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数学物理学报, 2020, 40(5): 1269-1281 doi:

论文

第二类完全p-椭圆积分关于Hölder平均的凹性

王淼坤,1, 何再银,2, 褚玉明,1

Concavity of the Complete p-Elliptic Integral of the Second Kind According to Hölder Mean

Wang Miaokun,1, He Zaiyin,2, Chu Yuming,1

通讯作者: 褚玉明, E-mail: chuyuming@zjhu.edu.cn

收稿日期: 2019-03-19  

基金资助: 国家自然科学基金.  11701176
国家自然科学基金.  61373169

Received: 2019-03-19  

Fund supported: NSFC.  11701176
NSFC.  61373169

作者简介 About authors

王淼坤,E-mail:wmk000@126.com , E-mail:wmk000@126.com

何再银,E-mail:hzy@zjhu.edu.cn , E-mail:hzy@zjhu.edu.cn

摘要

该文给出了第二类完全p-椭圆积分满足Hölder凹性的充分必要条件,从而推广了先前关于第二类完全椭圆积分的相应结果.

关键词: 完全p-椭圆积分 ; 广义椭圆积分 ; Hölder平均值 ; 凹性 ; 凸性

Abstract

In the article, we provide a necessary and sufficient condition such that the complete p-elliptic integral of the second kind is concave with respect to Hölder mean, which is a generalization of the previously known result on the complete elliptic integral of the second kind.

Keywords: Complete p-elliptic integral ; Generalized elliptic integral ; Hölder mean ; Concavity ; Convexity

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本文引用格式

王淼坤, 何再银, 褚玉明. 第二类完全p-椭圆积分关于Hölder平均的凹性. 数学物理学报[J], 2020, 40(5): 1269-1281 doi:

Wang Miaokun, He Zaiyin, Chu Yuming. Concavity of the Complete p-Elliptic Integral of the Second Kind According to Hölder Mean. Acta Mathematica Scientia[J], 2020, 40(5): 1269-1281 doi:

1 引言

0<r<1,勒让德第一类和第二类完全椭圆积分K(r)E(r)[1]分别定义为

K=K(r)=π/20dθ1r2sin2θ,K(0+)=π2,K(1)=,
(1.1)

E=E(r)=π/201r2sin2θdθ,E(0+)=π2,E(1)=1.
(1.2)

椭圆积分如此命名是因为它们出现在椭圆周长的计算公式.众所周知,完全椭圆积分在数学、物理及其其它的一些学科中具有广泛的应用.例如,圆周率π的计算,一些微分方程的解的表达式,电磁场和单摆周期的计算公式等都与两类完全椭圆积分K(r)E(r)密切相关.

在过去的三十多年,完全椭圆积分的研究主要集中于它们在几何函数论中的应用.许多共形不变量、拟共形映射中的偏差函数都可以用完全椭圆积分来表示[2-3].特别地,关于K(r)E(r)的许多优美的性质和上下界被证明[4-21].

p(1,), r(0,1), Takeuchi[22]引入了二类完全p -椭圆积分Kp(r)Ep(r),其定义如下

Kp=Kp(r)=πp/20dθ(1rpsinppθ)11/p,Kp(0+)=πp2,Kp(1)=,
(1.3)

Ep=Ep(r)=πp/20(1rpsinppθ)1/pdθ,Ep(0+)=πp2,Ep(1)=1,
(1.4)

其中sinpθ是广义三角正弦函数,具有反函数

sinp1θ=θ0dt(1tp)1/p,0θ1,

πp=210dt(1tp)1/p=2πpsin(π/p)

是广义圆周率.

易见sin2θ=sinθ, π2=π,且由(1.1)–(1.4)式可得K2(r)=K(r), E2(r)=E(r). sinpθπp首先被给出在1维的p-Laplacian方程的特征值问题[23-25].

