A RELATION TO HILBERT’S INTEGRAL INEQUALITY AND SOME BASE HILBERT-TYPE INEQUALITIES Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 BICHENG YANG Department of Mathematics Guangdong Education Institute Guangzhou, Guangdong 510303, P.R. China Title Page Contents EMail: bcyang@pub.guangzhou.gd.cn Received: 17 April, 2008 Accepted: 18 May, 2008 Communicated by: J. Pečarić 2000 AMS Sub. Class.: 26D15. Key words: Base Hilbert-type integral inequality; Parameter; Weight function. Abstract: In this paper, by using the way of weight function and real analysis techniques, a new integral inequality with some parameters and a best constant factor is given, which is a relation to Hilbert’s integral inequality and some base Hilbert-type integral inequalities. The equivalent form and the reverse forms are considered. JJ II J I Page 1 of 17 Go Back Full Screen Close Contents 1 Introduction 3 2 Some Lemmas 6 3 Main Results 10 Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 2 of 17 Go Back Full Screen Close 1. Introduction R∞ R∞ If f, g ≥ 0, 0 < 0 f 2 (x)dx < ∞ and 0 < 0 g 2 (x)dx < ∞ then we have the following Hilbert’s integral inequality [1]: Z ∞ 12 Z ∞Z ∞ Z ∞ f (x)g(y) 2 2 (1.1) dxdy < π f (x)dx g (x)dx , x+y 0 0 0 0 where the constant factor π is the best possible. Under the same condition of (1.1), we also have the following basic Hilbert-type integral inequalities [1, 2, 3]: Z ∞ 12 Z ∞Z ∞ Z ∞ f (x)g(y) 2 2 (1.2) dxdy < 4 f (x)dx g (x)dx ; max{x, y} 0 0 0 0 Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents ∞Z Z ∞ (1.3) 0 0 | ln(x/y)|f (x)g(y) dxdy < c0 x+y ∞ Z f 2 (x)dx 0 ∞ Z g 2 (x)dx 12 ; 0 Z ∞ 12 Z ∞ | ln(x/y)|f (x)g(y) (1.4) dxdy < 8 f 2 (x)dx g 2 (x)dx , max{x, y} 0 0 0 0 P ∞ 8(−1)k−1 + where the constant factors 4, c0 = k=1 (2k−1)2 = 7.3277 and 8 are the best possible. In 2005, Hardy-Riesz gave a best extension of (1.1) by introducing one pair of conjugate exponents (p, q) (p > 1, p1 + 1q = 1) as [4] Z Z ∞ ∞ Z Z (1.5) 0 0 ∞ ∞ f (x)g(y) π dxdy < x+y sin πp Z ∞ p p1 Z f (x)dx 0 ∞ q g (x)dx 0 1q , JJ II J I Page 3 of 17 Go Back Full Screen Close π where the constant factor sin(π/p) is the best possible. Inequality (1.5) is referred to as Hardy-Hilbert’s integral inequality, which is important in analysis and its applications [5]. In 1998, Yang gave a best extension of (1.1) by introducing an independent parameter λ > 0 as [6, 7] 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 where the constant factor B λ2 , λ2 is the best possible and the Beta function B(u, v) is defined by [8]: Z ∞ 1 (1.7) B(u, v) := tu−1 dt (u, v > 0). u+v (1 + t) 0 In 2004-2005, by introducing two pairs of conjugate exponents and an independent parameter, Yang et al. [9, 10] gave two different extensions of (1.1) and (1.5) as: If λ λ p, r > 1, p1 + 1q = 1, 1r + 1s = 1, λ > 0, φ(x) = xp(1− r )−1 , ψ(x) = xq(1− s )−1 , f, g ≥ 0, Z ∞ p1 p(1− λ )−1 p r 0 < ||f ||p,φ := x f (x)dx <∞ 0 and Z 0 < ||g||q,ψ := ∞ x q(1− λs )−1 q g (x)dx < ∞, then ∞ Z (1.8) 0 0 ∞ Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 4 of 17 Go Back Full Screen Close 1q 0 Z Hilbert’s Integral Inequality f (x)g(y) π dxdy < ||f ||p,φ ||g||q,ψ ; xλ + y λ λ sin( πr ) Z ∞ Z (1.9) 0 0 ∞ f (x)g(y) dxdy < B (x + y)λ λ λ , r s ||f ||p,φ ||g||q,ψ , π λ λ where the constant factors λ sin( , are the best possible. Yang [11] also π and B ) r s r considered the reverse of (1.8) and (1.9). In this paper, by using weight functions and real analysis techniques, a new integral inequality with the homogeneous kernel of −λ degree kλ (x, y) = | ln(x/y)|β (x + y)λ−α (max{x, y})α Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 (λ > 0, α ∈ R, β > −1) Title Page is given, which is a relation to (1.1) and the above basic Hilbert-type integral inequalities (1.2), (1.3) and (1.4). The equivalent and reverse forms are considered. All the new inequalities possess the best constant factors. Contents JJ II J I Page 5 of 17 Go Back Full Screen Close 2. Some Lemmas We introduce the following Gamma function [8]: Z ∞ (2.1) Γ(s) = e−t ts−1 dt (s > 0). 0 Lemma 2.1. For a, b > 0, it follows that Z 1 Z ∞ 1 a−1 b−1 (2.2) x (− ln x) dx = b Γ(b) = y −a−1 (ln y)b−1 dy. a 0 1 Proof. Setting x = e−t/a in first integral of (2.2), by (2.1), we find the first equation of (2.2). Setting y = 1/x in the first integral of (2.2), we obtain the second equation of (2.2). The lemma is hence proved. Lemma 2.2. If r > 1, function as (2.3) 1 r λ + Z $λ (s, x) := x r 0 1 s = 1, λ > 0, α ∈ R and β > −1, define the weight β λ ln xy y s −1 ∞ (x + y)λ−α (max{x, y})α dy (x ∈ (0, ∞)). Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 6 of 17 Go Back Then we have Z (2.4) $λ (s, x) = kλ (r) := 0 ∞ λ | ln u|β u r −1 du, (u + 1)λ−α (max{u, 1})α where kλ (r) is a positive number and # " ∞ X α−λ 1 1 (2.5) kλ (r) = Γ(β + 1) + . λ β+1 λ β+1 k k + k + k=0 r s Full Screen Close Proof. Setting u = x/y in (2.3), by simplification, we obtain (2.4). In view of (2.2), we obtain Z 1 Z ∞ λ λ (− ln u)β u r −1 (ln u)β u r −α−1 0 < kλ (r) = du + du (u + 1)λ−α (u + 1)λ−α 0 1 Z 1 Z ∞ −λ |α−λ| −1 (β+1)−1 −1 (β+1)−1 λ ≤2 (− ln u) u r du + (ln u) u s du 0 1 r β+1 s β+1 |α−λ| =2 + Γ(β + 1) < ∞. λ λ Hence kλ (r) is a positive number. Using the property of power series, we find Z ∞ Z 1 −λ λ (ln u)β u s −1 (− ln u)β u r −1 du kλ (r) = du + (1 + u−1 )λ−α (u + 1)λ−α 0 1 Z 1X Z ∞X ∞ ∞ −λ α−λ α−λ +k−1 β λ = du + (ln u)β u s −k−1 du (− ln u) u r k k 0 k=0 1 k=0 Z 1 Z ∞ ∞ X −λ α−λ β λ +k−1 β −k−1 = (− ln u) u r du + (ln u) u s du . k 0 1 k=0 Then in view of (2.2), we have (2.5). The lemma is proved. Lemma 2.3. If p > 0 (p 6= 1), r > 1, p1 + 1q = 1, 1r + 1s = 1, λ > 0, α ∈ R, β > −1, r n ∈ N, n > |q|λ , then for n → ∞, we have (2.6) In := 1 n Z 1 ∞ Z 1 ∞ β λ 1 − −1 λ − 1 −1 ln xy x r np y s nq (x + y)λ−α (max{x, y})α dxdy = kλ (r) + o(1). Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 7 of 17 Go Back Full Screen Close Proof. Setting u = y/x, by Fubini’s theorem [12], we obtain β 1 1 −1 −1 λs − nq Z ∞ Z ∞ ln x x λr − np y y 1 In = dx dy λ−α (max{x, y})α n 1 (x + y) 1 # λ 1 | ln u|β u s + np −1 y du dy λ−α (max{1, u})α 1 0 (1 + u) "Z # 1 1 λ λ Z Z y 1 (ln u)β u s + np −1 1 ∞ − 1 −1 (− ln u)β u s + np −1 y n du + du dy = n 1 (1 + u)λ−α (1 + u)λ−α uα 0 1 "Z # λ 1 λ 1 Z Z 1 y 1 ∞ − 1 −1 (ln u)β u s + np −1 (− ln u)β u s + np −1 = du + y n du dy (1 + u)λ−α n 1 (1 + u)λ−α uα 0 1 λ λ 1 1 Z Z ∞ Z 1 1 1 ∞ (ln u)β u s + np −1 (− ln u)β u s + np −1 −n −1 = du + y dy du (1 + u)λ−α n 1 (1 + u)λ−α uα 0 u λ 1 1 λ Z ∞ Z 1 (ln u)β u s − nq −1 (− ln u)β u s + np −1 (2.7) = du + du. (1 + u)λ−α (1 + u)λ−α uα 1 0 1 = n Z ∞ 1 −n −1 "Z y (i) If p > 0 (p 6= 1) and q > 0, then by Levi’s theorem [12], we find λ 1 0 (− ln u)β u s + np −1 du = (1 + u)λ−α Z ∞ 1 Z 1 β λ 1 − nq −1 s (ln u) u du = (1 + u)λ−α uα Z 0 Z 1 1 λ λ (ln u)β u s −1 du + o2 (1) (1 + u)λ−α uα Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 8 of 17 Go Back Full Screen Close (− ln u)β u s −1 du + o1 (1), (1 + u)λ−α ∞ Hilbert’s Integral Inequality (n → ∞); (ii) If q < 0, setting n0 ∈ N, n0 > n ≥ n0 , we find Z 1 ∞ λ 1 (ln u)β u s − nq −1 du ≤ (1 + u)λ−α uα r , 1 |q|λ s0 Z 1 ∞ = 1 s − λ 1 , 1 n0 qλ r 0 − 1 = 1 r + 1 , n0 qλ then for −1 (ln u)β u s n0 q du ≤ kλ (r0 ), (1 + u)λ−α uα and by Lebesgue’s control convergence theorem, we have Hilbert’s Integral Inequality Z 1 ∞ β λ 1 − nq −1 s (ln u) u du = (1 + u)λ−α uα Z 1 ∞ β λ −1 s (ln u) u du + o3 (1) (1 + u)λ−α uα Bicheng Yang (n → ∞). Hence by the above results and (2.7), we obtain (2.6). The lemma is proved. vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 9 of 17 Go Back Full Screen Close 3. Main Results 1 + 1q p q(1− λs )−1 Theorem 3.1. Assume that p > 0 (p 6= 1), r > 1, p(1− λ )−1 r = 1, 1 r + 1 s = 1, λ > 0, α ∈ R, β > −1, φ(x) = x , ψ(x) = x (x ∈ (0, ∞)), f, g ≥ 0, 1 Z ∞ p p(1− λ )−1 p r 0 < ||f ||p,φ = x f (x)dx < ∞, 0 < ||g||q,ψ < ∞. 0 Hilbert’s Integral Inequality (i) For p > 1, we have the following inequality: β Z ∞ Z ∞ ln x f (x)g(y) y (3.1) I := dxdy < kλ (r)||f ||p,φ ||g||q,ψ ; λ−α (x + y) (max{x, y})α 0 0 (ii) For 0 < p < 1, we have the reverse of (3.1), where the constant factor kλ (r) expressed by (2.5) in (3.1) and its reverse is the best possible. Proof. (i) By Hölder’s inequality with weight [13], in view of (2.3), we find β " #" # Z ∞Z ∞ λ λ ln xy x(1− r )/q y (1− s )/p I= f (x) g(y) dxdy λ (x + y)λ−α (max{x, y})α y (1− λs )/p x(1− r )/q 0 0 p1 β x Z Z λ ∞ ∞ ln y x(1− r )(p−1) p ≤ · f (x)dxdy λ (x + y)λ−α (max{x, y})α y 1− s 0 0 × Z 0 ∞Z 0 β ln xy ∞ (x + y)λ−α (max{x, y})α · y (1− λs )(q−1) λ x1− r g q (y)dxdy 1q Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 10 of 17 Go Back Full Screen Close Z = (3.2) p1 Z ∞ p $λ (s, x)φ(x)f (x)dx 0 ∞ q 1q $λ (r, y)ψ(y)g (y)dy . 