ON A DECOMPOSITION OF HILBERT’S INEQUALITY Hilbert’s Inequality BICHENG YANG Department of Mathematics Guangdong Education Institute Guangzhou, Guangdong 510303 P.R. CHINA Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page EMail: bcyang@pub.guangzhou.gd.cn Contents Received: 07 October, 2008 JJ II Accepted: 20 March, 2009 J I Communicated by: B. Opic Page 1 of 18 2000 AMS Sub. Class.: 26D15. Key words: Hilbert’s inequality; Weight coefficient; Equivalent form; Hilbert-type inequality. Abstract: By using the Euler-Maclaurin’s summation formula and the weight coefficient, a pair of new inequalities is given, which is a decomposition of Hilbert’s inequality. The equivalent forms and the extended inequalities with a pair of conjugate exponents are built. Acknowledgement: The author expresses his sincerest thanks to the referee for his thoughtful suggestions in improving this work. Go Back Full Screen Close Contents 1 Introduction 3 2 Some Lemmas 6 3 Main Results and their Equivalent Forms 12 Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 2 of 18 Go Back Full Screen Close 1. Introduction In 1908, H. Weyl the following Hilbert inequality: If {an }, {bn } are real Ppublished P∞ ∞ 2 sequences, 0 < n=1 an < ∞ and 0 < n=1 b2n < ∞, then [1] (1.1) ∞ ∞ X ∞ ∞ X X X am bn 2 b2n <π an m+n n=1 m=1 n=1 n=1 ! 12 , Hilbert’s Inequality Bicheng Yang where the constant factor π is the best possible. In 1925, G. H. Hardy gave an extension of (1.1) by introducing one pair of conjugate exponents (p, q) ( p1 + 1q = 1) P P∞ q p as [2]: If p > 1, an , bn ≥ 0, 0 < ∞ n=1 an < ∞ and 0 < n=1 bn < ∞, then (1.2) ∞ X ∞ X am b n π < m+n sin( πp ) n=1 m=1 ∞ X ! p1 apn n=1 ∞ X (1.3) ∞ X am b n ≤ K(p, q) apn λ (m + n) n=1 m=1 n=1 bqn Contents , n=1 ! p1 Title Page ! 1q where the constant factor π/ sin( πp ) is the best possible. We refer to (1.2) as the Hardy-Hilbert inequality. In 1934, Hardy et al. [3] gave some applications of (1.1) and (1.2). By introducing a pair of non-conjugate exponents (p, q) in (1.1), Hardy et al. [3] gave: If p, q > 1, p1 + 1q ≥ 1, 0 < λ = 2 − ( p1 + 1q ) ≤ 1, then ∞ X ∞ X vol. 10, iss. 1, art. 25, 2009 ∞ X ! 1q bqn JJ II J I Page 3 of 18 Go Back Full Screen , n=1 where the constant factor K(p, q) is the best value only for λ = 1. In 1951, Bonsall [4] considered (1.3) in the case of a general kernel. In 1991, Mitrinović et al. [5] summarized the above method and results. Close In 1997-1998, by using weight coefficients, Yang and Gao [6], [7] gave a strengthened version of (1.2) as: (1.4) ∞ ∞ X X am b n m+n n=1 m=1 (∞ " X < n=1 # ) p1 ( ∞ " # ) 1q X π π 1−γ p 1−γ q , − 1/p an − 1/q bn sin( πp ) n sin( πp ) n n=1 where, 1 − γ = 0.42278433+ (γ is the Euler constant). In 2001, Yang [8] gave an extension of (1.1) by introducing an independent parameter 0 < λ ≤ 4 as (1.5) ∞ X ∞ X am b n <B (m + n)λ n=1 m=1 ! 