volume 6, issue 4, article 98, 2005.
Received 08 April, 2005;
accepted 03 September, 2005.
Communicated by:D. Stefanescu
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Journal of Inequalities in Pure and Applied Mathematics
INEQUALITIES FOR WALSH POLYNOMIALS WITH SEMI-MONOTONE AND SEMI-CONVEX COEFFICIENTS
ŽIVORAD TOMOVSKI
University "St. Cyril and Methodius"
Faculty of Natural Sciences and Mathematics Institute of Mathematics
PO Box 162, 91000 Skopje Repubic of Macedonia.
EMail:tomovski@iunona.pmf.ukim.edu.mk
c
2000Victoria University ISSN (electronic): 1443-5756 112-05
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
Živorad Tomovski
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Abstract
Using the concept of majorant sequences (see [4, ch. XXI], [5], [7], [8]) some new inequalities for Walsh polynomials with complex semi-monotone, complex semi-convex, complex monotone and complex convex coefficients are given.
2000 Mathematics Subject Classification:26D05, 42C10.
Key words: Petrovic inequality, Walsh polynomial, Complex semi-convex coeffi- cients, Complex convex coefficients, Complex semi-monotone coeffi- cients, Complex monotone coefficients, Fine inequality.
Contents
1 Introduction and Preliminaries. . . 3 2 Main Results . . . 6
References
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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1. Introduction and Preliminaries
We consider the Walsh orthonormal system{wn(x)}∞n=0 defined on[0,1)in the Paley enumeration. Thus w0(x) ≡ 1and for each positive integer with dyadic development
n=
p
X
i=1
2νi, ν1 > ν2 >· · ·> νp ≥0, we have
wn(x) =
p
Y
i=1
rνi(x),
where{rn(x)}∞n=0denotes the Rademacher system of functions defined by (see, e.g. [1, p. 60], [3, p. 9-10])
rν(x) = sign sin 2νπ(x) (ν = 0,1,2, . . .; 0≤x <1).
In this paper we shall consider the Walsh polynomialsPm
k=nλkwk(x) with complex-valued coefficients{λk}.
Let ∆λn = λn −λn+1 and ∆2λn = ∆(∆λn) = ∆λn−∆λn+1 = λn− 2λn+1+λn+2, for alln = 1,2,3. . ..
Petrovi´c [6] proved the following complementary triangle inequality for a sequence of complex numbers{z1, z2, . . . , zn}.
Theorem A. Let α be a real number and0 < θ < π2. If{z1, z2, . . . , zn} are complex numbers such thatα−θ ≤argzν ≤α+θ, ν = 1,2, . . . , n, then
n
X
ν=1
zν
≥(cosθ)
n
X
ν=1
|zν|.
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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For0< θ < π2 denote byK(θ)the coneK(θ) ={z :|argz| ≤θ}.
Let {bk} be a positive nondecreasing sequence. The following definitions are given in [7] and [8]. The sequence of complex numbers {uk} is said to be complex semi−monotone if there exists a coneK(θ) such that∆
uk
bk
∈ K(θ) or ∆(ukbk) ∈ K(θ). For bk = 1, the sequence {uk} shall be called a complex monotone sequence. On the other hand, the sequence {uk} is said to be complex semi−convex if there exists a coneK(θ), such that∆2
uk
bk
∈ K(θ) or ∆2(ukbk) ∈ K(θ). For bk = 1, the sequence {uk} shall be called a complex convex sequence.
The following two Theorems were proved by Tomovski in [7] and [8].
Theorem B ([7]). Let{zk}be a sequence such that|Pm
k=nzk| ≤ A, (∀n, m∈ N, m > n),whereAis a positive number.
(i) If∆
uk
bk
∈K(θ),then
m
X
k=n
ukzk
≤A
1 + 1 cosθ
|um|+ 1 cosθ
bm bn
|un|
, (∀n, m∈N, m > n).
(ii) If∆(ukbk)∈K(θ), then
m
X
k=n
ukzk
≤A
1 + 1 cosθ
|un|+ 1 cosθ
bm
bn|um|
, (∀n, m∈N, m > n).