0 We confirm that the middle of (3.2) keeps the form of strict inequality. Otherwise, there exist constants A and B, such that they are not all zero and [13] λ A x(1− r )(p−1) y 1− λs Hilbert’s Integral Inequality λ f p (x) = B λ y (1− s )(q−1) x 1− λ r g q (y) a.e. in (0, ∞) × (0, ∞). Bicheng Yang vol. 9, iss. 2, art. 59, 2008 λ It follows that Axp(1− r ) f p (x) = By q(1− s ) g q (y) a.e. in (0, ∞) h × (0,λ ∞). Assuming i 1 )−1 p(1− λ p r f (x) = By q(1− s ) g q (y) Ax that A 6= 0, there exists y > 0, such that x a.e. in x ∈ (0, ∞). This contradicts the fact that 0 < ||f ||p,φ < ∞. Then inequality (3.1) is valid by using (2.4) and (2.5). r For n ∈ N, n > |q|λ , setting fn , gn as ( ( 0, 0 < x ≤ 1; 0, 0 < x ≤ 1; fn (x) := g (x) := n λ 1 λ 1 x r − np −1 , x > 1; x r − nq −1 , x > 1; if there exists a constant factor 0 < k ≤ kλ (r), such that (3.1) is still valid if we replace kλ (r) by k, then by (2.6), we have β Z ∞ Z ∞ ln x fn (x)gn (y) y 1 kλ (r) + o(1) = In = dxdy n 0 (x + y)λ−α (max{x, y})α 0 1 < k||fn ||p,φ ||gn ||q,ψ = k, n Title Page Contents JJ II J I Page 11 of 17 Go Back Full Screen Close and kλ (r) ≤ k (n → ∞). Hence k = kλ (r) is the best constant factor of (3.1). (ii) For 0 < p < 1, by the reverse Hölder’s inequality with weight [13], in view of (2.3), we find the reverse of (3.2), which still keeps the strict form. Then by (2.4) and (2.5), we have the reverse of (3.1). By using (2.6) and the same manner as mentioned above , we can show that the constant factor in the reverse of (3.1) is still the best possible. The theorem is proved. 1 + 1q = p q(1− λs )−1 Theorem 3.2. Assume that p > 0 (p 6= 1), r > 1, λ α ∈ R, β > −1, φ(x) = xp(1− r )−1 , ψ(x) = x 0 < ||f ||p,φ < ∞. 1, 1 r + 1 s = 1, λ > 0, (x ∈ (0, ∞)), f ≥ 0, (i) For p > 1, we have the following inequality, which is equivalent to (3.1) and with the best constant factor kλp (r): p β x Z ∞ Z ∞ ln f (x) pλ y (3.3) dx dy J := y s −1 λ−α (max{x, y})α (x + y) 0 0 < kλp (r)||f ||pp,φ ; Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 12 of 17 Go Back (ii) For 0 < p < 1, we have the reverse of (3.3), which is equivalent to the reverse of (3.1), with the best constant factor kλp (r). Proof. (i) For p > 1, x > 0, setting a bounded measurable function as ( f (x), for f (x) < n; [f (x)]n := min{f (x), n} = n, for f (x) ≥ n, Full Screen Close Rn since ||f ||p,φ > 0, there exists n0 ∈ N, such that 1 n φ(x)[f (x)]pn dx > 0 (n ≥ n0 ). Setting gen (y) (y ∈ ( n1 , n); n ≥ n0 ) as p−1 β x Z n ln pλ y (3.4) gen (y) := y s −1 [f (x)]n dx , 1 (x + y)λ−α (max{x, y})α n Hilbert’s Integral Inequality then by (3.1), we find Z n ψ(y)e gnq (y)dy 0< Bicheng Yang vol. 9, iss. 2, art. 59, 2008 1 n Z = Z n pλ −1 s y 1 n Z n n Z = n 1 n β ln xy [f (x)]n dx (x + Contents dy β ln xy [f (x)]n gen (y) n < kλ (r) 1 n φ(x)[f (x)]pn dx 1 n ψ(y)e gnq (y)dy JJ II J I Page 13 of 17 1 n n (3.5) y)λ−α (max{x, y})α dxdy (x + y)λ−α (max{x, y})α (Z ) p1 (Z 1 n Title Page p ) 1q Go Back < ∞; Full Screen Close Z (3.6) n 0< 1 n ψ(y)e gnq (y)dy < kλp (r) Z ∞ φ(x)f p (x)dx < ∞. 