12 X ∞ ∞ X λ λ , , n1−λ a2n n1−λ b2n 2 2 n=1 n=1 where the constant factor B( λ2 , λ2 ) is the best possible (B(u, v) is the Beta function). In 2004, Yang [9] published the dual form of (1.2) as follows Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 4 of 18 (1.6) ∞ X ∞ X am b n π < m+n sin( πp ) n=1 m=1 ∞ X n=1 ! p1 np−2 apn ∞ X ! 1q nq−2 bqn . n=1 For p = q = 2, both (1.6) and (1.2) reduce to (1.1). It means that there are two different best extensions of (1.1). To generalize (1.2) and (1.6), in 2005, Yang [10] gave an extension of (1.2) and (1.6) with two pairs of conjugate exponents (p, q), (r, s)(p, r > P p(1− αλ −1 p r ) 1) and parameters α, λ > 0 (αλ ≤ min{r, s}) as: If 0 < ∞ an < ∞ n=1 n Go Back Full Screen Close and 0 < (1.7) P∞ n=1 αλ nq(1− s )−1 bqn < ∞, then ∞ X ∞ X am b n (mα + nα )λ n=1 m=1 < kαλ (r) (∞ X n p(1− αλ )−1 r apn ) p1 ( ∞ X n=1 ) 1q n q(1− αλ )−1 s bqn , Hilbert’s Inequality n=1 Bicheng Yang where the constant factor kαλ (r) = α1 B( λr , λs ) is the best possible. T. K. Pogány [11] also considered a best extension of (1.2) with the non-homogeneous kernel as 1 (µ, λm , ρn > 0). (λm +ρn )µ We have a non-negative decomposition of kernel in (1.1): 1 max{m, n} min{m, n} = + m+n (m + n)2 (m + n)2 vol. 10, iss. 1, art. 25, 2009 Title Page Contents (m, n ∈ N) (N is the set of positive integer numbers). In this paper, by using the Euler-Maclaurin summation formula and the weight coefficient as in [8], we give a pair of new Hilbert-type inequalities as ! 12 ∞ X ∞ ∞ ∞ π X X X max{m, n} am b n < +1 a2n b2n (1.8) ; 2 (m + n) 2 n=1 m=1 n=1 n=1 ! 12 ∞ ∞ ∞ X ∞ π X X X min{m, n} (1.9) am b n < −1 a2n b2n , 2 (m + n) 2 n=1 n=1 n=1 m=1 where the sum of two best constant factors is π. The equivalent forms and extended inequalities with a pair of conjugate exponents are considered. JJ II J I Page 5 of 18 Go Back Full Screen Close 2. Some Lemmas Lemma 2.1 (Euler-Maclaurin’s summation formula, cf. [8, 12, Lemma 1]). If f (x) ∈ C 1 [1, ∞), then we have Z ∞ Z ∞ ∞ X 1 (2.1) f (k) = f (x)dx + f (1) + P1 (x)f 0 (x)dx, 2 1 1 k=1 x−[x]− 21 (i) 3 where P1 (x) = is the Bernoulli function of the first order; if g ∈ C [1, ∞), i (i) (−1) g (x) > 0, g (∞) = 0, (i = 0, 1, 2, 3), then Z n 1 (2.2) [g(n) − g(1)] < P1 (x)g(x)dx < 0, 12 1 Z ∞ 1 − g(n) < P1 (x)g(x)dx < 0. 12 n Lemma 2.2. If 1 2 ≤ α < 1, setting the weight coefficient ω(α, m) as ω(α, m) := (2.3) ∞ X n=1 max{m, n}mα (m + n)2 nα (m ∈ N), Title Page Contents JJ II J I Page 6 of 18 Full Screen k(α) = Aα (m) + ω(α, m); where 1 k(α) := α and Aα (m) = O(m Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Go Back then we have (2.4) Hilbert’s Inequality α−1 Z ∞ 0 ), (m → ∞). ω 1 ,m 2 1 π <k = + 1, 2 2 max u1/α , 1 1 −2 u α du (u1/α + 1)2 Close Proof. For fixed 12 ≤ α < 1, m ∈ N, setting f (x) := (2.1), it follows that ω(α, m) = m (2.5) α ∞ X max{m,x} , (m+x)2 xα x ∈ (0, ∞), then by f (n) n=1 =m α = mα Z ∞ 1 ∞ Z 1 f (x)dx + f (1) + 2 Z ∞ 0 P1 (x)f (x)dx Hilbert’s Inequality 1 Bicheng Yang f (x)dx − mα ρ(α, m), P1 (x)f 0 (x)dx. vol. 10, iss. 1, art. 25, 2009 0 Z ρ(α, m) := (2.6) 0 We find 1 1 f (x)dx − f (1) − 2 Z Title Page ∞ 0 P1 (x)f (x)dx. Contents 1 1 −m −1 1 − f (1) = = + , 2 2 2(m + 1) 2(m + 1) 2(m + 1)2 JJ II J I Page 7 of 18 and Z 1 m m 1 f (x)dx = dx ≥ dx = ; 2 α 2 m+1 0 0 (m + x) x 0 (m + x) Z 1 Z 1 m 1 f (x)dx ≤ dx = . 