近几年,完全p -椭圆积分已经被Takeuchi等从不同的角度进行深入研究. 2016年, Takeuchi[22, 26]证明完全p -椭圆积分KpEp满足如下导数公式及其广义Legendre恒等式

dKp(r)dr=EprpKprrp,dEp(r)dr=EpKpr,

d(KpEp)dr=rp1Ep(r)rp,d(EprpKp)dr=(p1)rp1Kp(r),

Kp(r)Ep(r)+Kp(r)Ep(r)Kp(r)Kp(r)=πp2,
(1.5)

其中对r(0,1), r=p1rp.利用式(1.5), Takeuchi[22, 27]发现了当p=3p=4πp的算法公式.此外,完全p -椭圆积分与经典的Gauss超几何函数具有如下关系(参见文献[22,命题2.8])

Kp(r)=πp2F(11p,1p;1;rp),Ep(r)=πp2F(1p,1p;1;rp),
(1.6)

其中

F(a,b;c;x)=2F1(a,b;c;x)=n=0(a)n(b)n(c)nxnn! (|x|<1)

是Gauss超几何函数[28-29], a,b,c为实参数且c0,1,2,, (a)0=1(a0), (a)n=a(a+1)(a+2)(a+3)(a+n1)=Γ(n+a)/Γ(a)(n=1,2,3), Γ(x)=0tx1etdt是Gamma函数[30-31].

最近,张孝惠[32]将完全椭圆积分K(r)E(r)满足的一些单调性质和不等式推广到了完全p -椭圆积分Kp(r)Ep(r).

完全椭圆积分的另一种重要的广义形式,称为广义椭圆积分Ka(r), Ea(r),已经被Vuorinen教授及其合作者引入并在Ramanujan模方程理论中广泛研究(参见文献[33]).事实上,对a(0,1), r(0,1),第一类和第二类广义椭圆积分Ka(r), Ea(r)分别定义为

Ka=Ka(r)=π2F(a,1a;1;r2),Ea=Ea(r)=π2F(a1,1a;1;r2).
(1.7)

易见K1/2(r)=K(r), E1/2(r)=E(r).比较(1.6)和(1.7)式,我们便知:令p=1/(1a),

Ka(r)=ππpKp(r2/p), Ea(r)=ππpEp(r2/p).
(1.8)

更多Ka(r), Ea(r)的性质及其应用可参见文献[34-40].

αR,两个正数xyα阶Hölder平均值定义为

Hα(x,y)={(xα+yα2)1/α,α0,xy,α=0.

特别令α=1, 01,便得到经典的调和平均值H(x,y)=H1(x,y),几何平均值G(x,y)=H0(x,y)及其算术平均值A(x,y)=H1(x,y).众所周知,对固定的x,y>0xy, Hölder平均值Hα(x,y)关于α在实数域R上是连续且严格单调增加的. Hölder平均值的一些基本性质及其不等式可参见文献[43].

α,βR,区间I(0,),函数f:I(0,)连续.若对于所有的x,yI都成立如下不等式

f(Hα(x,y))()Hβ(f(x),f(y)),
(1.9)

则称fIHα,β -凸的(凹的).而且,若不等式(1.9)成立等号当且仅当x=y,便称fI是严格Hα,β -凸的(凹的).特别当α=β=1时, Hα,β -凸性(凹性)就退化为经典的凸性(凹性).

2007年,对α=0,1,1β=0,1,1, Anderson, Vamanamurthy和Vuorinen[44]探究了由Maclaurin级数定义的函数的Hα,β -凸性和Hα,β -凹性.运用该结果, Gauss超几何函数的Hα,β -凸性(凹性)及其一类新的不等式被证得.

Baricz[45]考察了零平衡超几何函数F(a,b;a+b;x)(a,b>0)的Hölder凸性,发现对(α,β){(α,β)|α2,β0},第一类完全椭圆积分K(r)是在(0,1)上严格Hα,β -凸的.随后,使得K(r)(0,1)上严格Hα,β -凸的完整区域被王淼坤等[46]证明.类似地,相同作者在文献[47]中得到了关于第二类完全椭圆积分在(0,1)满足Hölder凹的充要条件:

定理1.1[47]  第二类完全椭圆积分E(r)(0,1)Hα,β -凹的当且仅当

(β,α){(β,α)|αC(β)},

其中C(β)=inf关于 \beta 连续,且当 \beta\leq 5/2 时, C(\beta) = 2 ,而当 \beta>5/2 时, C(\beta)<2 .此外,不存在 \alpha, \beta\in {{\Bbb R}} 使得 {\cal E}(r) (0, 1) H_{\alpha, \beta} -凸的.

