Hindawi Publishing Corporation
Journal of Inequalities and Applications
Volume 2010, Article ID 837951, 14 pages
doi:10.1155/2010/837951
Research Article
On Boundedness of Weighted Hardy Operator in
Lp· and Regularity Condition
Aziz Harman1 and Farman Imran Mamedov1, 2
1
2
Education Faculty, Dicle University, 21280 Diyarbakir, Turkey
Institute of Mathematics and Mechanics of National Academy of Science, Azerbaijan
Correspondence should be addressed to Farman Imran Mamedov, m.farman@dicle.edu.tr
Received 22 September 2010; Accepted 26 November 2010
Academic Editor: P. J. Y. Wong
Copyright q 2010 A. Harman and F. I. Mamedov. This is an open access article distributed under
the Creative Commons Attribution License, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
We give a new proof for power-type weighted Hardy inequality in the norms of generalized
Lebesgue spaces Lp· Rn . Assuming the logarithmic conditions of regularity in a neighborhood
of zero and at infinity for the exponents px ≤ qx, βx, necessary and sufficient conditions are
p·
proved for the boundedness of the Hardy operator Hfx |y|≤|x| fydy from L β· Rn into
|x|
q·
L
|x|
β·−n/p ·−n/q·
RN . Also a separate statement on the exactness of logarithmic conditions at zero
and at infinity is given. This shows that logarithmic regularity conditions for the functions β, p at
the origin and infinity are essentially one.
1. Introduction
The object of this investigation is the Hardy-type weighted inequality
β·−n/p ·−n/q·
Hf |x|
Lq· Rn ≤ C|x|β· f Lp· Rn ,
Hfx |y|≤|x|
f y dy
1.1
in the norms of generalized Lebesgue spaces Lp· Rn . This subject was investigated in the
papers 1–7. For the one-dimensional Hardy operator in 1, the necessary and sufficient
condition was obtained for the exponents β, p, q. We give a new proof for this result in
more general settings for the multidimensional Hardy operator. Also we prove that the
logarithmic regularity conditions are essential one for such kind of inequalities to hold. In
that proposal, we improve a result sort of 8 since, there is an estimation by the maximal
function |x|−n Hfx ≤ CMfx.
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Journal of Inequalities and Applications
At the beginning, a one-dimensional Hardy inequality was considered assuming the
the local log condition at the finite interval 0, l. Subsequently, the logarithmic condition
was assumed in an arbitrarily small neighborhood of zero, where an additional restriction
px ≥ p0 was imposed on the exponent. In 3, 9 it was shown that it is sufficient to assume
the logarithmic condition only at the zero point. In 10 the case of an entire semiaxis was
considered without using the condition px ≥ p0. However, a more rigid condition β <
1 − 1/p− was introduced for a range of exponents. The exact condition was found in 1.
They proved this result by using of interpolation approaches. In this paper, we use other
approaches, analogous to those in 10, based on the property of triangles for px-norms and
binary decomposition near the origin and infinity. We consider the multidimensional case,
and the condition βx const is not obligatory, while the necessary and sufficient condition
is obtained by a set of exponents p, q, β without imposing any preliminary restrictions on their
values Theorems 3.1 and 3.2. In Theorem 3.3, it has been proved that logarithmic conditions
at zero and at infinity are exact for the Hardy inequality to be valid in the case q p.
Problems of the boundedness of classical integral operators such as maximal and
singular operators, the Riesz potential, and others in Lebesgue spaces with variable exponent,
as well as the investigation of problems of regularity of nonlinear equations with nonstandard
growth condition have become of late the arena of an intensive attack of many authors see
11–18.
2. Lebesgue Spaces with a Variable Exponent
As to the basic properties of spaces Lp· , we refer to 19. Throughout this paper, it is assumed
that px is a measurable function in Ω, where Ω ∈ Rn is an open domain, taking its values
from the interval 1, ∞ with p supx∈Rn p < ∞. The space of functions Lp· Ω is introduced
as the class of measurable functions fx in Ω, which have a finite Ip f : Ω |fx|px dxmodular. A norm in Lp· Ω is given in the form
f
f p·
≤
1
.
inf
λ
>
0
:
I
p
L Ω
λ
2.1
For p− > 1, p < ∞ the space Lp· Ω is a reflexive Banach space.
