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两类解析函数的三阶Hankel行列式的上界估计

郭栋, 汤获, 文传军, 李宗涛

郭栋, 汤获, 文传军, 李宗涛. 两类解析函数的三阶Hankel行列式的上界估计[J]. 华南师范大学学报(自然科学版), 2024, 56(1): 118-122. DOI: 10.6054/j.jscnun.2024014
引用本文: 郭栋, 汤获, 文传军, 李宗涛. 两类解析函数的三阶Hankel行列式的上界估计[J]. 华南师范大学学报(自然科学版), 2024, 56(1): 118-122. DOI: 10.6054/j.jscnun.2024014
GUO Dong, TANG Huo, WEN Chuanjun, LI Zongtao. The Upper Bounds of the Third Hankel Determinant for Two Subclasses of Analytic Functions[J]. Journal of South China Normal University (Natural Science Edition), 2024, 56(1): 118-122. DOI: 10.6054/j.jscnun.2024014
Citation: GUO Dong, TANG Huo, WEN Chuanjun, LI Zongtao. The Upper Bounds of the Third Hankel Determinant for Two Subclasses of Analytic Functions[J]. Journal of South China Normal University (Natural Science Edition), 2024, 56(1): 118-122. DOI: 10.6054/j.jscnun.2024014

两类解析函数的三阶Hankel行列式的上界估计

基金项目: 

国家自然科学基金项目 11561001

安徽省高校自然科学基金项目 KJ2018A0833

安徽省高校自然科学基金项目 KJ2020A0993

安徽省高校自然科学基金项目 KJ2020ZD74

内蒙古自治区高等学校科学研究项目 NJZY19211

详细信息
    通讯作者:

    郭栋, Email: gd791217@163.com

  • 中图分类号: O174.51

The Upper Bounds of the Third Hankel Determinant for Two Subclasses of Analytic Functions

  • 摘要:

    H表示形如f(z)=z+a2z2+a3z3+且在U={z:|z|<1}内解析的函数类,研究了在单位圆盘上的2类解析函数类STs={f?H:Re2zf(z)f(z)f(z)>0,z?U}R(12)={f?H:Ref(z)z>12,z?U}的三阶Hankel行列式|H3,1(f)|的上界估计。

    Abstract: Let H denote the family of all analytic functions with the form f(z)=z+a2z2+a3z3+ in the unit disk U={z:|z|<1}. Two subclasses of analytic functions STs and R(1/2) which are difined in the unit disk U are introduced, respectively, i.e., STs={f?H:Re2zf(z)f(z)f(z)>0,z?U},R(12)={f?H:Ref(z)z>12,z?U}. And the bounds of |H3,1(f)| for subfamilies of STs and R(1/2) are obtained.
  • U={z: |z| < 1}, H表示在U内解析且具有形式

    f(z)=z+a2z2+a3z3+ (1)

    的函数类; P表示在U内解析且满足Re(p(z))>0的解析函数类,其泰勒展式为

    p(z)=1+c1z+c2z2+c3z3+ (2)

    1966年,POMMERENKE[1]引入函数f(z)的q阶Hankel行列式:

    Hq,n(f)=|anan+1an+q1an+1an+2an+qan+q1an+qan+2q2|(n1,q1,a1=1)

    自此,Hankel行列式的上界估计成为几何函数论的一个重要问题。

    2010年,BABALOLA[2]首次研究了星像函数类S*和凸像函数类K的三阶Hankel行列式:

    |H3,1(f)|=|a22a5+2a2a3a4a33+a3a5a24| (3)

    的上界估计。2017年,ZAPRAWA[3]改进了文献[2]的结论:若f?S,则有|H3,1(f)|≤1;若f?k,则有|H3,1(f)|≤49/540。但文献[3]的结果仍不精确,2018、2022年,KOWALCZYK等[4-5]分别给出了星像函数类、凸像函数类的|H3,1(f)|的精确估计:若f?S,则有|H3,1(f)|≤4/9;若f ? K,则有|H3,1(f)|≤4/135。