2012年,周丽明等[48]考虑了两类广义椭圆积分,给出了 {\cal K}_{a}(r) (0, 1) H_{\alpha, \alpha} -凸的及其 {\cal E}_{a}(r) ( a\in(0, 1/2] ) (0, 1) H_{\alpha, \alpha} -凹的充分必要条件.最近, Baricz和Vuorinen在文献[49-50]中也研究广义三角函数的 H_{\alpha, \alpha} -凸性(凹性).

本文的主要目的是将定理1.1推广到第二类 p -完全椭圆积分和第二类广义椭圆积分的情形.主要结果是如下的定理1.2和推论1.1.

定理1.2  对 p\in(1, \infty) ,第二类 p -完全椭圆积分 E_{p}(r) (0, 1) H_{\alpha, \beta} -凹的,即不等式

E_{p}(H_{\alpha}(x, y))\geq H_{\beta}(E_{p}(x), E_{p}(y))

对所有的 x, y\in (0, 1) 恒成立,当且仅当

(\beta, \alpha)\in D = \{(\beta, \alpha)|\alpha\leq L(\beta)\},

其中 L(\beta) = \inf\limits_{r\in(0, 1)}\left\{r^{p}E_{p}/[{r'}^{p}(K_{p}-E_{p})]+(1-\beta)(K_{p}-E_{p})/E_{p}\right\} 关于 \beta 连续,且当 \beta\leq (p^2+1)/2 时, L(\beta) = p ,而当 \beta>(p^2+1)/2 时, L(\beta)<p (见图 1).此外,不存在 \alpha, \beta\in {{\Bbb R}} 使得 E_{p}(r) (0, 1) 上是 H_{\alpha, \beta} -凸的.

图 1

图 1   参数 p = \frac{3}{2}, 2, \frac{5}{2}, 4 时的区域 D 及其对应的边界曲线 \alpha = L(\beta)


推论1.1  对 a\in(0, 1) ,第二类广义椭圆积分 {\cal E}_{a}(r) (0, 1) 上严格 H_{\alpha, \beta} -凸的,即不等式

{\cal E}_{a}(H_{\alpha}(x, y))\geq H_{\beta}({\cal E}_{a}(x), {\cal E}_{a}(y))

对所有 x, y\in (0, 1) 恒成立,当且仅当

(\beta, \alpha)\in\{(\beta, \alpha)|\alpha\leq L^*(\beta)\},

其中 L^*(\beta) = \inf\limits_{r\in(0, 1)}2(1-a)\left\{r^{2}{\cal E}_{a}/[(1-r^2)({\cal K}_{a}-{\cal E}_{a})]+(1-\beta)({\cal K}_{a}-{\cal E}_{a})/{\cal E}_{a}\right\} 关于 \beta 连续,且当 \beta\leq (a^2-2a+2)/[2(1-a)^2] 时, L^*(\beta) = 2 ,而当 \beta>(a^2-2a+2)/[2(1-a)^2] 时, L^*(\beta)<2 .此外,不存在 \alpha, \beta\in {{\Bbb R}} 使得 {\cal E}_{a}(r) (0, 1) 上是 H_{\alpha, \beta} -凸的.

2 引理

为证明本文的主要结果,需要在本节中建立几则引理.

引理2.1[3]  对 -\infty<a<b<+\infty ,设 f g 是两个实值函数,都在 [a, b] 上连续,在 (a, b) 上可微,且在 (a, b) g'\neq 0 .如果 f'/g' (a, b) 上(严格)单调增加(减小),那么函数

\frac{f(x)-f(a)}{g(x)-g(a)}\quad\mbox{和}\quad \frac{f(x)-f(b)}{g(x)-g(b)}

也在 (a, b) 上(严格)单调增加(减小).

