






摘要: 基于有界算子的本質之一: 當原象集有界時象集一定有界, 提出算子有界的反問題, 即當算子T的象集有界時, 如何判斷其原象集有界. 先引入算子反向有界的概念, 再利用權函數方法和實分析技巧, 討論積分算子反向有界的等價參數條件, 并給出反向有界積分算子的構造定理. 最后給出一些特例.
關鍵詞: 非齊次核; 積分算子; 反向有界算子; 逆向Hilbert型積分不等式; 構造定理
中圖分類號: O178" 文獻標志碼: A" 文章編號: 1671-5489(2024)04-0858-08
Inverse Problem for a Class of Bounded IntegralOperators with Non-homogeneous Kernel
ZHANG Lijuan1, HONG Yong1, LIAO Jianquan2
(1. College of Data Science, Guangzhou Huashang College, Guangzhou 511300, China;
2. College of Mathematics, Guangdong University of Education, Guangzhou 510303, China)
Abstract: One of the essence of" bounded operators" is that the" image set must be bounded when the original" image set is bounded, we propose
the inverse problem of operator boundedness: how to determine the boundedness of"" the original image set of an operator T "when its image set is bo
unded. We first introduce the concept of operator reverse boundedness, and then use weight" function method and real analysis techniques to discuss" the equivalent parametric conditions for
reverse boundedness of integral operators, and give" a construction theorem for reverse boundedness of" integral operators. Finally, some special cases are given.
Keywords: non-homogeneous kernel; integral operator; reverse bounded operator; inverse Hilbert-type integral inequality; construction theorem
1 引言與預備知識
設1p+1q=1(pgt;1), Hardy等[1]給出了Hilbert積分不等式: 若f∈Lp(0,+∞), g∈Lq(0,+∞), 則有
∫+∞0∫+∞01x+yf(x)g(y)dxdy≤πsin(π/p)‖f‖p‖g‖q,
其中常數因子πsin(π/p)是最佳值. 由該不等式可導出具有相同核1x+y的積分算子T:
T(f)(y)=∫+∞01x+yf(x)dx
是Lp(0,+∞)中的有界算子, 且T的算子范數‖T‖=πsin(π/p). 目前, 關于Hilbert型不等式及相關算子的研究已取得豐碩成果
[2-14], 并形成了較完整的理論體系[15]. 由于一個算子T有界是指存在常數Mgt;0, 使得‖T(f)‖≤M‖f‖, 因此其本質是T將有界
集映射成有界集, 即當原象集有界時, 象集也有界. 顯然, 其反問題是: 當T的象集有界時, 如何保證T的原象集有界. 這需要研究反向不等式‖f‖≤
‖T(f)‖, 由此需引入算子反向有界的概念, 也需要探討相關的逆向Hilbert型不等式. 近年來, 關于逆向Hilbert型不等式的研究已有許多成果
[16-19], 但尚不完善. 文獻[20]探討了擬齊次核積分算子有界的等價參數條件, 本文在此基礎上, 討論非齊次核積分算子反向有界的問題, 得到積分算子反向有界的充分必要條件.
將Lebesgue空間Lr(0,+∞)進行帶權推廣: 設r≠0, α∈瘙綆, 記
Lαr(0,+∞)=f(x): ∫+∞0xαf(x)rdx1/rlt;+∞.
當rgt;1時, Lαr(0,+∞)是賦范向量空間, 其范數是‖f‖r,α=∫+∞0xαf(x)rdx1
/r. 由于當rlt;1(r≠0)時, Lαr(0,+∞)不能構成通常意義上的向量空間, 為區別, 當rlt;1(r≠0)時, 記‖f‖
*r,α=∫+∞0xαf(x)rdx1/r.
若1p+1q=1(0lt;plt;1, qlt;0), α,β∈瘙綆, K(x,y)≥0, gt;0, f∈Lαp(0,+∞), g∈Lβq(0,+∞), 則以K(x,y)為核的不等式
‖f‖*p,α‖g‖*q,β≤∫+∞0∫+∞0K(x,y)f(x)g(y)dxdy,(1)
稱為逆向Hilbert型積分不等式, 稱為常數因子. 本文記
W1(s)=∫+∞0K(1,t)tsdt," W2(s)=∫+∞0K(t,1)tsdt,
A(K,f,g)=∫+∞0∫+∞0K(x,y)f(x)g(y)dxdy.
