6] respectively. On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents JJ J II I Go Back Close Quit Page 18 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au References [1] G.H. HARDY, J.E. LITTLEWOOD AND G. POLYA, Inequalities, Cambridge Univ. Press, Cambridge, UK, 1952. [2] BICHENG YANG, On Hardy-Hilbert’s integral inequality, J. Math. Anal. Appl., 261 (2001), 295–306. [3] MINGZHE GAO, LI TAN AND L. DEBNATH, Some improvements on Hilbert’s integral inequality, J. Math. Anal. Appl., 229 (1999), 682–689. [4] MINGZHE GAO, On the Hilbert inequality, Zeitschrift für Analysis und ihre Anwendungen, 18(4) (1999), 1117–1122. [5] MINGZHE GAO, SHANG RONG WEI AND LEPING HE, On the Hilbert inequality with weights, Zeitschrift für Analysis und ihre Anwendungen, 21(1) (2002), 257–263. On Hardy-Hilbert’s Integral Inequality with Parameters Leping He, Mingzhe Gao and Weijian Jia Title Page Contents [6] BICHENG YANG, On Hilbert’s integral inequality, J. Math. Anal. Appl., 220 (1998), 778–785. JJ J II I Go Back Close Quit Page 19 of 19 J. Ineq. Pure and Appl. Math. 4(5) Art. 94, 2003 http://jipam.vu.edu.au