Denote by Λ a class of measurable functions f : Rn → R satisfying the following
conditions:
∃m ∈
1
0,
,
2
∃M > 1,
∃f0 ∈ R,
∃f∞ ∈ R,
1
< ∞,
sup fx − f0 ln
|x|
x∈B0,m
sup fx − f∞ ln|x| < ∞.
x∈Rn \B0,M
For the exponential functions βx, px, and qx, we further assume β, p, q ∈ Λ.
We will many times use the following statement in the proof of main results.
2.2
2.3
Journal of Inequalities and Applications
3
Lemma 2.1. Let s ∈ Λ be a measurable function such that −∞ < s− , s < ∞. Then the condition
2.2 for the function sx is equivalent to the estimate
C3−1 |x|s0 ≤ |x|sx ≤ C3 |x|s0
2.4
when |x| ≤ m and the condition 2.3 for sx is equivalent to the estimate
C4−1 |x|s∞ ≤ |x|sx ≤ C4 |x|s∞
2.5
when |x| ≥ M. Where the constants C3 , C4 > 1 depend on s0, s∞, s− , s , s0, s∞, m, M, C1 ,
C2 .
To prove Lemma 2.1, for example 2.4, it suffices to rewrite the inequality 2.4 in the
form
C3−1 ≤ |x|sx−s0 ≤ C3
2.6
and pass to logarithmic in this inequality see also, 1, 7, 17.
For 1 < p < ∞, p denotes the conjugate number of p, p p/p − 1. It is further
assumed that p ∞ for p 1, and p 1 for p ∞, 1/∞ 0, 1/0 ∞. We denote by C, C1 , C2
various positive constants whose values may vary at each appearance. Bx, r denotes a ball
with center at x and radius r > 0. We write u ∼ v if there exist positive constants C3 , C4 such
that C3 ux ≤ vx ≤ C4 ux. By χE , we denote the characteristic function of the set E.
3. The Main Results
The main results of the paper are contained in the next statements. The theorem below gives
a solution of the two-weighted problem for the multidimensional Hardy operator in the case
of power-type weights.
Theorem 3.1. Let qx ≥ px and βx be measurable functions taken from the class Λ. Let the
following conditions be fulfilled:
0 < p− ≤ px,
qx ≤ q < ∞,
−∞ < β− ≤ βx ≤ β < ∞.
3.1
Then the inequality 1.1 for any positive measurable function f is fulfilled if and only if
p0 > 1,
p∞ > 1,
β0 < n 1 −
1
,
p0
β∞ < n 1 −
1
.
p∞
3.2
We have the following analogous result for the conjugate Hardy operator Hfx fydy.
|y|≥|x|
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Journal of Inequalities and Applications
Theorem 3.2. Let qx ≥ px and βx be measurable functions taken from the class Λ. Let the
conditions 3.1 be fulfilled. Then the inequality 1.1 for any positive measurable function f and
operator Hf is fulfilled if and only if
p0 > 1,
p∞ > 1,
β0 > n 1 −
1
,
p0
β∞ > n 1 −
1
.
p∞
3.3
In the next theorem, we prove that the logarithmic conditions near zero and at infinity
are essentially one.
Theorem 3.3. If condition 2.2 or 2.3 does not hold, then there exists an example of functions p, β,
and a sequence f below index k violating the inequality
β·−n
Hf |x|
Lp· Rn ≤ C|x|β· f Lp· Rn 3.4
.
4. Proofs of the Main Results
Proof of Theorem 3.1.
Sufficiency. Let fx ≥ 0 be a measurable function such that
β· |x| f Lp· Rn ≤ 1.
4.1
We will prove that
β·−n/p ·−n/q·
Hf |x|
Lq· Rn ≤ C5 .
4.2
Assume that 0 < δ < m is a sufficiently small number such that n/p x > n/p 0 − ε
for all x ∈ B0, δ, where ε n/p 0−β0/2. Let, furthermore, M < N < ∞ be a sufficiently
large number such that n/p x > n/p ∞ − δ1 for all x ∈ Rn \ B0, N, where δ1 n/p ∞ −
β∞/2.