引理1 設1p+1q=1(0lt;plt;1, qlt;0), α,β∈瘙綆, λ1λ2gt;0, K(x,y)=G(xλ1yλ2)是非負可測函數, 則
ω1(x,β,q)=∫+∞0K(x,y)y-(β+1)/qdy=x(λ1/λ2)\W1-β+1q,
ω2(y,α,p)=∫+∞0K(x,y)x-(α+1)/pdx=y(λ2/λ1)\W2-α+1p.
若αλ1p+1λ2p=βλ2q+1λ1q, 則λ1W2-α+1p=λ2W1-β+1q.
證明: 易知K(x,y)=G(xλ1yλ2)具有如下性質: 對tgt;0, 有
K(tx,y)=K(x,tλ1/λ2y)," K(x,ty)=K(tλ2/λ1x,y).
特別地, K(t,1)=K(1,tλ1/λ2), K(1,t)=K(tλ2/λ1,1). 于是
ω1(x,β,q)= "∫+∞0K(1,xλ1/λ2y)y-(β+1)/qdy=x(λ1/λ2)
\∫+∞0K(1,t)t-(β+1)/qdt= "x(λ1/λ2)\W1-β+1q.
根據對稱性, 同理可得ω2(y,α,p)=y(λ2/λ1)\W2-α+1p.
若αλ1p+1λ2p=βλ2q+1λ1q, 則-λ1λ2β+
1q-1-1=-α+1p, 于是
W1-β+1q= "∫+∞0K(tλ2/λ1,1)t-(β+1)/qdt
=λ1λ2∫+∞0K(u,1)u-(λ1/λ2)\-1du= "λ1λ2∫+∞0K(u,1)u-
(α+1)/pdu=λ1λ2W2-α+1p,
故λ1W2-α+1p=λ2W1-β+1q.
引理2[21] 設1p+1q=1(0lt;plt;1, qlt;0), x∈Ω瘙綆n, f(x)≥0, g(x)
≥0, ω(x)≥0, 則有帶權的逆向Hlder積分不等式
∫Ωf(x)g(x)ω(x)dx≥∫Ωfp(x)ω(x)dx1/p∫Ωgq(x)ω(x)dx1/q,
當且僅當存在常數C使得fp(x)=Cgq(x)(x∈Ω)時, 不等式取等號.
2 非齊次核逆向Hilbert型積分不等式構造定理
定理1 設1p+1q=1(0lt;plt;1, qlt;0), λ1λ2gt;0, α,β∈瘙綆, K(x,y)=G(xλ1yλ
2)是非負可測函數, 0lt;W1-β+1qlt;+∞, 0lt;W2-α+1plt;+∞, 且存在常數σgt;0, 使得W1-
β+1q±σlt;+∞或W2-α+1p±σlt;+∞. 則:
1) 當且僅當αλ1p+1λ2p=βλ2q+1λ1q時, 存在常數gt;0, 使得
‖f‖*p,α‖g‖*q,β≤A(K,f,g),(2)
其中f∈Lαp(0,+∞), g∈Lβq(0,+∞);
2) 當αλ1p+1λ2p=βλ2q+1λ1q時, 式(2)的最佳常數因子為
inf{}=λ11/qλ21/pW0 W0=λ2W1-β+1q=λ1W2-α+1p.
證明: 由于對稱性, 不妨設W1-β+1q±σlt;+∞.