引理2.2  令 p\in(1, \infty) .

(1) 函数 r\mapsto(E_{p}-{r'}^pK_{p})/r^p (0, 1) ({(p-1)\pi_{p}}/{2p}, 1) 严格单调增加;

(2) 函数 r\mapsto(K_{p}-E_{p})/(r^p K_{p}) (0, 1) ({1}/{p}, 1) 严格单调增加;

(3) 函数 r\mapsto{r'}^p(K_{p}-E_{p})/(r^pE_{p}) (0, 1) (0, 1/p) 严格单调减小;

(4) 当且仅当 c\geq (p-1)/p ,函数 r\mapsto{r'}^cK_{p} (0, 1) 上严格单调减小,且此时函数的值域为 (0, \pi_{p}/2) ;

(5) 当且仅当 c\leq -1/p ,函数 r\mapsto{r'}^cE_{p} (0, 1) 上严格单调增加,且此时函数的值域为 (\pi_{p}/2, \infty) ;

(6) 函数 r\mapsto[(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})]/r^{2p} 具有正的 \rm Maclaurin 级数,在 (0, 1) 的值域为 (\pi_{p}(1-p)^2/(4p^2), \infty) ;

(7) 函数 r\mapsto[E_{p}-{r'}^pK_{p}-(p-1){r'}^p(K_{p}-E_{p})]/[r^{p}(K_{p}-E_{p})] (0, 1) (0, (p^2-1)/(2p)) 严格单调减小.

  (1)–(5)可参见文献[32,引理3.4(1), (4)–(7)].下证明(6),利用Gauss超几何函数的级数展开可得

\begin{eqnarray*} \frac{K_{p}-E_{p}}{r^p}& = &\frac{\pi_{p}}{2}\cdot\frac{\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n}}{n!}\frac{r^{np}}{n!}-\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n}(-1/p)_{n}}{n!}\frac{r^{np}}{n!}}{r^{p}}\\ & = &\frac{\pi_{p}}{2}\cdot\frac{\sum\limits_{n = 1}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n-1}}{n!}(n-1/p+1/p)\frac{r^{np}}{n!}}{r^{p}}\\ & = &\frac{\pi_{p}}{2}\cdot\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!}, \\ \frac{E_{p}-{r'}^pK_{p}}{r^p}& = &K_{p}-\frac{K_{p}-E_{p}}{r^p}\\ & = &\frac{\pi_{p}}{2}\left(\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n}}{n!}\frac{r^{np}}{n!}-\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!}\right)\\ & = &\frac{\pi_{p}}{2}\left(1-\frac{1}{p}\right)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!}, \end{eqnarray*}

及其

\begin{eqnarray} &&\frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{2p}} = \frac{(p-1)(K_{p}-E_{p})/r^p-(E_{p}-{r'}^pK_{p})/r^p}{r^{p}}{}\\ & = &\frac{\pi_{p}}{2}\bigg[(p-1)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!} -\big(1-\frac{1}{p}\big)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!}\bigg]\bigg/r^p{}\\ & = &\frac{\pi_{p}}{2}\bigg[(p-1)\sum\limits_{n = 1}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!} -\big(1-\frac{1}{p}\big)\sum\limits_{n = 1}^{\infty}\frac{(1/p)_{n}(1-1/p)_{n}}{n!}\frac{r^{np}}{(n+1)!}\bigg]\bigg/r^p{}\\ & = &\frac{\pi_{p}}{2}\bigg[(p-1)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+2}(1-1/p)_{n+1}}{(n+1)!}\frac{r^{np}}{(n+2)!} {}\\ && -\big(1-\frac{1}{p}\big)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n+1}}{(n+1)!}\frac{r^{np}}{(n+2)!}\bigg]{}\\ & = &\frac{\pi_{p}}{2}\cdot\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n+1}}{(n+1)!}\frac{r^{np}}{(n+2)!} \big[(p-1)(n+1+1/p)-(1-1/p)\big]{}\\ & = &\frac{\pi_{p}}{2}(p-1)\sum\limits_{n = 0}^{\infty}\frac{(1/p)_{n+1}(1-1/p)_{n+1}}{(n+2)!}\frac{r^{np}}{n!}{}\\ & = &\frac{\pi_{p}(1-1/p)^2}{4}F(1+1/p, 2-1/p;3;r^p). \end{eqnarray}
(2.1)

因此,由等式(2.1)知(6)中定义的函数具有正的Maclaurin级数且值域为

(\pi_{p}(1-p)^2/(4p^2), \infty).