By Minkowski inequality, for px-norms, we have
β·−n/p ·−n/q·
Hf |x|
Lq· Rn ≤ |x|β·−n/p ·−n/q· Hf Lq· B0,δ
|x|β·−n/p ·−n/q· Hf Lq· B0,N\B0,δ
β·−n/p ·−n/q·
|x|
ftdt
{t:|t|<N}
Lq· Rn \B0,N
β·−n/p ·−n/q·
|x|
ftdt
{t:N<|t|<|x|}
Lq· RN \B0,N
: i1 i2 i3 i4 .
4.3
Journal of Inequalities and Applications
5
The estimate near zero i1 .
By Minkowski inequality, we have the inequalities
∞ β·−n/p ·−n/q· ftdt
i1 ≤ |x|
{t:2−k−1 |x|<|t|<2−k |x|}
k0
Lq· B0,δ
∞ β·−n/p ·−n/q·
≤
ftdt
|x|
−k−1
−k
{t:2
|x|<|t|<2 |x|}
k0
4.4
.
Lq· B0,δ
−
minpx, infy∈Bx,k py.
Denote Bx,k {y ∈ Rn : 2−k−1 |x| < |y| < 2−k |x|} and px,k
By 2.2 and Lemma 2.1, for x ∈ B0, δ, t ∈ Bx,k , we have |x|βx ∼ 2kβ0 tβt . To prove this
equivalence, we use that |t| ∼ |x|2−k , |x|βx ∼ |x|β0 and |t|βt ∼ |t|β0 . Therefore, and due to
Holder’s inequality, for x ∈ B0, δ, we get
|x|
βx−n/p x−n/qx
ftdt
Bx,k
≤ C6 2kβ0 |x|−n/p x−n/qx
|t|βt ftdt
4.5
Bx,k
≤ C6 2
kβ0
|x|
−n/p 0−n/qx
−
1/px,k
|t|
βt
−
px,k
ft
dt
− n/px,k
2−k |x|
.
Bx,k
−
a If px,k
px, then by 2.2 and Lemma 2.1,
/
− n/px,k
2−k |x|
∼ tn/p t ∼ tn/p 0 ∼ 2−kn/p 0 |x|n/p 0 ∼ 2−kn/p 0 |x|n/p x .
4.6
Demonstrate details in proof of 4.6. For t ∈ Bx,k and x ∈ B0, δ, we have 2−k−1 |x| <
|t| ≤ 2−k |x|. Then
2−k |x|
− n/px,k
−
4.7
∼ |t|n/px,k .
−
By hypothesis a, px,k
attains in the interval Bx,k , because there exists a point y ∈ Bx,k where
−
−
px,k
∼ py. Obviously, the point y depends on x, k. Then |t|n/px,k ∼ |t|n/p y . By virtue of
2−k−1 |x| < |y| ≤ 2−k−1 |x|, we have |t|/2 < |y| ≤ 2|t|. Hence, |t|n/p y ∼ |y|n/p y , by Lemma 2.1,
|y|n/p y ∼ |y|n/p 0 ∼ |t|n/p 0 .
−
px, then by choice of δ,
b If px,k
− n/px,k
2−k |x|
∼ 2−kn/p x |x|n/p x ≤ 2−kn/p 0εk |x|n/p x ;
x ∈ B0, δ.
4.8
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Journal of Inequalities and Applications
Applying estimate 4.8 to both hypotheses a and b, by choosing of ε and δ, the right-hand
part of 4.5 is less than
C7 |x|
−n/qx −kε
|t|
2
βt
ft
−
px,k
−
1/px,k
dt
4.9
.
Bx,k
Simultaneously,
−
px,k
dt
|t|βt ft
Bx,k
≤
|t|βt ft
Bx,k ∩{t∈Rn :|t|βt ft≥1}
pt
4.9 dt ≤ 1 2−kn δn C8 .
dt Bx,k
By 4.5 and 4.9 , we have
Iq;B0,δ |x|
β·−n/p ·−n/q·
ftdt
Bx,k
−kεq−
−n
≤ C9 2
|x|
B0,δ
q /p− −1 −kεq−
≤ C9 C8
|t|
Bx,k
ft
−
px,k
−
qx/px,k
dt
dx
pt
βt
1 dt |x|−n dx
|t| ft
4.10
2
B0,δ
βt
Bx,k
which, due to Fubini’s theorem, yields
≤
q /p− −1 −kεq−
C9 C8
2
−kεq−
C10 2
ln 2
{t:|t|<2−k δ}
βt
ft|t|
ft|t|
{t:|t|<2−k δ}
βt
pt pt
−n
B0,2k1 |t|\B0,2k |t|
|x| dx dt
4.11
−
1 dt ≤ C11 2−kεq .
