Journal of Inequalities in Pure and Applied Mathematics FOUR INEQUALITIES SIMILAR TO HARDY-HILBERT’S INTEGRAL INEQUALITY volume 7, issue 2, article 76, 2006. W.T. SULAIMAN College of Computer Sciences and Mathematics University of Mosul, Iraq. Received 22 September, 2005; accepted 05 January, 2006. Communicated by: B. Yang EMail: waadsulaiman@hotmail.com Abstract Contents JJ J II I Home Page Go Back Close c 2000 Victoria University ISSN (electronic): 1443-5756 285-05 Quit Abstract Four new different types of inequalities similar to Hardy-Hilbert’s inequality are given. 2000 Mathematics Subject Classification: 26D15. Key words: Hardy-Hilbert integral inequality. Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality The author is grateful to the referee who read through the paper very carefully and correct many typing mistakes. W.T. Sulaiman Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 New Inequalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References 3 6 Title Page Contents JJ J II I Go Back Close Quit Page 2 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au 1. Introduction Suppose f and g are real functions, such that 0 < R ∞ that 2 0 < 0 g (t)dt < ∞, then Z ∞ Z (1.1) 0 0 ∞ f (x)g(y) <π x+y Z ∞ 2 Z ∞ f (t)dt 0 R∞ 0 f 2 (t)dt < ∞ and 12 g (t)dt , 2 0 where π P is best possible. If (an )P and (bn ) are sequences of real numbers such ∞ 2 2 that 0 < ∞ a < ∞ and 0 < n=1 n n=1 bn < ∞, then (1.2) ∞ X ∞ ∞ ∞ X X X am b n 2 <π an b2n m+n n=1 m=1 n=1 n=1 ! 12 Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman . Title Page The inequalities (1.1) and (1.2) are called Hilbert’s inequalities. These inequalities play an important role in analysis (cf. [1, Chap. 9]). In their recent papers Hu [5] and Gao [3] gave two distinct improvements of (1.1) and Gao [4] gave a strengthened version of (1.2). The following definitions are given: Contents JJ J II I Go Back r+λ−2 ϕλ (r) = (r = p, q), r kλ (p) = B (ϕλ (p), ϕλ (q)) , and B is the beta function. Recently, by introducing some parameters, Yang and Debnath [2] gave the following extensions: Close Quit Page 3 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Theorem A. If f, g ≥ 0, p > 1, 1 p + 1 q = 1, λ > 2 − min {p, q} , such that ∞ Z 0< t ∞ Z 1−λ p f (t)dt < ∞ and t1−λ g q (t)dt < ∞, 0< 0 0 then Z ∞ ∞ Z (1.3) 0 0 < f (x)g(y) (Ax + By)λ kλ (p) ϕ λ A (p) B ϕλ (q) dxdy Z ∞ x1−λ f p (x)dx 0 p1 Z ∞ y 1−λ g q (y)dy 1q , 0 where the constant factor [kλ (p)/A ϕλ (p) B ϕλ (q) ∞ y (λ−1)(p−1) Z ∞ f (x) !p dx dy (Ax + By)λ p Z ∞ kλ (p) < x1−λ f p (x)dx, ϕ (p) ϕ (q) λ λ A B 0 p where the constant factor kλ (p)/Aϕλ (p) B ϕλ (q) is the best possible. The inequalities (1.3) and (1.4) are equivalent. (1.4) 0 0 W.T. Sulaiman ] is the best possible. Theorem B. If f ≥ 0, p > 1, p1 + 1q = 1, λ > 2 − min {p, q} , A, B > 0 such R∞ that 0 < 0 t1−λ f p (t)dt < ∞, then Z Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality Title Page Contents JJ J II I Go Back Close Quit Page 4 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Theorem C. If an , bn > 0 (n ∈ N), p > 1, p1 + 1q = 1, 2 − min {p, q} < λ < 2, A, B > 0 such that 0< ∞ X n1−λ apn <∞ n=1 and 0< ∞ X n1−λ bqn < ∞, n=1 then (1.5) ∞ X ∞ X am b n n=1 m=1 (Am + Bn)λ ! p1 ∞ ! 1q ∞ X X kλ (p) n1−λ bqn , < ϕ (p) ϕ (q) n1−λ apn A λ B λ n=1 n=1 where the constant factor kλ (p)/Aϕλ (p) B ϕλ (q) is the best possible. Theorem D. If an ≥ 0 (n ∈ N), p > 1, p1 + 1q = 1, 2 − min {p, q} < λ ≤ 2, P 1−λ p A, B > 0 such that 0 < ∞ an < ∞, then n=1 n !p ∞ ∞ X X a m (1.6) n(λ−1)(p−1) λ n=1 m=1 (Am + Bn) p X ∞ kλ (p) < n1−λ apn , Aϕλ (p) B ϕλ (q) n=1 p where the constant factor kλ (p)/Aϕλ (p) B ϕλ (q) is the best possible. The inequalities (1.5) and (1.6) are equivalent. Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman Title Page Contents JJ J II I Go Back Close Quit Page 5 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au 2. New Inequalities The aim of this paper is to give the following results: Theorem 2.1. Let ln f , ln g be convex for nonnegative functions f and g such that f (0) = g(0) = 0, f (∞) = g(∞) = ∞, f 0 (s) ≥ 0, g 0 (s) ≥ 0, s ∈ {xp , y q } . Let λ > max {p, q} , p > 1, p1 + 1q = 1. Let Z 0< ∞ −p2 /q 2 t p [f 0 (t)] q 0 Z 0< ∞ −q 2 /p2 t 0 0 [g(tq )]2−λ+q/p q 0 then we have Z ∞Z (2.1) [f (tp )]2−λ+p/q [g 0 (t)] p dt < ∞, dt < ∞, Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman Title Page ∞ f (xy)g(xy) dxdy (f (xp ) + g(y q ))λ 1 1 1 p (p, λ − p) B q (q, λ − q) ≤ √ B √ p p q q ! p1 Z ∞ −p2 /q2 t [f (tp )]2−λ+p/q × dt p [f 0 (t)] q 0 ! 1q Z ∞ −q2 /p2 t [g(tq )]2−λ+q/p × dt . q [g 0 (t)] p 0 Contents JJ J II I Go Back Close Quit Page 6 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Proof. Since ln f is convex and xy ≤ ln f (xy) f (xy) = e ln f ≤e Therefore, we have Z ∞Z ∞ f (xy)g(xy) 0 (f (xp ) + g(y q ))λ 0 1 ∞ Z ∞ Z q xp + yq p xp p + ≤e yq , q then ln f (xp ) ln f (y q ) + p q 0 1 f p (xp )g q (y q ) [g (y 1 q )] p 1 [f 0 (xp )] q q−1 p p−1 x q y 1 ∞Z ∞ ≤ 0 × λ q 1 dxdy Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman f (xp )g p/q (y q )g 0 (y q )y q−1 0 = M pN q, p−1 q q−1 y p x (f (xp ) + g(y q )) q ! p1 dxdy p x(p−1)p/q [f 0 (xp )] q (f (xp ) + g(y q ))λ Z ∞Z ∞ f (y q )g q/p (xp )f 0 (xp )xp−1 0 1 1 p )] q 1 [g 0 (y q )] p λ Z 0 1 f q (y q )g p (xp ) [f (x (f (xp ) + g(y q )) p 0 1 dxdy ≤ 0 1 = f p (xp )f q (y q ). 0 y (q−1)q/p [g 0 (y q )] p (f (xp ) + g(y q ))λ Title Page ! 