volume 5, issue 3, article 72, 2004.
Received 08 March, 2004;
accepted 11 April, 2004.
Communicated by:N. Elezovi´c
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Journal of Inequalities in Pure and Applied Mathematics
ON LANDAU TYPE INEQUALITIES FOR FUNCTIONS WITH HÖLDER CONTINUOUS DERIVATIVES
LJ. MARANGUNI ´C AND J. PE ˇCARI ´C
Department of Applied Mathematics
Faculty of Electrical Engineering and Computing University of Zagreb
Unska 3, Zagreb, Croatia.
EMail:ljubo.marangunic@fer.hr Faculty of Textile Technology University of Zagreb Pierottijeva 6, Zagreb Croatia.
EMail:pecaric@element.hr
2000c Victoria University ISSN (electronic): 1443-5756 079-04
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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Abstract
An inequality of Landau type for functions whose derivatives satisfy Hölder’s condition is studied.
2000 Mathematics Subject Classification:26D15 Key words: Landau inequality, Hölder continuity
Contents
1 Introduction. . . 3 2 Main Results . . . 4
References
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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1. Introduction
S.S. Dragomir and C.I. Preda have proved the following theorem (see [1]):
Theorem A. Let I be an interval in R and f : I → R locally absolutely continuous function onI. Iff ∈L∞(I)and the derivativef0 :I →Rsatisfies Hölder’s condition
(1.1) |f0(t)−f0(s)| ≤H· |t−s|α for any t, s∈I,
where H > 0 and α ∈ (0,1] are given, then f0 ∈ L∞(I) and one has the inequalities:
(1.2) ||f0|| ≤
2 1 + α1α+1α
· ||f||α+1α ·Hα+11 if m(I)≥2α+2α+1||f||
H
α+11
1 + α1α+11
;
4·||f||
m(I) + 2α(α+1)H [m(I)]α
if 0< m(I)≤2α+2α+1 ||f||
H
α+11
(1 + α1)α+11 , where|| · ||is the∞-norm on the intervalI, andm(I)is the length ofI.
In our paper we shall give an improvement of this theorem.
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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2. Main Results
Theorem 2.1. Let I be an interval and f : I → R function on I satisfying conditions of TheoremA. Thenf0 ∈L∞(I)and the following inequlities hold:
(2.1) ||f0|| ≤
2 1 + α1α+1α
· ||f||α+1α ·Hα+11 if m(I)≥2α+11 ||f||
H
α+11
1 + α1α+11
;
2||f||
m(I) +α+1H [m(I)]α
if 0< m(I)≤2α+11 ||f||
H
α+11
1 + α1α+11 ,
where|| · ||is the∞-norm on the intervalI, andm(I)is the length ofI.
In our proof and in the subsequent discussion we use three lemmas.
Lemma 2.2. Let a, b ∈ R, a < b, α ∈ (0,1]. Then the following inequality holds:
(2.2) (b−x)α+1+ (x−a)α+1 ≤(b−a)α+1, ∀x∈[a, b].
Proof. Consider the functiony: [a, b]→Rgiven by:
y(x) = (b−x)α+1+ (x−a)α+1. We observe that the unique solution of the equation
y0(x) = (α+ 1) [(x−a)α−(b−x)α] = 0
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isx0 = a+b2 ∈ [a, b]. The functiony0(x)is decreasing on(a, x0)and increasing on (x0, b). Thus, the maximal values fory(x)are attained on the boundary of [a, b] :y(a) =y(b) = (b−a)α+1, which proves the lemma.
A generalization of the following lemma is proved in [1]:
Lemma 2.3. LetA, B >0andα∈(0,1]. Consider the functiongα: (0,∞)→ Rgiven by:
(2.3) gα(λ) = A
λ +B·λα. Defineλ0 := αBA α+11
∈(0,∞). Then forλ1 ∈(0,∞)we have the bound
(2.4) inf
λ∈(0,λ1]gα(λ) =
A
λ1 +B·λα1 if 0< λ1 < λ0 (α+ 1)α−α+1α ·Aα+1α ·Bα+11 if λ1 ≥λ0.
Proof. We have:
g0α(λ) =−A
λ2 +α·B·λα−1.
The unique solution of the equationg0α(λ) = 0,λ∈(0,∞), isλ0 = αBA α+11
∈ (0,∞). The functiongα(λ)is decreasing on(0, λ0)and increasing on(λ0,∞).
The global minimum forgα(λ)on(0,∞)is:
(2.5) gα(λ0) = A αB
A α+11
+B A
αB α+1α
= (α+ 1)α−α+1α ·Aα+1α ·Bα+11 , which proves (2.4).
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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Lemma 2.4. Let A, B > 0 and α ∈ (0,1]. Consider the functions gα : (0,∞)→Randhα : (0,∞)→Rdefined by:
(2.6)
gα(λ) = Aλ +B·λα hα(λ) = 2Aλ + 2Bαλα. Defineλ0 := αBA α+11
∈(0,∞). Then forλ1 ∈(0,∞)we have:
(2.7)
λ∈(0,λinf1]gα(λ)< inf
λ∈(0,λ1]hα(λ) if 0< λ1 <2λ0
inf
λ∈(0,λ1]gα(λ) = inf
λ∈(0,λ1]hα(λ) if λ1 ≥2λ0.
Proof. In Lemma2.3, we found that the global minimum forgα(λ)is obtained forλ=λ0. Similarly we find that the global minimum forhα(λ)is obtained for λ = 2λ0, and its value is equal to the minimal value ofgα(λ), i.e. hα(2λ0) = gα(λ0).