2 α (1 − α)m 0 0 m x Z 1 Z For x ∈ (0, m), f (x) = 1 m , (m+x)2 xα it follows f 0 (x) = −2m (m+x)3 xα − αm ; (m+x)2 xα+1 Go Back Full Screen Close for x ∈ (m, ∞), f (x) = x1−α , (m+x)2 we find −2x1−α 1−α + 3 (m + x) (m + x)2 xα −2(x + m − m) 1−α = + 3 α (m + x) x (m + x)2 xα −2 2m 1−α = + + . (m + x)2 xα (m + x)3 xα (m + x)2 xα f 0 (x) = Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 1 1 In the following, it is obvious that g1 (x) = (m+x) 3 xα , g2 (x) = (m+x)2 xα+1 and 1 g3 (x) = (m+x) 2 xα are suited to apply in (2.2). Then by (2.2), we obtain Z m − (2.7) P1 (x)f 0 (x)dx 1 Z m Z m αmP1 (x)dx 2mP1 (x)dx = + 3 α (m + x) x (m + x)2 xα+1 1 1 2m 1 1 αm 1 1 > − + − 12 8m3+α (m + 1)3 12 4m3+α (m + 1)2 α+1 α 2−α 1 = − − + ; 2+α 2 48m 12(m + 1) 12(m + 1) 6(m + 1)3 Z m Contents JJ II J I Page 8 of 18 Go Back Full Screen Close ∞ (2.8) − Title Page P1 (x)f 0 (x)dx Z ∞ Z ∞ Z ∞ 2P1 (x)dx 2mP1 (x)dx P1 (x)dx = − − (1 − α) 2 α 3 α 2 α m (m + x) x m (m + x) x m (m + x) x Z ∞ −1 > − P1 (x)f 0 (x)dx 24m2+α 1 Z m Z ∞ 0 =− P1 (x)f (x)dx − P1 (x)f 0 (x)dx 1 m α 2−α 1 α−1 > − − + . 2+α 2 48m 12(m + 1) 12(m + 1) 6(m + 1)3 Hence by (2.6), for α = 12 , it follows that 1 −1 1 1 (2.9) ρ ,m > + + 2 2 2(m + 1) 2(m + 1) m+1 1 1 2 − 12 −1 1 2 + 2 2+1/2 − − + 48m 12(m + 1) 12(m + 1)2 6(m + 1)3 9 −1 1 11 = + + + 2 2+1/2 24(m + 1) 24(m + 1) 96m 6(m + 1)3 11 9 −1 1 ≥ + + + > 0. 2 2 24(m + 1) 96m 96m 6(m + 1)3 By (2.7) and (2.8), we obtain Z ∞ Z 0 − P1 (x)f (x)dx = − 1 Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 9 of 18 Go Back m P1 (x)f 0 (x)dx − 1 1−α 1 + < 2+α 48m 48m2+α 2−α = . 48m2+α Z ∞ m P1 (x)f 0 (x)dx Full Screen Close Then by (2.6), it follows 0 < m1−α [mα ρ(α, m)] −m m 1 2−α < + + + 2(m + 1) 2(m + 1)2 1 − α 48m1+α 1 1 → − (m → ∞). 1−α 2 Setting u = (x/m)α , we find Z ∞ Z ∞ max{m, x} α α (2.11) m f (x)dx = m dx (m + x)2 xα 0 0 Z 1 ∞ max{u1/α , 1} 1 −2 u α du = k(α), = α 0 (u1/α + 1)2 Z ∞ Z 1 1 max{u2 , 1} du (2.12) k =2 du = 4 2 2 2 (u2 + 1)2 0 0 (u + 1) Z π 4 π =4 cos2 θdθ = + 1. 2 0 Hence by (2.5), (2.9), (2.10) and (2.11), (2.4) is valid and the lemma is proved. (2.10) 1 2 ≤ α < 1, setting the weight coefficient $(α, m) as $(α, m) := (2.13) ∞ X min{m, n}mα n=1 (m ∈ N), then we have (2.14) Title Page Contents JJ II J I Page 10 of 18 $ Full Screen Close (m + n)2 nα e k(α) = Bα (m) + $(α, m); Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Go Back Similar to Lemma 2.2, we still have Lemma 2.3. If Hilbert’s Inequality 1 ,m 2 1 π e <k = − 1, 2 2 where 1 e k(α) = α and Bα (m) = O(m α−2 Z ∞ 0 min{u1/α , 1} 1 −2 u α du (u1/α + 1)2 ), (m → ∞). Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 11 of 18 Go Back Full Screen Close 3. Main Results and their Equivalent Forms P∞ p −1 p 2 Theorem 3.1. If p > 1, p1 + 1q = 1, an , bn ≥ 0, 0 < an < ∞ and n=1 n P∞ q −1 q 0 < n=1 n 2 bn < ∞, then we have the following equivalent inequalities ∞ X ∞ X max{m, n} I := am b n (m + n)2 n=1 m=1 ! p1 ∞ X π p < +1 n 2 −1 apn 2 n=1 (3.1) Hilbert’s Inequality ∞ X ! 