Therefore,
β·−n/p ·−n/q·
ftdt
|x|
Bx,k
≤ C12 2−kεq
−
/q
.
4.12
Lq· B0,δ
By 4.12 and 4.4, we get
i1 ≤ C12
∞
−
2−kεq /q C13 < ∞.
k0
The estimate at infinity i4 .
4.13
Journal of Inequalities and Applications
7
Put fN t ftχ|t|>N . Analogously to the case of 4.4, we have
∞ β·−n/p ·−n/q·
i4 ≤
fN tdt
|x|
−k−1 |x|<|t|<2−k |x|}
{t:2
k0
4.14
.
Lq· Rn \B0,N
By |t| ∼ |x|2−k , condition 2.3 and Lemma 2.1 for x ∈ Rn \ B0, N, t ∈ Bx,k , we have
4.15
|x|βx ∼ |x|β∞ ∼ 2kβ∞ tβ∞ ∼ 2kβ∞ tβt .
Therefore, by virtue of Holder’s inequality,
|x|
βx−n/p x−n/qx
fN tdt
Bx,k
≤ C14 2kβ∞ |x|−n/p x−n/qx
|t|βt fN tdt
4.16
Bx,k
≤ C14 2
kβ∞
|x|
−n/p x−n/qx
|t|
βt
fN t
−
px,k
−
1/px,k
dt
2−k |x|
− n/px,k
.
Bx,k
−
i If px,k
/ px and t ∈ Bx,k , by 2.3 and Lemma 2.1, we have
2−k |x|
− n/px,k
4.17
∼ tn/p t ∼ tn/p ∞ ∼ 2−kn/p ∞ |x|n/p ∞ ∼ 2−kn/p ∞ |x|n/p x .
−
ii If px,k
px, then by choice of δ1 ,
2−k |x|
− n/px,k
4.18
∼ 2−kn/p x |x|n/p x ≤ 2−kn/p ∞δ1 k |x|n/p x .
In both hypotheses i and ii by choosing of δ1 , we have
βx−n/p x−n/qx
−n/qx −kδ1
fN tdt ≤ C15 |x|
|x|
|t|
2
Bx,k
βt
fN t
−
px,k
−
1/px,k
dt
4.19
.
Bx,k
On the other hand,
Bx,k
|t|
βt
ft
−
px,k
dt ≤
Bx,k
∩{t∈Rn :|t|βt ft≥Gt}
|t|βt ft
Gt
−
px,k
−
px,k
Gt
−
Gtp dt,
dt Bx,k
4.20
8
Journal of Inequalities and Applications
where Gt 1/1 t2 . Hence,
≤
fN t|t|βt
pt
−
Gtpx,k −pt 4.21
Gtdt.
Bx,k
Bx,k
By 2.3, for t ∈ Bx,k , we have
−
pt−px,k
−
Gtpx,k −pt ≤ 1 t2
≤ C16 .
4.22
Then 4.21 implies
|t|βt fN t
−
px,k
dt ≤ C17 .
4.23
Bx,k
Therefore,
I
|x|
q;Rn \B0,N
βx−n/p x−n/qx
fN tdt
Bx,k
q /p
≤ C17
−
2−kδ1 q
−
Rn \B0,N
|x|−n
|t|βt fN t
pt
4.24
dt dx,
Bx,k
by Fubini’s theorem,
≤
q /p− −1 −kδ1 q−
2
C17
ln 2
{t:|t|>2−k N}
fN t|t|βt
pt
−
dt ≤ C18 2−kδ1 q .
4.25
From 4.25 and expansion 4.14, we get
i4 ≤ C18
∞
−
2−kq δ1 /q C19 .