(7)令 \varphi(r) = E_{p}-{r'}^pK_{p}-(p-1){r'}^p(K_{p}-E_{p}) , \phi(r) = r^p(K_{p}-E_{p}) .

\varphi(0) = \phi(0) = 0,

\begin{eqnarray*} \varphi'(r)& = &(p-1)r^{p-1}K_{p}-(p-1)(-pr^{p-1})(K_{p}-E_{p})-(p-1)r^{p-1}E_{p}\\ & = &(p^2-1)r^{p-1}(K_{p}-E_{p}), \end{eqnarray*}

\phi'(r) = pr^{p-1}(K_{p}-E_{p})+(r^{2p-1}E_{p})/{r'}^p,

及其

\begin{equation} \frac{\varphi'(r)}{\phi'(r)} = \frac{p^2-1}{p+{r}^pE_{p}/[{r'}^p(K_{p}-E_{p})]}. \end{equation}
(2.2)

式(2.2)和本引理(3)表明 \varphi'(r)/\phi'(r) (0, 1) 上严格单调减小

\lim\limits_{r\rightarrow 0^+}[\varphi(r)/\phi(r)] = \lim\limits_{r\rightarrow 0^+}[\varphi'(r)/\phi'(r)] = (p^2-1)/(2p).

应用引理2.1,可推断 \varphi(r)/\phi(r) (0, 1) 上也严格单调减小.此外,显然有 \lim\limits_{r\rightarrow 1^-}[\varphi(r)/\phi(r)] = 0 .引理2.2证毕.

引理2.3  令 p\in(1, \infty) , c\in {{\Bbb R}}

\begin{eqnarray*} f_{c}(r) = \frac{r'^{c}(K_{p}-E_{p})}{r^p}. \end{eqnarray*}

则以下论断成立

(1) 当且仅当 c\geq (p^2-1)/(2p) ,函数 f_{c}(r) (0, 1) (0, \pi_{p}/(2p)) 严格单调减小;

(2) 当且仅当 c\leq 0 ,函数 f_{c}(r) (0, 1) (\pi_{p}/(2p), \infty) 严格单调增加;

(3) 0<c<(p^2-1)/(2p) ,则存在 r_{0} = r_{0}(c)\in(0, 1) 使得 f_{c}(r) (0, r_{0}) 上严格单调增加,在 (r_{0}, 1) 严格单调减小.

  显然当 c\leq0 时, f_{c}(1^-) = \infty ,而当 c>0 时, f_{c}(1^-) = 0 .利用引理2.2(2), f_{c}(0^+) = \pi_{p}/(2p) . f_{c} 求导得

\begin{eqnarray} f_{c}'(r)& = &-c{r'}^{c-p}r^{p-1}\left(\frac{K_{p}-E_{p}}{r^p}\right)+{r'}^c\left[\frac{r^pE_{p}-p{r'}^p(K_{p}-E_{p})}{r^{p+1}{r'}^p}\right]\\ & = &\frac{{r'}^{c-p}(K_{p}-E_{p})}{r}\left[-c+\frac{E_{p}-{r'}^pK_{p}-(p-1){r'}^p(K_{p}-E_{p})}{r^p(K_{p}-E_{p})}\right]. \end{eqnarray}
(2.3)

由引理2.2(7)和(2.3)式立即可得第(1)、(2)部分关于 f_{c} 的单调性论断.此外,若 0<c<(p^2-1)/(2p) ,又可由引理2.2(7)和(2.3)式知存在依赖于参数 c r_{0} = r_{0}(c)\in(0, 1) 使得当 r\in(0, r_{0}) 时, f_{c}'(r)>0 ,当 r\in(r_{0}, 1) 时, f_{c}'(r)<0 ,故 f_{c}(r) (0, r_{0}) 上严格单调增加,在 (r_{0}, 1) 上严格单调减小.