Theorem C ([8]). LetA= max
n≤p≤q≤m
q
P
j=p j
P
k=i
zk
.
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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(i) If{uk}is a sequence of complex numbers such that∆2 uk
bk
∈K(θ), then
m
X
k=n
ukzk
≤A
|um|+bm
1 + 1 cosθ
∆
um−1
bm−1
+ bm cosθ
∆ un
bn
, (∀n, m∈N, m > n).
(ii) If {uk} is a sequence of complex numbers such that ∆2(ukbk) ∈ K(θ), then
m
X
k=n
ukzk
≤A
|un|+b−1n
1 + 1 cosθ
(|∆(unbn)|+|∆(um−1bm−1)|)
, (∀n, m∈N, m > n).
Using the concept of majorant sequences we shall give some estimates for Walsh polynomials with complex semi-monotone, complex monotone, complex semi-convex and complex convex coefficients.
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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2. Main Results
For the main results we require the following Lemma.
Lemma 2.1. For allp, q, r∈N, p < qthe following inequalities hold:
q
X
k=p
ωk(x)
≤ 2
x,0< x <1.
(i)
q
X
j=p j
X
k=l
ωk(x) (ii)
≤
2(q−p+1)
x =C1(p, q, x) : 0 < x <1
8
x(x−2−r)+ x82 + 2(q−p+1)x + 1 =C2(p, q, r, x) :x∈(2−r,2−r+1) Proof. (i) Let Dq(x) = Pq−1
i=0 wi(x) be the Dirichlet kernel. Then it is known that (see [3, p. 28])|Dq(x)| ≤ x1,0< x <1. Hence
q
X
k=p
wk(x)
=|Dq+1(x)−Dp(x)| ≤ |Dq+1(x)|+|Dp(x)| ≤ 2 x. (ii) By (i) we get
q
X
j=p j
X
k=l
wk(x)
≤
q
X
j=p
j
X
k=l
wk(x)
≤ 2(q−p+ 1)
x , 0< x <1.
LetFn(x) = n+11 Pn
k=0Dk(x)be the Fejer kernel. Applying Fine’s inequality (see [2])
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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(n+ 1)Fn(x)< 4
x(x−2−r)+ 4
x2, x∈(2−r,2−r+1), we get
q
X
j=p j
X
k=l
wk(x)
=
q
X
j=p
(Dj+1(x)−Dl(x))
≤
q
X
j=p
Dj+1(x)
+q−p+ 1 x
≤ |(q+ 1)Fq(x)|+|Dq+1(x)|+|D0(x)|
+|pFp−1(x)|+ q−p+ 1 x
< 8
x(x−2−r)+ 8
x2 + 2(q−p+ 1)
x + 1, x∈(2−r,2−r+1).
Applying the inequality (i) of the above lemma and TheoremB, we obtain following theorem.
Theorem 2.2. Let0< x <1.
(i) If{uk}is a sequence of complex numbers such that∆
uk
bk
∈K(θ), then
m
X
k=n
ukwk(x)
≤ 2 x
1 + 1 cosθ
|um|+ 1 cosθ
bm bn|un|
, (∀n, m∈N, m > n).
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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(ii) If{uk}is a sequence of complex numbers such that∆(ukbk)∈K(θ), then
m
X
k=n
ukwk(x)
≤ 2 x
1 + 1 cosθ
|un|+ 1 cosθ
bm bn|um|
, (∀n, m∈N, m > n).
Specially forbk= 1we get the following inequalities for Walsh polynomials with complex monotone coefficients.
Corollary 2.3. Let0< x <1. If{uk}is a sequence of complex numbers such that∆uk ∈K(θ), then
m
X
k=n
ukωk(x)
≤ 2 x
1 + 1 cosθ
|um|+ 1 cosθ|un|
, (∀n, m∈N, m > n).
Corollary 2.4. Let 0 < x < 1. If{uk}is a complex monotone sequence such that lim
k→∞uk = 0, then
∞
X
k=n
ukωk(x)
≤ 2
xcosθ|un|.