    近几年,学者们研究了各类解析函数的三阶Hankel行列式|H3,1(f)|,估计方法为计算|a3|、|a4|、|a5|及3个二阶行列式的上界,但所得结论不精确[2, 6-8]。如KRISHNA VAMSHEE等[7]研究了函数类STs的二阶、三阶Hankel行列式,得到如下结果:若f ? STs,则有|H3,1(f)|≤5/2。近年来,学者们将解析函数的三阶Hankel行列式|H3,1(f)|的上界归为一个求多元函数的最值问题[9-10],得到了几类解析函数子类的三阶Hankel行列式的精确上界[11-12]。在文献[7]的基础上,本文利用多元函数的最值法重新研究函数类STs的三阶Hankel行列式,得到了更好的结果:若f ? STs,则有|H3,1(f)|≤0.251 5···。

    定义1[13]  若f(z)由式(1)给出且满足条件

    Re(2zf(z)f(z)f(z))>0(z?U),

    则称此函数类为有关对称点的星像函数类,记为f ? STs

    定义2[14]  若f(z)由式(1)给出且满足条件

    Re(f(z)z)>12(z?U),

    则称f ? R(1/2)。

    下面给出本文证明需要的引理:

    引理1[15-17]  假定p?P,c10, 则对ζ,η,ξ?ˉD={z?≫:|z|1}, 有

    2c2=c21+(4c21)ζ, (4)
    4c3=c31+c1ζ(4c21)(2ζ)+2(4c21)(1|ζ|2)η, (5)
    8c4=c41+(4c21)ζ[c21(ζ23ζ+3)+4ζ]4(4c21)(1|ζ|2)[c1(ζ1)η+ˉζη2(1|η|2)ξ] (6)

    定理1  如果f(z) ? STs,则有

    |H3,1(f)|64.3896256=0.2515

    证明  因为f(z) ? STs, 则存在p ? P, 满足

    2zf(z)=p(z)[f(z)f(z)]

    从而可得

    1+2a2z+3a3z2+4a4z3+5a5z4+=1+c1z+(c2+a3)z2+(c3+c1a3)z3+(c4+c2a3+a5)z4+

    比较两边的系数,可得

    a2=c12,a3=c22,a4=18(2c3+c1c2),a5=18(2c4+c22) (7)

    不妨假定c=c1?[0,2],由式(3)和式(7)得

    H3,1(f)=164[c2c22+4cc2c34c324c23+4c4(2c2c2)] (8)

    为简化计算,令t=4c2,式(4)~(6)可以写成

    c2=12(c+tζ),c3=14(c3+2ctζctζ2+2t(1|ζ|2)η),c4=18[c4+3c2tζ+(43c2)tζ2+c2tζ3+4t(1|ζ|2)×(cηcζηˉζη2)+4t(1|ζ|2)(1|η|2)ξ] (9)

    则有

    c2c22=14[c6+2c4tζ+c2t2ζ2] (10)
    4cc2c3=12[c6+3c4tζ+2c2t2ζ2c4tζ2c2t2ζ3+2t(c3+ctζ)×(1|ζ|2)η], (11)
    4c32=12[c6+3c4tζ+3c2t2ζ2+t3ζ3], (12)
    4c23=14[c6+4c4tζ+4c2t2ζ22c4tζ24c2t2ζ3+c2t2ζ4+4t(c3+2ctζctζ2)(1|ζ|2)η+4t2(1|ζ|2)2η2], (13)
    4c4(2c2c2)=12[c4tζ+3c2t2ζ2+(43c2)t2ζ3+c2t2ζ4+4t2cζ(1ζ)(1|ζ|2)η4t2(1|ζ|2)|ζ|2η2+4t2(1|ζ|2)(1|η|2)ζξ] (14)

    由式(10)~(14)和式(8),可得

    H3,1(f)=1256(4c2)2[r1(c,ζ)+r2(c,ζ)η+r3(c,ζ)η2+r4(c,ζ,η)ξ] (15)