1q dxdy say. Contents JJ J II I Go Back Then Close 1 M= q Z ∞ x (1−p)p/q 2−λ+p/q p [f (x )] p 0 [f 0 (x)] q Z dx 0 ∞ g(y q ) f (xp ) pq 1+ q−1 g 0 (y q ) qy f (xp ) dy λ q g(y ) f (xp ) Quit Page 7 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au 1 = q ∞ Z 2 /q 2 x−p [f (xp )]2−λ+p/q Z ∞ up/q dx du p (1 + u)λ [f 0 (x)] q 0 Z ∞ −p2 /q2 1 x [f (xp )]2−λ+p/q = B (p, λ − p) dx. p q [f 0 (x)] q 0 0 Similarly, 1 N = B (q, λ − q) p Therefore Z ∞Z ∞ 0 0 Z ∞ y −q 2 /p2 [g(y q )]2−λ+q/p q 0 [g 0 (y)] p dy. Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman f (xy)g(xy) dxdy (f (xp ) + g(y q ))λ 1 1 1 p (p, λ − p) B q (q, λ − q) ≤ √ B √ q p p q ! p1 Z ∞ −p2 /q2 p 2−λ+p/q t [f (t )] × t dt p [f 0 (t)] q 0 ! 1q Z ∞ −q2 /p2 q 2−λ+q/p t [g(t )] . dt × q [g 0 (t)] p 0 Title Page Contents JJ J II I Go Back Close Quit Page 8 of 18 Theorem 2.2. Let f, g, h be nonnegative functions, h(x, y) is homogeneous of order n such that h(0, 1) = h(1, 0) = 0, h(∞, 1) = h(1, ∞) = ∞. Let p > 1, J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au 1 p + 1 q = 1, 0 < 1 + µ − r < λ, where hx = dh/dx, 0 < Z (2.2) ∞ Z ∞ r∈ R∞ 0 n p q , q p o , hx (x, y) ≥ 0, tp/q f p (t)dt < ∞, 0 < R∞ 0 hy (x, y) ≥ 0, tq/p g q (t)dt < ∞, then f (x)g(y)hµ (x, y) dxdy (1 + h(x, y))λ 1 1 p1 p p q q q ≤ B 1 + µ− , λ − 1 − µ+ B 1 + µ− , λ − 1 − µ− n q q p p 1q p1 Z ∞ Z ∞ q p q p p−1 q−1 t g (t)dt . × t f (t)dt 0 0 0 Proof. Observe that Z ∞Z ∞ f (x)g(y)hµ (x, y) 0 0 λ 0 1 q =M N , W.T. Sulaiman Title Page Contents dxdy (1 + h(x, y)) ! p1 Z ∞Z ∞ p µ f (x)h (x, y)hy (x, y) ≤ dxdy p/q hx (x, y) (1 + h(x, y))λ 0 0 ! 1q Z ∞Z ∞ q µ g (y)h (x, y)hx (x, y) × dxdy q/p hy (x, y) (1 + h(x, y))λ 0 0 1 p Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality say. JJ J II I Go Back Close Quit Page 9 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Then ∞ Z Z p M= ∞ f (x)dx 0 0 hµ (x, y)hy (x, y) p/q hx (x, y) (1 + h(x, y))λ dy. Let y = xv, dy = xdv and hence dh(x, xv) dh(1, v) dh(1, v) = xn = xn−1 = xn−1 hv (1, v), dy dy dv dh(x, xv) d n hx (x, y) = = x h(1, v) = nxn−1 h(1, v), dx dx hy (x, y) = therefore 1 M= Z ∞ x p/q−1 p Z f (x)dx ∞ [xn h(1, v)]µ−p/q xn hv (1, v) (1 + xn h(1, v))λ Z ∞ p 1 p = p/q B 1 + µ− , λ − 1 − µ+ xp/q−1 f p (x)dx. n q q 0 np/q 0 W.T. Sulaiman dv 0 q q N = q/p B 1 + µ − , λ − 1 − µ + n p p This implies Z 0 ∞Z 0 ∞ Z 0 Title Page Contents JJ J Similarly, 1 Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality ∞ y q/p−1 g q (y)dy. II I Go Back Close Quit f (x)g(y)hµ (x, y) λ (1 + h(x, y)) dxdy Page 10 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au 1 1 ≤ Bp n 1 p p q q q B 1 + µ − , λ − 1 − µ+ 1 + µ − , λ − 1 − µ+ q q p p Z ∞ p1 Z ∞ 1q p/q−1 p q/p−1 q × t f (t)dt t g (t)dt . 