引理2.4  令 p\in(1, \infty) ,有

f(r) = E_{p}(r)+\frac{{r'}^p(K_{p}-E_{p})^2}{r^pE_{p}(r)},

f(r) (0, 1) (1, {\pi_{p}}/{2}) 严格单调减小.

  对 f 求导得

\begin{eqnarray} f'(r)& = &\Big\{[-pr^{p-1}(K_{p}-E_{p})^2+2r^{p-1}E_{p}(K_{p}-E_{p})]r^pE_{p}{}\\ &&-{r'}^p(K_{p}-E_{p})^2 \left[pr^{p-1}E_{p}+r^{p-1}(E_{p}-K_{p})\right]\Big\}\Big/ \big\{r^{2p}{E_{p}}^2\big\} +\frac{E_{p}-K_{p}}{r}{}\\ & = &\frac{E_{p}-K_{p}}{r}+\frac{K_{p}-E_{p}}{r^{p+1}{E_{p}}^2}\left[\left(-p(K_{p}-E_{p})+2E_{p}\right)r^{p}E_{p}-{r'}^p(K_{p}-E_{p})(pE_{p}+E_{p}-K_{p})\right]{}\\ & = &\frac{E_{p}-K_{p}}{r}+\frac{K_{p}-E_{p}}{r^{p+1}{E_{p}}^2}\left[-pE_{p}(K_{p}-E_{p})+2r^{p}{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2\right]{}\\ & = &-\frac{K_{p}-E_{p}}{r}\left[1-\frac{-pE_{p}(K_{p}-E_{p})+2r^{p}{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2}{r^p{E_{p}}^2}\right]{}\\ & = &-\frac{K_{p}-E_{p}}{r}\left[-1-\frac{-pE_{p}(K_{p}-E_{p})+{r'}^p(K_{p}-E_{p})K_{p}-{r'}^p(K_{p}-E_{p})E_{p}}{r^p{E_{p}}^2}\right]{}\\ & = &-\frac{K_{p}-E_{p}}{r}\left[-\frac{{r'}^p(K_{p}-E_{p})K_{p}}{r^p{E_{p}}^2}+\frac{{r'}^p(K_{p}-E_{p})}{r^p{E_{p}}}-1-\frac{K_{p}-E_{p}}{r^p{E_{p}}}+\frac{(p+1)(K_{p}-E_{p})}{r^p{E_{p}}}\right]{}\\ & = &-\frac{K_{p}-E_{p}}{r}\left[-\frac{{r'}^p(K_{p}-E_{p})K_{p}}{r^p{E_{p}}^2}-\frac{K_{p}}{E_{p}}+\frac{(p+1)(K_{p}-E_{p})}{r^p{E_{p}}}\right]{}\\ & = &-\frac{(K_{p}-E_{p})K_{p}}{rE_{p}}\left[-\frac{{r'}^p(K_{p}-E_{p})}{r^pE_{p}}-1+(p+1)\frac{K_{p}-E_{p}}{r^pK_{p}}\right]. \end{eqnarray}
(2.4)

引理2.2(2)和(3)表明函数 r\mapsto -{r'}^p(K_{p}-E_{p})/(r^pE_{p})-1+(p+1)(K_{p}-E_{p})/(r^pK_{p}) (0, 1) (0, p) 严格单调增加.结合式(2.4)可推断对所有 r\in(0, 1) ,有 f'(r)<0 ,故 f(r) (0, 1) 上严格单调减小.此外,不难计算得 f(0^+) = E_{p}(0) = \pi_{p}/2 , f(1^-) = E_{p}(1) = 1 .

引理2.5  令 p\in(1, +\infty) ,有

g(r) = \frac{(K_{p}-E_{p})(E_{p}-{r'}^p K_{p})}{r^{2p}}+E_{p}\frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{2p}}.

g(r) (0, 1) ((p^2-1){\pi_{p}}^2/(8p^2), \infty) 严格单调增加.