The only solution of equationgα(λ) =hα(λ),λ∈(0,∞), is:
λS =
A B(1−2−α)
α+11 ,
and we can easily check that λ0 < λS < 2λ0. Thus, for λ1 < λ0 we have gα(λ1)< hα(λ1)and inf
λ∈(0,λ1]gα(λ)< inf
λ∈(0,λ1]hα(λ), and the rest of the proof is obvious.
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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Proof of Theorem2.1. Now we start proving our theorem using the identity:
(2.8) f(x) = f(a) + (x−a)f0(a) + Z x
a
[f0(s)−f0(a)]ds; a, x∈I
or, by changingxwithaandawithx:
(2.9) f(a) =f(x) + (a−x)f0(x) + Z a
x
[f0(s)−f0(x)]ds; a, x∈I.
Analogously, we have forb ∈I:
(2.10) f(b) =f(x) + (b−x)f0(x) + Z b
x
[f0(s)−f0(x)]ds; b, x∈I.
From (2.9) and (2.10) we obtain:
(2.11) f(b)−f(a) = (b−a)f0(x) + Z b
x
[f0(s)−f0(x)]ds +
Z x a
[f0(s)−f0(x)]ds; a, b, x∈I
and
(2.12) f0(x) = f(b)−f(a)
b−a − 1
b−a Z b
x
[f0(s)−f0(x)]ds
− 1 b−a
Z x a
[f0(s)−f0(x)]ds.
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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Assuming thatb > awe have the inequality:
(2.13) |f0(x)| ≤ |f(b)−f(a)|
b−a + 1
b−a
Z b x
|f0(s)−f0(x)|ds
+ 1
b−a
Z x a
|f0(s)−f0(x)|ds . Sincef0 is ofα−HHölder type, then:
Z b x
|f0(s)−f0(x)|ds
≤H·
Z b x
|s−x|αds (2.14)
=H Z b
x
(s−x)αds
= H
α+ 1(b−x)α+1; b, x∈I, b > x
Z x a
|f0(s)−f0(x)|ds
≤H·
Z x a
|s−x|αds (2.15)
=H Z x
a
(x−s)αds
= H
α+ 1(x−a)α+1; a, x∈I, a < x.
From (2.13), (2.14) and (2.15) we deduce:
(2.16) |f0(x)| ≤ |f(b)−f(a)|
b−a
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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+ H
(b−a)(α+ 1)[(b−x)α+1+ (x−a)α+1];
a, b, x∈I, a < x < b.
Sincef ∈L∞(I)then|f(b)−f(a)| ≤2· ||f||. Using Lemma2.2we obviously get that:
(2.17) |f0(x)| ≤ 2||f||
b−a + H
α+ 1(b−a)α; a, b, x∈I, a < x < b.
Denoteb−a =λ. Sincea, b ∈ I, b > a, we haveλ ∈ (0, m(I)), and we can analyze the right-hand side of the inequality (2.17) as a function of variableλ.
Thus we obtain:
(2.18) |f0(x)| ≤ 2||f||
λ + H
α+ 1λα =gα(λ) forx∈Iand for everyλ∈(0, m(I)).
Taking the infimum overλ∈(0, m(I))in (2.18), we get:
(2.19) |f0(x)| ≤ inf
λ∈(0,m(I))gα(λ).
If we take the supremum overx∈Iin (2.19) we conclude that
(2.20) sup
x∈I
|f0(x)|=||f0|| ≤ inf
λ∈(0,m(I))gα(λ).
Making use of Lemma2.3we obtain the desired result (2.1).
On Landau Type Inequalities for Functions with HÖlder Continuous Derivatives Lj. Maranguni´c and J. Peˇcari´c
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Remark 2.1. Denoteλ0 = h
2 1 + α1||f||
H
iα+11
. Comparing the results of Theo- remAand Theorem2.1we can see that in the case ofm(I)≥2λ0the estimated values for ||f0|| in both theorems coincide. If0 < m(I) < 2λ0 the estimated value for||f0||given by (2.1) is better than the one given by (1.2). Namely, using Lemma2.4we have:
(2.21) 2||f||
m(I)+ H
α+ 1[m(I)]α < 4||f||
m(I)+ H
2α(α+ 1)[m(I)]α; m(I)∈(0, λ0] and
(2.22)
2
1 + 1 α
α+1α
· ||f||α+1α ·Hα+11
< 4||f||
m(I) + H
2α(α+ 1)[m(I)]α; m(I)∈[λ0,2λ0).
Remark 2.2. Let the conditions of Theorem 2.1 be fulfilled. Then a simple consequence of (2.11) is the following inequality:
|(b−a)f0(x)−f(b) +f(a)| ≤ H α+ 1
(b−x)α+1+ (x−a)α+1
;
a, b, x∈I, a < x < b.
This result is an extension of the result obtained by V.G. Avakumovi´c and S.
Aljanˇci´c in [2] (see also [3]).
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References
[1] S.S. DRAGOMIR AND C.J. PREDA, Some Landau type inequalities for functions whose derivatives are Hölder continuous, RGMIA Res. Rep. Coll., 6(2) (2003), Article 3. ONLINE [http://rgmia.vu.edu.au/v6n2.
html].
[2] V.G. AVAKUMOVI ´C AND S. ALJAN ˇCI ´C, Sur la meilleure limite de la dérivée d’une function assujetie à des conditions supplementaires, Acad.
Serbe Sci. Publ. Inst. Math., 3 (1950), 235–242.
[3] D.S. MITRINOVI ´C, J.E. PE ˇCARI ´CANDA.M. FINK, Inequalities Involv- ing Functions and Their Integrals and Derivatives, Kluwer Academic Pub- lishers, Dordrecht, Boston, London, 1991.