1) 充分性. 設αλ1p+1λ2p=βλ2q+1λ1q, 根據引理1和引理2并注意到qlt;0, 有
A(K,f,g)= "∫+∞0∫+∞0x(α+1)/(pq)y(β+1)/(pq)f(x)y(β+1)/(pq)x(α+1)/(pq)g(y)K(x,y)dxdy≥ "∫+∞0∫+∞0x(α+1)/
qy(β+1)/qf(x)pK(x,y)dxdy1/p∫+∞0∫+∞0y(β+1)/
px(α+1)/pg(y)qK(x,y)dxdy1/q= "∫+∞0x(α+1)/qf(x)pω
1(x,β,q)dx1/p∫+∞0y(β+1)/pg(y)qω2(y,α,p)dy1/q
= "W1/p1-β+1qW1/q2-α+1p∫+∞0x
(α+1)/q+(λ1/λ2)\f(x)pdx1/p× "∫+∞0y(β+1)/p+(λ2/λ1)
\g(y)qdy1/q= "W1/p1-β+1qW1/q
2-α+1p∫+∞0xαf(x)pdx1/p∫+∞0yβg(y)qdy1/q= "W1/p1-β+1qW1/q
2-α+1p‖f‖*p,α‖g‖*q,β,
從而
‖f‖*p,α‖g‖*q,β≤W-1/p1-β+1qW-1/q2-α+1pA(K,f,g),
任取≥W-1/p1-β+1qW-1/q2-α+1p, 式(2)成立.
必要性. 設式(2)成立. 記αλ1p+1λ2p-βλ2q+1λ1q=c, 下面只
需證明c=0即可. 若cλ1lt;0, 取充分小的εgt;0, 令
f(x)=x-(α+1-λ1ε)/p,0lt;x≤1,0,xgt;1," g(y)=
y-(β+1+λ2ε)/q,y≥1,0,0lt;ylt;1,
則計算可得
‖f‖*p,α‖g‖*q,β=∫10x-1+λ1εdx1/p∫+∞1y-1
-λ2εdy1/q=1ελ11/pλ21/q.
由cλ1lt;0有-cλ1gt;0, 于是
A(K,f,g)= "∫10x-(α+1)/p+λ1ε/p∫+∞1K(x,y)y-(β+1)/q-λ2ε/qdydx
= "∫10x-(α+1)/p+λ1ε/p∫+∞1K(1,xλ1/λ2y)y-(β+1)/q-λ2ε/qdydx
= "∫10x-(α+1)/p+λ1ε/p+(λ1/λ2)\λ2ε/q-1\〗∫+∞xλ1/λ2K(1,t)t-(β+1)/
q-λ2ε/qdtdx≤ "∫10x-1-cλ1+λ1εdx∫+∞0K(1,t)t-(β+1)/q-λ2ε/qdt
= "1-cλ1+λ1εW1-β+1q-λ2εq,
從而可得
1λ11/pλ21/q≤ε-cλ1+λ1εW
1-β+1q-λ2εq.(3)
令
F(x)=K(1,t)t-(β+1)/q+σ,t≥1,K(1,t)t-(β+1)/q,0lt;tlt;1,
因為εgt;0充分小, qlt;0, 故0lt;λ2ε-qlt;σ, 于是0≤K(1,t)t-(β+1)/q-λ2ε/q≤F(t)(tgt;0), 且
∫+∞0F(t)dt= "∫10K(1,t)t-(β+1)/qdt+∫+∞1K(1,t)t-(β+1)/q+σdt
≤ "W1-β+1q+W1-β+1q+σlt;+∞.
綜上并根據Lebesgue控制收斂定理, 有
limε→0+ W1-β+1q-λ2εq= "limε→0+ ∫+∞0K(1,t)t-(β+1)/q-λ2ε/qdt= "∫+∞0K(1,t)t-(β+1)/qdt=W1-β+1qlt;+∞.
于是在式(3)中令ε→0+, 得
0lt;1λ11/pλ21/q≤0,(4)
矛盾. 故cλ1lt;0不成立.
若cλ1gt;0, 取充分小的εgt;0, 令
f(x)=x-(α+1+λ1ε)/p,x≥1,0,0lt;xlt;1," g(y)=y-(β+1-λ1ε)/q,0lt;y≤1,0,ygt;1,
則類似式(3)的推導方法, 可得
1λ11/pλ21/q≤ελ1c+λ1εW
1-β+1q+λ2εq.(5)
同理, 利用W1-β+1q-σlt;+∞及Lebesgue控制收斂定理, 有
limε→0+ W1-β+1q+λ2εq=W1-β+1qlt;+∞,
于是在式(5)中令ε→0+, 可得式(4), 矛盾, 因此cλ1gt;0也不成立.