  根据引理2.2(1), (2)及(6)得 \lim\limits_{r\rightarrow 1^-}g(r) = \infty

\begin{eqnarray*} \lim\limits_{r\rightarrow 0^+}g(r)& = &\lim\limits_{r\rightarrow 0^+}\left(\frac{K_{p}-E_{p}}{r^p}\right)\lim\limits_{r\rightarrow 0^+}\left(\frac{E_{p}-{r'}^pK_{p}}{r^p}\right)\nonumber\\ &&+\lim\limits_{r\rightarrow 0^+}E_{p}\lim\limits_{r\rightarrow 0^+}\left[\frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{2p}}\right]\nonumber\\ & = &\frac{(p-1){\pi_{p}}^2}{4p^2}+\frac{(1-p)^2{\pi_{p}}^2}{8p^2} = \frac{{\pi_{p}}^2(p^2-1)}{8p^2}. \end{eqnarray*}

注意到

\frac{{\rm d}[(E_{p}-{r'}^pK_{p})/r^p]}{{\rm d}r} = \frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{p+1}}.

g 求导并结合引理2.2(2), (6)可推断

\begin{eqnarray*} g'(r)& = &\frac{{\rm d}[(K_{p}-E_{p})/r^p]}{{\rm d}r}\cdot\left(\frac{E_{p}-{r'}^pK_{p}}{r^p}\right)+\left(\frac{K_{p}-E_{p}}{r^p}\right)\cdot\frac{{\rm d}[(E_{p}-{r'}^pK_{p})/r^p]}{{\rm d}r}\nonumber\\ &&+\frac{{\rm d}E_{p}}{{\rm d}r}\left[\frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{2p}}\right]\nonumber\\ &&+E_{p}\frac{{\rm d}\left\{\left[(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})\right]/r^{2p}\right\}}{{\rm d}r}\nonumber\\ & = &\frac{{\rm d}[(K_{p}-E_{p})/r^p]}{{\rm d}r}\cdot\left(\frac{E_{p}-{r'}^pK_{p}}{r^p}\right)+E_{p}\frac{{\rm d}\left\{\left[(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})\right]/r^{2p}\right\}}{{\rm d}r}\\ &>&0. \end{eqnarray*}

引理2.5成立.

引理2.6  令 p\in(1, +\infty) ,有

\begin{equation} h(r) = \frac{r^p{{E}_{p}}^2\left\{({K}_{p}-{E}_{p}) ({E}_{p}-{r'}^p{K}_{p})+{{E}_{p}}\left[(p-1)({K}_{p}-{E}_{p}) -({E}_{p}-{r'}^p{K}_{p})\right]\right\}}{{r'}^p({K}_{p}-{E}_{p})^2\left[{r'}^p ({K}_{p}-{E}_{p})^2+r^p{{E}_{p}}^2\right]}, \end{equation}
(2.5)

h(r) (0, 1) ((p^2-1)/2, \infty) 严格单调增加.

  将 h(r) 改写成

\begin{eqnarray*} h(r) = \frac{\frac{(K_{p}-E_{p})(E_{p}-{r'}^p K_{p})}{r^{2p}}+E_{p}\frac{(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})}{r^{2p}}} {\left[E_{p}(r)+\frac{{r'}^p(K_{p}-E_{p})^2}{r^pE_{p}(r)}\right]\left[\frac{{r'}^{(p^2-1)/(2p)}(K_{p}-E_{p})}{r^p}\right]^2\left(\frac{{r'}^{1/p}}{E_{p}}\right)}. \end{eqnarray*}

根据引理2.2(5),引理2.3(1),引理2.4和引理2.5便得引理2.6.

引理2.7  令 p\in(1, \infty) , \beta\in{{\Bbb R}} ,有

\begin{equation} G_{\beta}(r) = (1-\beta)\frac{K_{p}-E_{p}}{E_{p}}+\frac{r^pE_{p}}{{r'}^p(K_{p}-E_{p})} \end{equation}
(2.6)

L(\beta) = \inf\limits_{r\in(0, 1)}G_{\beta}(r) .