1q n q −1 2 bqn Bicheng Yang ; vol. 10, iss. 1, art. 25, 2009 n=1 Title Page (3.2) J := ∞ X " p n 2 −1 n=1 where the constant factors π 2 ∞ X max{m, n} am (m + n)2 m=1 + 1 and π 2 #p < π 2 ∞ p X q +1 n 2 −1 apn , n=1 p + 1 are the best possible. Proof. By Hölder’s inequality and (2.3) – (2.4), we find " ∞ #p X max{m, n} (3.3) am (m + n)2 m=1 ( ∞ )p X max{m, n} m1/(2q) n1/(2p) = am (m + n)2 n1/(2p) m1/(2q) m=1 " ∞ #" ∞ #p−1 X max{m, n} mp/(2q) X max{m, n} nq/(2p) ≤ apm 2 1/2 (m + n) n (m + n)2 m1/2 m=1 m=1 Contents JJ II J I Page 12 of 18 Go Back Full Screen Close ∞ X 1 max{m, n} mp/(2q) p 1− p2 =ω ,n n a 2 (m + n)2 n1/2 m m=1 ∞ π p−1 X max{m, n} mp/(2q) p 1− p2 ≤ +1 n a ; 2 (m + n)2 n1/2 m m=1 p−1 J≤ = = π 2 π 2 π 2 ∞ X ∞ p−1 X max{m, n} mp/(2q) p +1 a (m + n)2 n1/2 m n=1 m=1 "∞ # ∞ p−1 X X max{m, n} mp/(2q) +1 apm 2 1/2 (m + n) n m=1 n=1 ∞ ∞ p−1 X π p X p p 1 −1 p +1 ω , m m 2 am < +1 m 2 −1 apm . 2 2 m=1 m=1 Therefore (3.2) is valid. By Hölder’s inequality, we find that " # ∞ ∞ h −1 1 i X 1 1 X max{m, n} I= n2−p (3.4) a n 2 + p bn m 2 (m + n) n=1 m=1 ! 1q ∞ X q 1 ≤ Jp n 2 −1 bqn . n=1 Then by (3.2), we have (3.1). On the other hand, suppose that (3.1) is valid. Setting #p−1 " ∞ X max{m, n} p bn := n 2 −1 am , n ∈ N, (m + n)2 m=1 Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 13 of 18 Go Back Full Screen Close P q −1 q 2 then it follows ∞ bn = J. By (3.3), we confirm that J < ∞. If J = 0, then n=1 n (3.2) is naturally valid; if 0 < J < ∞, then by (3.1), we find ∞ X n q −1 2 bqn = J = I < n=1 ∞ X π 2 ∞ X p +1 n 2 −1 apn n=1 ! p1 n n=1 q −1 2 bqn ! p1 1 p =J < π 2 ∞ X ! 1q n q −1 2 bqn ; n=1 ∞ X p +1 n 2 −1 apn ! p1 , n=1 Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 and inequality (3.2) is valid, which is equivalent to (3.1). −1 −1 ε − pε 2 ebn2 − q , for n ∈ N, if eb = {ebn }∞ as e , For 0 < ε < 2q , setting e a = {e an } ∞ , a n n=1 n=1 there exists a constant 0 < k ≤ π2 + 1, such that (3.1) is still valid when we replace π + 1 by k, then we find 2 ∞ X ∞ X max{m, n}e amebn (m + n)2 n=1 m=1 ! p1 ∞ ! 1q ∞ ∞ X X q X p 1 −1 p −1eq 2 2 <k n e an n bn ; =k 1+ε n n=1 n=1 n=1 Ie := Title Page Contents JJ II J I Page 14 of 18 Go Back Full Screen Ie = ∞ X n=1 ∞ X " ∞ X # −1 ε ε max{m, n} −1 m 2 −p n 2 −q 2 (m + n) m=1 1 ε ∞ ∞ X 1 X max{m, n}m 2 + q 1 1 ε = = ω + ,m . m1+ε n=1 (m + n)2 n 12 + qε m1+ε 2 q m=1 m=1 Close And then by (2.4) and the above results, it follows that X ∞ ∞ X 1 ε 1 1 1 A 1 + ε (m) 1− >k + (3.5) k 1+ε 1+ε 2 q 1 ε n 2 q m k + n=1 m=1 2 q X ∞ ∞ X 1 ε 1 1 1 A 1 ε (m) =k − + 1+ε 2 + q 1+ε ε 1 m 2 q m k + m=1 2 q m=1 P ∞ X 1 ∞ 1 ε m=1 m1+ε A 2 + q (m)] 1 1 ε ; P =k + 1 − ∞ 1 1 ε 2 q m=1 m1+ε k 2+q m=1 m1+ε 1 ε P∞ 1 2−q 1 O m=1 m1+ε m 1 ε . P k>k + 1− ∞ 1 2 q k 1+ε 2 Setting α = 1 2 q m=1 m1+ε + qε , by Fatou’s Lemma, it follows that Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 15 of 18 lim k ε→0+ 1 ε + 2 q Z 1 ∞ max{u1/α , 1} 1 −2 = lim+ u α du (u1/α + 1)2 α→ 12 α 0 Z ∞ max{u1/α , 1} 1 −2 π 1 α u du = k = + 1. ≥2 lim+ (u1/α + 1)2 2 2 α→ 12 0 Then by (3.5), we have k ≥ π2 + 1 (ε → 0+ ). Hence k = π2 + 1 is the best value of (3.1). We confirm that the constant factor in (3.2) is the best, otherwise we would obtain a contradiction by (3.4) that the constant factor in (3.1) is not the best possible. The theorem is proved. Go Back Full Screen Close In the same manner, by Lemma 2.3, we have: P∞ p −1 p 2 an < ∞ and Theorem 3.2. If p > 1, p1 + 1q = 1, an , bn ≥ 0, 0 < n=1 n P∞ q −1 q 0 < n=1 n 2 bn < ∞, then we have the following equivalent inequalities (3.6) ∞ X ∞ ∞ π X X p min{m, n} a b < − 1 n 2 −1 apn m n 2 (m + n) 2 n=1 n=1 m=1 ∞ X n=1 " n p −1 2 ∞ X min{m, n} am (m + n)2 m=1 where the constant factors π 2 #p < π 2 ! p1 ∞ X ! 1q n q −1 2 bqn n=1 ∞ p X q −1 n 2 −1 apn , ; Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 n=1 − 1 and ( π2 − 1)p are the best possible. Title Page Contents Remark 1. For p = q = 2, (3.1) reduces to (1.8) and (3.6) reduces to (1.9). JJ II J I Page 16 of 18 Go Back Full Screen Close References [1] H. WEYL, Singuläre Integralgleichungen mit besonderer Berücksichtigung des Fourierschen Integraltheorems, Göttingen : Inaugural–Dissertation, 1908. [2] G.H. HARDY, Note on a theorem of Hilbert concerning series of positive term, Proceedings of the London Math. Society, 23 (1925),45–46. [3] G.H. HARDY, J.E. LITTLEWOOD University Press, Cambridge,1934. AND G. PÓLYA, Inequalities, Cambridge [4] F.F. BONSALL, Inequalities with non-conjugate parameter, J. Math. Oxford Ser., 2(2) (1951), 135–150. [5] D. S. MITRINOVIĆ, J.E. PEČARIĆ AND A.M. FINK, Inequalities Involving Functions and their Integrals and Derivatives, Kluwer Academic Publishers, Boston, 1991. [6] BICHENG YANG AND MINGZHE GAO, On a best value of Hardy-Hilbert’s inequality, Advances in Math. (China), 26(2) (1997), 159–164. [7] MINGZHE GAO AND BICHENG YANG, On the extended Hilbert’s inequality, Proc. Amer. Math. Soc., 126(3) (1998), 751–759. [8] BICHENG YANG, On a generalization of Hilbert double series theorem, Journal of Nanjing University Mathematical Biquarterly (China), 18(1) (2001), 145–152. [9] BICHENG YANG, On new extensions of Hilbert’s inequality, Acta Math. Hungar., 104(4) (2004), 291–299. Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 17 of 18 Go Back Full Screen Close [10] BICHENG YANG, On best extensions of Hardy-Hilbert’s inequality with two parameters, J. Ineq. Pure & Applied Math., 6(3) (2005), Art. 81. [ONLINE http://jipam.vu.edu.au/article.php?sid=554]. [11] T.K. POGÁNY, Hilbert’s double series theorem extended to the case of nonhomogeneous kernels, J. Math. Anal. Appl., 342 (2008), 1485–1489. [12] BICHENG YANG, On a strengthened version of the more accurate HardyHilbert’s inequality, Acta Mathematica Sinica (Chin. Ser.), 42(6) (1999), 1103– 1110. Hilbert’s Inequality Bicheng Yang vol. 10, iss. 1, art. 25, 2009 Title Page Contents JJ II J I Page 18 of 18 Go Back Full Screen Close