4.26
k0
The estimate in the middle i2 , i3 .
We have
β·−n/p ·−n/q·
i2 |x|
ftdt
n
{t∈R :|t|<|x|}
ftdt |x|β·−n/p ·−n/q· ≤
B0,N
Lq· B0,N\B0,δ
≤ C20
ftdt,
B0,N
Lq· B0,N\B0,δ
4.27
Journal of Inequalities and Applications
9
from which, by virtue of Holder’s inequality, for px-norms, we obtain the estimate
ftdt ≤ |t|β· ft
Lp· B0,N
B0,N
−β· |t|
Lp · B0,N
.
4.27 Using t−βtp t ∼ t−β0p 0 by Lemma 2.1 for t ∈ B0, N and taking the condition β0 <
n/p 0 into account, we find
Ip ;B0,N |t|−β· |t|−βtp t dt ≤ C21
B0,N
|t|−β0p 0 dt C22 .
B0,N
4.28
From 4.27 and 4.28, it follows that
i2 ≤ C23 .
4.29
Furthermore, we have
ftdt |x|βx−n/p x−n/qx i3 ≤
B0,N
.
4.30
Lq· Rn \B0,δ
The boundedness of the first term follows by 4.27 . Due to 2.3 and Lemma 2.1, for x ∈
Rn \ B0, N, we have
|x|βx−n/p xqx−n ∼ |x|β∞−n/p ∞qx−n .
4.31
Applying condition 4.31, we get
Iq;Rn /B0,N |x|β·−n/p ·−n/q· ≤ C24
Rn \B0,N
|x|−n−2δ1 dx C25 .
4.32
Then
1/p−
i3 ≤ C25 .
4.33
Necessity. Let β0 > n/p 0. Fix a sufficiently large τ > 0 and apply inequality 1.1 by the
test function
fτ t t−n/pt−βt χB0,δ/τ\B0,δ/2τ t.
4.34
10
Journal of Inequalities and Applications
We come to a contradiction
β·
Ip |t|
|x|−n dx C0 ln 2 < ∞,
fτ B0,δ/τ\B0,δ/2τ
Iq |t|
β·−n/p ·−n/q·
fτ y dy
B0,t
|t|βt−n/p t−n/qtqt
≥
B0,1\B0,δ/τ
≥
δ
2τ
−n/p0−β0
y
dy
4.35
qt
dt
B0,δ/τ\B0,δ/2τ
n/p 0−β0q− |t|β0−n/p 0qt−n dt −→ ∞
B0,1\B0,δ/τ
as τ → ∞.
If 0 < p0 ≤ 1, then by virtue of inequalities 4.35 and 3.2 we obtain
Iq |t|
βt−n/p t−n/qt
fτ y dy
−→ ∞,
as τ −→ ∞.
4.36
B0,t
Also,
Ip |t|βt fτ t C0 ln 2,
4.37
and we come to a contradiction.
If β∞ ≥ n/p ∞, then, using condition 2.3 and Lemma 2.1 assuming 0 < τ < 1, we
again obtain
Ip |t|βt fτ t C0 ln 2,
Iq |t|
βt−n/p t−n/qt
fτ tdy
B0,t
≥
Rn \B0,δ/τ
≥
βt−n/p tqt−n
δ
2τ
−n/p∞−β∞
y
dy dt
|t|
B0,δ/τ\B0,δ/2τ
n/p ∞−β∞q Rn \B0,δ/τ
|t|β∞−n/p ∞qt−n dt −→ ∞
4.38
Journal of Inequalities and Applications
11
as τ → ∞. If β∞ n/p ∞, then from 4.38 we have
βt−n/p t−n/qt
fτ tdy
Iq |t|
∞.
4.39
B0,t
From 4.38 and 3.2, we derive, as above, the necessity of the condition p∞ > 1.
This completes the proof of Theorem 3.1.