In [4] (chapter XXI), [5] Mitrinovi´c and Peˇcari´c obtained inequalities for cosine and sine polynomials with monotone nonnegative coefficients. Applying Theorem 2.2, we get analogical results for Walsh polynomials with monotone nonnegative coefficients.
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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Corollary 2.5. Let0< x <1.
(i) If {ak} is a nonnegative sequence such that {akb−1k } is a decreasing se- quence, then
m
X
k=n
akwk(x)
≤ an x
bm bn
, (∀n, m∈N, m > n).
(ii) If {ak} is a nonnegative sequence such that {akbk} is an increasing se- quence, then
m
X
k=n
akwk(x)
≤ am x
bm bn
, (∀n, m∈N, m > n).
Now, applying the inequality (ii) of Lemma2.1, we obtain new inequalities for Walsh polynomials with complex semi-convex coefficients.
Theorem 2.6.
(i) If{uk}is a sequence of complex numbers such that∆2
uk
bk
∈K(θ), then
m
X
k=n
ukwk(x)
≤
C1(m, n, x)h
|um|+bm−1 1 + cos1θ ∆u
m−1
bm−1
+bcosm−2θ
∆
un
bn
i
: 0< x < 1 C2(m, n, r, x)h
|um|+bm−1 1 + cos1θ ∆
um−1
bm−1
+bcosm−2θ
∆
un
bn
i
:x∈(2−r,2−r+1) for all n, m, r ∈N, m > n.
Inequalities for Walsh Polynomials with Semi-Monotone and Semi-Convex Coefficients
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(ii) If {uk} is a sequence of complex numbers such that ∆2(ukbk) ∈ K(θ), then
m
X
k=n
ukwk(x)
≤
C1(m, n, x)
|un|+b−1n 1 + cos1θ
×(|∆(unbn)|+|∆(um−1bm−1)|)] : 0< x <1 C2(m, n, r, x)
|un|+b−1n 1 + cos1θ
×(|∆(unbn)|+|∆(um−1bm−1)|)] :x∈(2−r,2−r+1) for all n, m, r ∈N, m > n.
Proof. (i) Applying Abel’s transformation twice and the triangle inequality, we get:
m
X
k=n
uk bk
(bkwk)
=
um bm
m
X
k=n
bkwk+ ∆
um−1
bm−1
m−1 X
j=n j
X
k=n
bkwk
+
m−2
X
r=n
∆2 ur
br r
X
j=n j
X
k=n
bkwk
≤ |um| bm bm
m
X
k=n
wk
+bm−1
∆
um−1
bm−1
m−1
X
j=n j
X
k=n
wk
+bm−2
m−2
X
r=n
∆2 ur
br
r
X
j=n j
X
k=n
wk .
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Using the Petrovi´c inequality and inequality (ii) of Lemma2.1, we obtain:
m
X
k=n
ukwk(x)
≤ |um|
m
X
k=n
wk
+bm−1
∆
um−1
bm−1
m−1
X
j=n j
X
k=n
wk
+bm−2
cosθ
m−2
X
r=n
∆2 ur
br r
X
j=n j
X
k=n
wk
≤
C1(m, n, x)h
|um|+bm−1 1 + cos1θ ∆
um−1
bm−1
+bcosm−2θ
∆
un
bn
i
: 0< x <1 C2(m, n, r, x)h
|um|+bm−1 1 + cos1θ ∆
um−1
bm−1
+bcosm−2θ
∆
un
bn
i
:x∈(2−r,2−r+1) (ii) Analogously as the proof of (i), we obtain:
m
X
k=n
(ukbk)b−1k wk
=
unbn
m
X
k=n
b−1k wk−
m−1
X
j=n+1
∆2(uj−1bj−1)
j
X
r=n m
X
k=r
b−1k wk
+∆(unbn)
m
X
k=n
b−1k wk−∆(um−1bm−1)
m
X
r=n m
X
k=r
b−1k wk
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≤ |un|bnb−1n
m
X
k=n
wk
+b−1n
m−1
X
j=n+1
∆2(uj−1bj−1)
j
X
r=n m
X
k=r
wk +b−1n |∆(unbn)|
m
X
k=n
wk
+b−1n |∆(um−1bm−1)|
m
X
r=n m
X
k=r
wk .