綜上可得cλ1=0, 從而c=0.
2) 設αλ1p+1λ2p=βλ2q+1λ1q, 則c=0. 根據引理1, 有λ1
W2-α+1p=λ2W1-β+1q=W0, 若式(2)的最佳常數因子不是
λ11/qλ21/p/W0, 則存在常數M0gt;0, 使得‖f‖*p,α‖g‖*q,β≤0A(K,f,g), 且
0lt;λ11/qλ21/pW0=λ11/q
λ21/pλ2W-11-β+1q=λ1λ21/qW-11-β+1q.
由于c=0, 類似推導式(3)的方法, 可得
1λ11/pλ21/q≤0λ1W
1-β+1q-λ2εq,
令ε→0+, 則有1λ11/pλ21/q≤0λ
1W1-β+1q, 于是可得
0≥λ1λ21/qW-11-β+1q,
與0lt;λ1λ21/qW-11-β+1q
矛盾, 故λ11/qλ21/pW0是式(2)的最佳常數因子.
注1 當0lt;plt;1, qlt;0, λ1λ2gt;0時, 1λ2p≠1λ1q. 根據定理1可知, 對非齊次核K(x,y)=G(x
λ1yλ2)(λ1λ2gt;0), 不存在常數gt;0, 使得‖f‖*p‖g‖*q≤A(K,f,g)成立, 即當
α=β=0時, 非齊次核K(x,y)=G(xλ1yλ2)(λ1λ2gt;0)不能構建逆向Hilbert型積分不等式A(K,f,g)≤A(K,f,g).
3 非齊次核積分算子反向有界的充要條件
設K(x,y)≥0, 定義以K(x,y)為核的積分算子T為
T(f)(y)=∫+∞0K(x,y)f(x)dx,(6)
若‖f‖*p,α≤‖T(f)‖*p,γ, 則稱T是Lαp(0,+∞)到Lγp(0,+∞)的反向有界算子, 并記
‖T‖*=sup‖f‖*p,α‖T(f)‖*p,γ: f∈Lαp(0,+∞), ‖T(f)‖*p,γ≠0.
根據Hilbert型不等式的基本理論, 逆向Hilbert型積分不等式(1)等價于算子不等式
‖f‖*p,α≤‖T(f)‖*p,β(1-p).(7)
于是根據定理1, 可得如下積分算子T反向有界的充分必要條件.
定理2 設1p+1q=1(0lt;plt;1, qlt;0), λ1λ2gt;0, α,β∈瘙綆, K(x,y)=G(xλ1y
λ2)是非負可測函數, 0lt;W1-β+1qlt;+∞, 0lt;W2-α+1plt;+∞, 存在常數σgt;0, 使得W1-β+1q±σlt;+∞或W2-α+1p±σlt;+∞, 積分算子T由式(6)定義. 則:
1) 當且僅當αλ1p+1λ2p=βλ2q+1λ1q時, T是Lαp(0,+∞)
到Lβ(1-p)p(0,+∞)的反向有界算子, 且‖T‖*≤W-1/p1-β+1qW-1/q2-α+1p;
2) 當αλ1p+1λ2p=βλ2q+1λ1q時, ‖T‖*=W-1/p
1-β+1qW-1/q2-α+1p=
λ11/qλ21/pW0.
在定理2中取λ1=λ2=1, 可得下列推論.
推論1 設1p+1q=1(0lt;plt;1, qlt;0), α,β∈瘙綆, K(x,y)=G(xy)是非負可測函數, 0lt;W1-β+1qlt;+∞, 0lt;W2-α+1plt;+∞, 存在σgt;0, 使得W1-β+1q±σlt;+∞或
W2-α+1p±σlt;+∞, 積分算子T由式(6)定義. 則:
1) 當且僅當α+1p=β+1q時, T是Lαp(0,+∞)到Lβ(1-p)p(0,+∞)的反向有界算子,
且‖T‖*≤W-1/p1-β+1qW-1/q2-α+1p;
2) 當α+1p=β+1q時, ‖T‖*=W1-β+1q=W2-α+1p.