    其中

    r1(c,ζ)=ζ2c2(1ζ)2,r2(c,ζ)=4cζ(1ζ)(1|ζ|2),r3(c,ζ)=4(1|ζ|4),r4(c,ζ,η)=8(1|ζ|2)(1|η|2)ζ

    x=|ζ|?[0,1],y=|η|?[0,1]。因|ξ|≤1,则由式(15)得

    |H3,1(f)|1256(4c2)2[|r1(c,ζ)|+|r2(c,ζ)||η|+|r3(c,ζ)||η|2+|r4(c,ζ,η)|]1256F(c,x,y)

    其中

    F(c,x,y)=f1(c,x)+f4(c,x)+f2(c,x)y+(f3(c,x)f4(c,x))y2,f1(c,x)=x2(4c2)2c2(1+x)2,f2(c,x)=4c(4c2)2x(1x2)(1+x),f3(c,x)=4(4c2)2(1x4),f4(c,x)=8(4c2)2(1x2)x

    下面证明FΩ={(c, x, y)|: 0≤c≤2, 0≤x≤1, 0≤y≤1}上的最大值为64.389 6···。对c?[0,2],有Fy=cx(1+x)2(1x)2<0, 则FΩ的内部无极值点,极值点只能出现在Ω的边界上。下面分3种情况讨论:

    (1) 若极值点在Ω的顶点上,则

    F(0,0,0)=F(2,0,0)=F(2,1,0)=F(2,1,1)=F(2,0,1)=F(0,1,0)=F(0,1,1)=0F(0,0,1)=64

    (2) 若极值点在Ω的棱上,则

    (ⅰ)当x=1, y=0或x=1, y=1时,有

    F(c,1,0)=F(c,1,1)=4c632c4+64c2=G1(c)(c?(0,2))

    G1(c)=24c5128c3+128c=0,c=1/3。又因为

    G1(c)=120c4384c3+128,G1(1.1547)=5723<0

    所以

    G1(c)G1(13)=10242737.9259

    (ⅱ)当c=0, y=0时,有

    F(0,x,0)=128x128x3=G2(x)(x?(0,1))

    G2(x)=128384x2=0,x=1/3。又因为G2(x)=768x,G2(1/3)=768/3<0, 所以

    G2(x)G2(13)=2563349.267

    (ⅲ)当x=0, y=1时,有

    F(c,0,1)=4(4c2)264(c?(0,2))

    (ⅳ)当c=0, y=1时,有

    F(0,x,1)=6464x464(x?(0,1))

    (ⅴ)当c=0, x=0时,有

    F(0,0,y)=64y464(y?(0,1))

    (ⅵ)当c=0且x=1,或x=0且y=0,或c=2且y=0,或c=2且y=1,或c=2且x=0,或c=2且x=1时,有

    F(0,1,y)=F(c,0,0)=F(2,x,0)=F(2,x,1)=F(2,0,y)=F(2,1,y)=0

    (3) 若极值点在Ω的面上,则

    (ⅰ)当c=2时,有F(2, x, y)=0。

    (ⅱ)当c=0时,有

    F(0,x,y)=128x128x3+64(1x2)(1x)2y6464x464(x?(0,1))

    (ⅲ)当x=0时,有

    F(c,0,y)=4(4c2)2y264(c?(0,2),y?(0,1))

    (ⅳ)当x=1时,有

    F(c,1,y)=4c632c4+64c2=G1(c)G1(13)37.9259(c?(0,2))

    (ⅴ)当y=0时,有

    F(c,x,0)=(4c2)2[c2x2(1+x)2+8x8x3]

    c2=t,则有

    G3(t,x)=(4t)2[tx2(1+x)2+8x8x3](t?(0,4),x?(0,1))

    下面求解G3在(0, 4)×(0, 1)上可能的极值点。

    G3t=(4t)x[3x(1+x)2t4x324x24x+16]=0,

    可得

    t(x)=4x3+24x2+4x163x(1+x)2 (16)

    G3x=(4t)2[2(x+x2)(1+2x)t+824x2]=0

    可得

    2(x+x2)(1+2x)t+824x2=0 (17)