Hence,
m
X
k=n
ukwk(x)
≤ |un|
m
X
k=n
wk
+ b−1n cosθ
m−1
X
j=n+1
∆2(uj−1bj−1)
j
X
r=n m
X
k=r
wk +b−1n |∆(unbn)|
m
X
k=n
wk
+b−1n |∆(um−1bm−1)|
m
X
r=n m
X
k=r
wk
≤
C1(m, n, x)
|un|+b−1n 1 + cos1θ
×(|∆(unbn)|+|∆(um−1bm−1)|)] : 0 < x <1, C2(m, n, r, x)
|un|+b−1n 1 + cos1θ
×(|∆(unbn)|+|∆(um−1bm−1)|)] :x∈(2−r,2−r+1).
Ifbk = 1, k = n, n+ 1, . . . , mfrom Theorem2.6, we obtain the following corollary.
Corollary 2.7. Let{uk}be a complex-convex sequence. Then,
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m
X
k=n
ukwk(x)
≤
C1(m, n, x)
|um|+ 1 + cos1θ
×|∆um−1|+cos1θ|∆un|
: 0< x <1 C2(m, n, r, x)
|um|+ 1 + cos1θ
×|∆um−1|+cos1θ|∆un|
:x∈(2−r,2−r+1) for all n, m, r ∈N, m > n.
Remark 1. Similarly, the results of Theorem 2.2, Theorem 2.6, Corollary 2.3, Corollary2.5and Corollary2.7were given by the author in [7,8] for trigono- metric polynomials with complex valued coefficients.
Corollary 2.8.
(i) If{ak}is a nonnegative sequence such that{akb−1k }is a convex sequence, then
m
X
k=n
akwk(x)
≤
C1(m, n, x)h
|am|+ 2bm−1
∆
am−1
bm−1
+bm−2
∆
an
bn
i
: 0< x <1 C2(m, n, r, x)
h
|am|+ 2bm−1
∆
am−1
bm−1
+bm−2
∆
an
bn
i
:x∈(2−r,2−r+1) for all n, m, r ∈N, m > n.
(ii) If {ak}is a nonnegative sequence such that{akbk}is a convex sequence,
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then
m
X
k=n
akwk(x)
≤
C1(m, n, x) [|an|+ 2b−1n |∆(anbn)|
+|∆(am−1bm−1)|] : 0< x <1 C2(m, n, r, x) [|an|+ 2b−1n |∆(anbn)|
+|∆(am−1bm−1)|] :x∈(2−r,2−r+1) for all n, m, r ∈N, m > n.
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References
[1] G. ALEXITS, Convergence problems of orthogonal series, Pergamon Press New York- Oxford-Paris, 1961.
[2] N.J. FINE, On the Walsh functions, Trans. Amer. Math. Soc., 65 (1949), 372–414.
[3] B.I. GOLUBOV, A.V. EFIMOVAND V.A. SKVORCOV, Walsh series and transformations, Science, Moscow (1987) (Russian).
[4] D.S. MITRINOVI ´C, J.E. PE ˇCARI ´C AND A.M. FINK, Classical and New Inequalities in Analysis, Kluwer, 1993.
[5] D.S. MITRINOVI ´C AND J.E. PE ˇCARI ´C, On an inequality of G.K.Lebed, Prilozi MANU, Od. Mat. Tehn. Nauki, 12 (1991), 15–19.
[6] M. PETROVI ´C, Theoreme sur les integrales curvilignes, Publ. Math. Univ.
Belgrade, 2 (1933), 45–59.
[7] Ž. TOMOVSKI, On some inequalities of Mitrinovic and Peˇcari´c, Prilozi MANU, Od. Mat. Tehn. Nauki, 22 (2001), 21–28
[8] Ž. TOMOVSKI, Some new inequalities for complex trigonometric polyno- mials with special coefficients, J. Inequal. in Pure and Appl. Math., 4(4) (2003), Art. 78. [ONLINE http://jipam.vu.edu.au/article.
php?sid=319].