0 It follows 0 < ||g||q,ψ < ∞. For n → ∞, by (3.1), both (3.5) and (3.6) still keep the forms of strict inequality. Hence we have (3.3). On the other-hand, suppose (3.3) is valid. By Hölder’s inequality, we have β x Z ∞ Z ∞ h i ln y f (x)dx −1 p1 − λs +λ (3.7) I= y g(y) dy y p s (x + y)λ−α (max{x, y})α 0 0 1 ≤ J p ||g||q,ψ . Hilbert’s Integral Inequality In view of (3.3), we have (3.1), which is equivalent to (3.3). We confirm that the constant factor in (3.3) is the best possible. Otherwise, we may get a contradiction by (3.7) that the constant factor in (3.1) is not the best possible. (ii) For 0 < p < 1, since ||f ||p,φ > 0, we confirm that J > 0. If J = ∞, then the reverse of (3.3) is naturally valid. Suppose 0 < J < ∞. Setting p−1 β x Z ∞ ln y pλ f (x)dx , g(y) := y s −1 λ−α (max{x, y})α (x + y) 0 Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 14 of 17 by the reverse of (3.1), we obtain Go Back ∞> ||g||qq,ψ = J = I > kλ (r)||f ||p,φ ||g||q,ψ > 0; 1 p J = ||g||q−1 q,ψ > kλ (r)||f ||p,φ . Hence we have the reverse of (3.3). On the other-hand, suppose the reverse of (3.3) is valid. By the reverse Hölder’s inequality, we can get the reverse of (3.7). Hence in view of the reverse of (3.3), we obtain the reverse of (3.1), which is equivalent to the reverse of (3.3). We confirm that the constant factor in the reverse of (3.3) is the best possible. Otherwise, we may get a contradiction by the reverse of (3.7) that Full Screen Close the constant factor in the reverse of (3.1) is not the best possible. The theorem is proved. Remark 1. For p = r = 2 in (3.1), setting α = β = 0, λ = 1, we obtain (1.1); setting α = 0, β = λ = 1, we obtain (1.3). For α = λ > 0, β > −1 in (3.1), we have β Z ∞ Z ∞ ln x f (x)g(y) y rβ+1 + sβ+1 dxdy < Γ(β + 1)||f ||p,φ ||g||q,ψ , (3.8) (max{x, y})λ λβ+1 0 0 1 β+1 β+1 where the constant factor λβ+1 (r + s )Γ(β + 1) is the best possible. For p = r = 2 in (3.8), setting λ = 1, β = 0, we obtain (1.2); setting λ = 1, β = 1, we obtain (1.4). Hence inequality (3.1) is a relation to (1.1), (1.2), (1.3) and (1.4). Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 15 of 17 Go Back Full Screen Close References [1] G.H. HARDY, J.E. LITTLEWOOD Univ. Press, Cambridge, 1952. AND G. PÓLYA, Inequalities, Cambridge [2] BICHENG YANG, On a base Hilbert-type inequality, Journal of Guangdong Education Institute, 26(3) (2006), 1–5. [3] BICHENG YANG, On a base Hilbert-type integral inequality and extensions, College Mathematics, 24(2) (2008), 87–92. [4] G.H. HARDY, Note on a theorem of Hilbert concerning series of positive terms, Proc. London Math. Soc., 23(2) (1925), Records of Proc. xlv-xlvi. [5] D.S. MINTRINOVIĆ, J.E. PEČARIĆ AND A.M. FINK, Inequalities Involving Functions and their Integrals and Derivatives, Kluwer Academic Publishers, Boston, 1991. [6] BICHENG YANG, On Hilbert’s integral inequality, J. Math. Anal. Appl., 220 (1998), 778–785. [7] BICHENG YANG, A note on Hilbert’s integral inequality, Chin. Quart. J. Math., 13(4) (1998), 83–86. 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Hilbert’s Integral Inequality Bicheng Yang vol. 9, iss. 2, art. 59, 2008 Title Page Contents JJ II J I Page 17 of 17 Go Back Full Screen Close