0 0 The following lemma is needed for the coming result. Lemma 2.3. Let s ≥ 1, 0 < 1 + µ ≤ min {α, λ} and define Z s tµ −α f (s) = s dt, λ 0 (1 + t) Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman then f (s) ≤ f (1). Title Page Proof. We have Contents 0 sµ Z s tµ dt (−α) s−α−1 λ λ (1 + s) 0 (1 + t) µ−α −α−1 Z s s αs ≤ − tµ dt λ (1 + s) (1 + s)λ 0 sµ−α α = 1− ≤ 0. 1+µ (1 + s)λ f (s) = s −α + JJ J II I Go Back Close Quit Page 11 of 18 This shows that f is nonincreasing and hence f (s) ≤ f (1). J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Rs TheoremR2.4. Let f, g, F, G be nonnegative functions such that F (s) = 0 f (t)dt, s G(s) = 0 g(t)dt, let, p > 1, p1 + 1q = 1, (1 − λ/2)r ≤ λ/2 + α ≤ 2α, o n r ∈ pq , pq , Z x (x − t)t(1−λ/2)p/q−λ/2−α F p−1 (t)f (t)dt < ∞, 0< Z0 x 0< (x − t)t(1−λ/2)q/p−λ/2−α Gq−1 (t)g(t)dt < ∞, 0 Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality then Z x Z (2.3) 0 0 x √ √ p p q q λ λ α dsdt ≤ x B , 2 2 2 (s + t)λ Z x p1 × (x − t)t(1−λ/2)p/q−λ/2−α F p−1 (t)f (t)dt F (s)G(t) W.T. Sulaiman Title Page Contents 0 Z × x (1−λ/2)q/p−λ/2−α (x − t)t 0 Proof. Observe that Z xZ x F (s)G(t) dsdt (s + t)λ 0 0 1/q λ/2−1 λ/2−1 Z x Z x F (s) t1/p G(t) st1/p s1/q = · dsdt (s + t)λ/p (s + t)λ/q 0 0 G q−1 1q (t)g(t)dt . JJ J II I Go Back Close Quit Page 12 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Z x Z ≤ 0 1 0 x ! p1 F p (s)tλ/2−1 (s + t)λ s(λ/2−1)p/q 1 = M pN q, Z x x Z dsdt 0 0 ! 1q Gq (t)sλ/2−1 (s + t)λ t(λ/2−1)q/p dsdt say. Then Z x s(1−λ/2)p/q−λ/2 F p (s)ds M= 0 Z 0 x = s (1−λ/2)p/q−λ/2 p F (s)ds 0 Z x Z x t λ/2−1 1 s s t λ 1+ s dt x α x −α Z s x α Z s 1 x/s 0 uλ/2−1 (1 + u)λ Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality du u s(1−λ/2)p/q−λ/2 F p (s)ds du λ s 0 0 (1 + u) Z x xα λ λ = B , s(1−λ/2)p/q−λ/2−α F p (s)ds, 2 2 2 0 ≤ by virtue of the lemma. As Z p F (s) = p W.T. Sulaiman λ/2−1 s Title Page Contents JJ J II I Go Back F p−1 (u)f (u)du, Close 0 then Quit p M = xα B 2 λ λ , 2 2 Z 0 x s(1−λ/2)p/q−λ/2−α ds Z s F p−1 (u)f (u)du Page 13 of 18 0 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Z xZ s p α λ λ = x B , u(1−λ/2)p/q−λ/2−α F p−1 (u)f (u)dsdu 2 2 2 Z0 x 0 λ λ p α = x B , (x − s)s(1−λ/2)p/q−λ/2−α F p−1 (s)f (s)ds. 2 2 2 0 Similarly, q N = xα B 2 λ λ , 2 2 Z x (x − t)t(1−λ/2)q/p−λ/2−α Gq−1 (t)g(t)dt. 0 Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality Therefore, we have x Z Z x W.T. Sulaiman F (s)G(t) dsdt (s + t)λ Z x p1 √ √ p p q q λ λ ≤ xα B , (x − t)t(1−λ/2)p/q−λ/2−α F p−1 (t)f (t)dt 2 2 2 0 Z x 1q (1−λ/2)q/p−λ/2−α q−1 × (x − t)t G (t)g(t)dt . 