The proof of Theorem 3.2 easily follows from Theorem 3.1 by using the equivalence of
inequalities
βx−n/p x−n/qx
Hfx
|x|
n− βz−2n/qz
|z|
Hfx
Lq· Rn Lq· Rn ≤ C|x|βx fx
Lp· Rn ≤ C|z|−βz−2n/pz fz
,
Lp· Rn 4.40
,
where px, qx, and βx stand for the functions px/|x|2 , qx/|x|2 , and βx/|x|2 ,
respectively. The equivalence readily follows from the equality
g Lp· Rn |z|−2n/pz g Lp· Rn 4.41
for any function g : Rn → R, where gz gz/|z|2 , which easily can be proved by changing
of variable x z/|z|2 in the definition of px-norm.
5. Exactness of the Logarithmic Conditions
Proof of Theorem 3.3. Assume δk 1/4k , k ∈ N, fk x |x|−n/px−βx χB0,2δk \B0,δk x, and
βx β0 . Define the function p : 0, ∞ → 1, ∞ as
px ⎧
⎨p0 ,
x ∈ B0, 2δk \ B0, δk ,
⎩p ,
k
x ∈ B0, 4δk \ B0, 2δk , k ∈ N
5.1
where p0 > 1, pk p0 αk , β0 ∈ R, and {αk } is an arbitrary sequence of positive numbers
satisfying the condition
kαk −→ ∞ as k −→ ∞.
5.2
12
Journal of Inequalities and Applications
Then αk ln1/δk → ∞, and condition 2.2 does not hold for the function px. Since
Ip |x|
βx
fk x |t|β0 · |t|−n/p0 −β0
B0,2δk \B0,δk −n
B0,2δk \B0,δk Ip H |·|β·−n fk · ≥
|t| dt C0
B0,2δk \B0,δk ≥C
B0,3δk \B0,2δk −nαk /p0
≥ Cδk
2δk
δk
B0,4δk \B0,2δk p0
dt
dt
ωn−1 ln 2,
t
pk
|t|−n/pt−β0 dt
|x|β0 −npk dx
5.3
δk n−n/p0 −β0 pk |x|β0 −np0 αk dx
enαk /p0 ln1/δk −→ ∞
as k → ∞, we see that this contradicts inequality 3.4.
The given function fk x and the exponential functions px and βx are also suitable
for proving the necessity of condition 2.3 for the function p. For this we define the numbers
δk from the equality δk 4k , k ∈ N. Let fk x |x|−n/px−β χB0,2δk \B0,δk x, βx β∞ , and
x ∈ Rn . We define the function p as
px ⎧
⎨p∞ , x ∈ B0, 2δk \ B0, δk ,
⎩p ,
k
5.4
x ∈ B0, 4δk \ B0, 2δk , k ∈ N
where p∞ > 1, β∞ ∈ R, pk p∞ − αk , and {αk } is an arbitrary sequence of positive numbers
satisfying the condition kαk → ∞ as k → ∞. Then αk ln δk → ∞; hence, condition 2.3
does not hold for the function px. Furthermore, we have
Ip |x|βx fk x Ip |x|
βx−n
B0,2δk \B0,δk fk x ≥
|t|β∞ · |t|−n/p∞ −β∞
B0,4δk \B0,2δk ≥C
B0,3δk \B0,2δk nαk /p∞
≥ Cδk
p∞
dt ωn−1 ln 2,
pk
−n/pt−β∞
B0,2δk \B0,δk |t|
dt
|x|β∞ −npk dx
δk n−n/p∞ −β∞ pk |x|β∞ −np∞ −αk dx
Cenαk /p∞ ln δk −→ ∞
as k → ∞, which contradicts inequality 3.4.
5.5
Journal of Inequalities and Applications
13
The same reasoning brings us to the proof of the exactness of conditions 2.2 and
2.3 for the function βx also. For instance, to show the necessity of condition 2.2, it can
be assumed that px ≡ p0 > 1, x ∈ Rn ,
βx ⎧
⎨β0 αk ,
x ∈ B0, 2δk \ B0, δk ,
⎩β ,
0
x ∈ B0, 4δk \ B0, 2δk k ∈ N.
5.6
Then
−p α
Ip |x|βx−n fk x ≥ Cδk 0 k −→ ∞
as k −→ ∞,
Ip |x|βx fk x ≤ C0 ln 2.
5.7
This completes the proof of Theorem 3.3.
Acknowledgment
F. I. Mamedov was supported partially by the INTAS Grant for the South-Caucasian
Republics, no. 8792.
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