    将式(16)代入式(17),化简得

    2x4+4x3x24x1=0

    解之得:x=±1?(0,1),x=1+2/2?(0,1),x=12/2?(0,1),则G3在(0, 4)×(0, 1)上无极值点。

    (ⅵ)当y=1时,有

    F(c,x,1)=(4c2)2c2(x2+2x3+x4)+8(4c2)2(xx3)+4c(4c2)2(x+x2x3x4)+4(4c2)2×(12x+2x3x4)=G(c,x)

    {Gc=(8+12c+10c23c3)x4+(8+8c+10c26c3)x3+(8+4c10c23c3)x2+(810c2)x8c=0Gx=(128128c+96c2+64c324c48c5+2c6)x3+(96c+48c2+48c324c46c5+3c6)x2+(64c+16c232c38c4+4c5+c6)x+2c516c3+32c=0

    利用Matlab可得:

    {x1=±2,y1=1;{x2=2,y2=1.5370;{x3=2,y3=1.9180;{x4=0,y4=0;;{x5=0.7196,y5=0.8319;;{x6=0.1741,y6=0.2534;{x7=11.3794y7=1.1717;{x8=0.5462,y8=0.5287

    由方程Gc=Gx=0可得G在(0, 4)×(0, 1)可能的极值点为(0.546 2, 0.528 7), 在该点的函数值为G(0.546 2, 0.528 7)=64.389 6···。

    综上所述, 可得

    |H3,1(f)|64.3896256=0.2515

    证毕。

    定理2  如果f ? R(1/2),则有|H3,1(f)|≤1,极值函数为f(z)=z1z3

    证明  因为f(z) ? R(1/2), 则存在p ? P, 满足

    f(z)z=12(p(z)+1)

    比较两边的系数, 可得

    a2=c12,a3=c22,a4=c32,a5=c42 (18)

    不妨假定c=c1?[0, 2],由式(3)和式(18)可得

    H3,1(f)=18[2cc2c3c322c23+c4(2c2c2)] (19)

    由式(9)和式(19)可得

    2cc2c3=14[c6+3c4tζ+2c2t2ζ2c4tζ2c2t2ζ3+2t(c3+ctζ)×(1|ζ|2)η], (20)
    c32=18[c6+3c4tζ+3c2t2ζ2+t3ζ3], (21)
    2c23=18[c6+4c4tζ+4c2t2ζ22c4tζ24c2t2ζ3+c2t2ζ4+4t(c3+2ctζctζ2)(1|ζ|2)η+4t2(1|ζ|2)2η2], (22)
    c4(2c2c2)=18[c4tζ+3c2t2ζ2+(43c2)t2ζ3+c2t2ζ4+4t2cζ(1ζ)(1|ζ|2)η4t2(1|ζ|2)|ζ|2η2+4t2(1|ζ|2)(1|η|2)ζξ] (23)

    由式(20)~(23)和式(12)可得

    H3,1(f)=116(4c2)2[r1(c,ζ)η2+r2(c,ζ,η)ξ], (24)

    其中, r1(c,ζ)=(1|ζ|2),r2(c,ζ,η)=(1|ζ|2)×(1|η|2)ζ

    x=|ζ|?[0,1],y=|η|?[0,1]。因|ξ|≤1,则由式(24)可得

    |H3,1(f)|116(4c2)2[|r1(c,ζ)||η|2+|r2(c,ζ,η)|]116(4c2)2F(x,y)

    其中, F(x,y)=(1x2)x+(1x2)(1x)y2

    下面证明F(x, y)≤1。对y?(0,1),x?(0,1),有Fy=2(1x2)(1x)y>0,则

    F(x,y)F(x,1)=1x21,

    所以

    |H3,1(f)|1

    由定理2的证明过程及引理知,c=c1=0, c2=0, c3=2, c4=0时等号成立。此时a2=0, a3=0, a4=1, a5=0, 代入式(3)得H3,1(f)=-1。证毕。

    由定理1的证明过程得到以下猜想:

    猜想1  若f(z) ? STs,则有|H3,1(f)|64256=14,极值函数为f(z)=2z2z3

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出版历程
  • 收稿日期:  2022-10-27
  • 网络出版日期:  2024-04-29
  • 刊出日期:  2024-02-24

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