0 0 0 Title Page Contents JJ J II I Go Back Theorem 2.5. Let f, g be nonnegative functions, f is submultiplicative and g is concave nonincreasing, f 0 (x), g 0 (y) ≥ 0, f (0) = g(0) = 0, f (∞) = g(∞) = Close Quit Page 14 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au ∞, p > 1, 1 p + 1 q = 1, 0 < a + 1 < λ, 0 < b + 1 < λ, Z ∞ 0< [f (x)]µp−bp/q [g(x)]1+a−λ p [f 0 (x)] q 0 Z ∞ 0< [g(y)]µq−aq/p [f (y)]1+b−λ q [g 0 (y)] p 0 dx < ∞, dy < ∞, then Z ∞Z ∞ (2.4) 0 0 f µ (xy) dxdy g λ/2 (xy) λ/2 ≤2 Proof. Since √ Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman 1 q 1 p B (a + 1, λ − a − 1) B (b + 1, λ − b − 1) ! p1 Z ∞ [f (t)]µp [g(t)]1+a−λ−bp/q × dt p [g 0 (t)] q 0 ! 1q Z ∞ [f (t)]µq [g(t)]1+b−λ−aq/p × dt . q [g 0 (t)] p 0 Title Page Contents JJ J II I Go Back xy ≤ x+y , 2 then 2 2 √ x+y g(x) + g(y) √ 2 2 g(xy) = g ( xy) ≥ (g ( xy)) ≥ g ≥ , 2 2 Close Quit Page 15 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au and hence Z ∞Z ∞ 0 0 f µ (xy) dxdy g λ/2 (xy) Z ∞Z λ/2 ≤2 0 ∞ f µ (x)f µ (y) (g(x) + g(y))λ 0 a/p = 2λ/2 ∞ Z 1/p 0 b/q [g (y)] f µ (x) [g(y)] [g(x)]b/q [g 0 (x)]1/q · [g(x)] f µ (y) [g(y)] a/p · [g 0 (x)]1/q [g 0 (y)]1/p λ (g(x) + g(y)) q ! p1 Z ∞Z ∞ f µp (x)g a (y)g 0 (y) dxdy p [g(x)]bp/q [g 0 (x)] q (g(x) + g(y))λ 0 0 ! 1q Z ∞Z ∞ f µq (y)g b (x)g 0 (x) × dxdy q [g(y)]aq/p [g 0 (y)] p (g(x) + g(y))λ 0 0 ≤ 2λ/2 =2 ∞ λ 0 λ/2 Z dxdy (g(x) + g(y)) p 0 1 p 1 q M N , say. M= ∞ µp 1+a−λ−bp/q [f (x)] [g(x)] ∞ Z dx p 0 W.T. Sulaiman Title Page Contents JJ J Then Z Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality [g 0 (x)] q 0 Z = B (a + 1, λ − a − 1) ∞ ηp g(y) g(x) 1+ g 0 (y) g(x) g(y) g(x) Go Back λ dy Close Quit 1+a−λ−bp/q [f (x)] [g(x)] p 0 a II I [g 0 (x)] q dx. Page 16 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au Similarly, we can show that Z N = B (b + 1, λ − b − 1) ∞ [f (y)]µq [g(y)]1+b−λ−aq/p q 0 [g 0 (y)] p dy. The result follows. Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman Title Page Contents JJ J II I Go Back Close Quit Page 17 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au References [1] G.H. HARDY, J.E. LITTLEWOOD AND G. POLYA, Inequalities, Cambridge Univ. Press. Cambridge, 1952. [2] B. YANG AND L. DEBNATH, On the extended Hardy-Hilbert’s inequality, J. Math. Anal. Appl., 272 (2002), 187–199. [3] M. GAO, TAN LI AND L. DEBNATH, Some improvements on Hilbert’s integral inequality, J. Math. Anal. Appl., 229 (1999), 682–689. [4] M. GAO, A note on Hilbert double series theorem, Hunan Ann. Math., 12(12) (1992), 142–147. [5] HU KE, On Hilbert’s inequality, Chinese Ann. of Math., 13B(1) (1992), 35–39. [6] W. RUDIN, Principles of Mathematical Analysis, Third Edition, McGrawHill Book Co. 1976. Four Inequalities Similar To Hardy-Hilbert’s Integral Inequality W.T. Sulaiman Title Page Contents JJ J II I Go Back Close Quit Page 18 of 18 J. Ineq. Pure and Appl. Math. 7(2) Art. 76, 2006 http://jipam.vu.edu.au