推論2 設1p+1q=1(0lt;plt;1, qlt;0), λ1gt;0, λ2gt;0, agt;0, -alt;1λ1
αp-1qlt;b, αλ1p+1λ2p=βλ2q+1λ1q, 積分算子T為
T(f)(y)=∫+∞0(min{1,xλ1yλ2})b(1+xλ1yλ2)af(x)dx,
則T是Lαp(0,+∞)到Lβ(1-p)p(0,+∞)的反向有界算子, 即
‖f‖*p,α≤0(a,b,α,β)‖T(f)‖*p,β(1-p),
且‖T‖*=0(a,b,α,β)=1λ1/q1λ1/p2∫101(1+t)a(ta-(1/λ1)
(1/q-α/p)-1+tb+(1/λ2)(1/p-β/q)-1)dt-1.
證明: 根據αλ1p+1λ2p=βλ2q+1λ1q和-alt;1λ1αp-1qlt;b可知, a-1λ11q-αpgt;0, b+1λ21p-βqgt;0, 從而0(a,b,α,β)中的積分收斂. 記
K(x,y)=G(xλ1yλ2)=(min{1,xλ1yλ2})b(1+xλ1yλ2)a (xgt;0, ygt;0),
則K(x,y)≥0, 且
W1-β+1q= "∫+∞0K(1,t)t-(β+1)/qdt=∫+∞0(min{1
,tλ2})b(1+tλ2)at-(β+1)/qdt= "∫101(1+tλ2)atbλ2-(β+
1)/qdt+∫+∞11(1+tλ2)at-(β+1)/qdt=
1λ2∫101(1+u)aub+(1/λ2)(1/p-β/q)-1du+1λ2∫+∞11(1+u)
au(1/λ2)(1/p-β/q)-1du= "1λ2∫101(1+u)aub+(1/λ2)
(1/p-β/q)-1du+1λ2∫101(1+t)ata-(1/λ2)(1/p-β/q)-1dt=
1λ2∫101(1+t)atb+(1/λ2)(1/p-β/q)-1d
t+1λ2∫101(1+t)ata-(1/λ1)(1/q-α/p)-1dt
= "1λ2∫101(1+t)a(ta-(1/λ1)(1/q-α/p)-1+tb+(1/λ2)(1/p-β/q)-1)dtlt;+∞.
類似可得W2-α+1plt;+∞.
因為a-1λ11q-αpgt;0, b+1λ21p-βqgt;0, 所以根據實數的稠密性, 可知存在常數σgt;0, 使得
a-1λ11q-αp±σλ2gt;0," b+1λ21p-βq±σλ2gt;0.
計算易得
W1-β+1q±σ=1λ2∫101(1+t)a(ta-(1/λ1)(1/q-α/p)±σ/λ2-1+tb+(1/λ2)(1/p-β/q)±σ/λ2-1)dt,
從而W1-β+1q±σlt;+∞. 又由于
W0=λ2W1-β+1q=∫101(1+t)a(ta-(1/λ1)(1/q-α/p)-1+tb+(1/λ2)(1/p-β/q)-1)dt,
故1W0λ1/q1λ1/p2=0(a,b,α,β).
綜上并根據定理2知結論成立. 證畢.
在推論2中取b=0, α=p1q-λ1τ, β=q1p-λ2τ, 0lt;τlt;a, 易驗證它們滿足推論2的所有條件, 且
a-1λ11q-αp
+b+1λ21p-βq=a,
a-1λ11q-αp=a-τ," b+1λ21p-βq=τ.
根據Beta函數的性質, 有0(a,b,α,β)=λ1/q1λ1/p2B(a-τ,τ), 于是根據推論2可得如下推論.
推論3 設1p+1q=1(0lt;plt;1, qlt;0), λ1gt;0, λ2gt;0, 0lt;τlt;a, 積分算子T為
T(f)(y)=∫+∞01(1+xλ1yλ2)af(x)dx,
則T是Lp(1/q-λ1τ)p(0,+∞)到Lλ2pτ-1p(0,+∞)的反向有界算子:
‖f‖*p,p(1/q-λ1τ)≤λ1/q1λ1/p2B(a-τ,τ)‖T(f)‖*p,λ2pτ-1,
且‖T‖*=λ1/q1λ1/p2B(a-τ,τ).
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