(1) 当且仅当 \beta\leq (p^2+1)/2 , G_{\beta}(r) (0, 1) (p, \infty) 严格单调增加, L(\beta) = p ;

(2) \beta>(p^2+1)/2 ,则存在 \lambda = \lambda(\beta)\in(0, 1) 使得 G_{\beta}(r) (0, \lambda) 上严格单调减小,在 (\lambda, 1) 上严格单调增加. G_{\beta}(r) (0, 1) 上的值域为 [L(\beta), \infty) L(\beta)<p .

  经计算得

\begin{equation} \lim\limits_{r\rightarrow 0^+}G_{\beta}(r) = p, \end{equation}
(2.7)

\begin{equation} \lim\limits_{r\rightarrow 1^-}G_{\beta}(r) = \lim\limits_{r\rightarrow 1^-}\left(\frac{K_{p}-E_{p}}{E_{p}}\right)\left[1-\beta+\frac{r^p{E_{p}}^2}{{r'}^p(K_{p}-E_{p})^2}\right] = \infty \end{equation}
(2.8)

及其

\begin{eqnarray} G_{\beta}'(r)& = &r^{p-1}\frac{{r'}^p(K_{p}-E_{p})(pE_{p}+E_{p}-K_{p}) -r^pE_{p}(E_{p}+pE_{p}-pK_{p})}{{r'}^{2p}(K_{p}-E_{p})^2}\\ &&+(1-\beta)\frac{r^p{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2}{r{r'}^p{E_{p}}^2}\\ & = &r^{p-1}\frac{pE_{p}(K_{p}-E_{p})-{r'}^p(K_{p}-E_{p})K_{p}+[{r'}^p(K_{p}-E_{p})E_{p}-r^p{E_{p}}^2]}{{r'}^{2p}(K_{p}-E_{p})^2}\\ &&+(1-\beta)\frac{r^p{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2}{r{r'}^p{E_{p}}^2}\\ & = &r^{p-1}\frac{E_{p}[(p-1)(K_{p}-E_{p})-(E_{p}-{r'}^pK_{p})]+(K_{p}-E_{p})(E_{p}-{r'}^pK_{p})}{{r'}^{2p}(K_{p}-E_{p})^2}\\ &&+(1-\beta)\frac{r^p{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2}{r{r'}^p{E_{p}}^2}\\ & = &\frac{r^p{E_{p}}^2+{r'}^p(K_{p}-E_{p})^2}{r{r'}^p{E_{p}}^2}\left[1-\beta+h(r)\right], \end{eqnarray}
(2.9)

其中 h(r) (0, 1) 上的严格单调增加函数,其定义见(2.5)式.

由引理2.6和(2.9)式知:当且仅当 \beta\leq \inf\limits_{r\in(0, 1)}[1+h(r)] = (p^2+1)/2 , {G_{\beta}}'(r)>0 对所有的 r\in(0, 1) 恒成立,即函数 G_{\beta}(r) (0, 1) 上严格单调增加,进而由(2.7)和(2.8)式可得引理第(1)部分.若 \beta>(p^2+1)/2 ,则引理2.6及(2.9)式表明存在 \lambda = \lambda(\beta)\in(0, 1) 使得当 r\in(0, \lambda) 时, G_{\beta}'(r)<0 ,当 r\in(\lambda, 1) , G_{\beta}'(r)>0 ,故 G_{\beta}(r) (0, 1) 分段单调的.结合(2.7)和(2.8)式即得 L(\beta)<p , G_{\beta}(r) (0, 1) 上的值域为 (L(\beta), \infty) .

引理2.8  令 p\in(1, +\infty) , \alpha, \beta\in{{\Bbb R}} ,有

\begin{eqnarray*} J(r) = {E_{p}}^{{\beta}-1}\frac{K_{p}(r)-E_{p}(r)}{r^{\alpha}}, \end{eqnarray*}

则以下论断成立

(1) \beta\leq (p^2+1)/2 ,则 J(r) (0, 1) 上严格单调增加当且仅当 \alpha\leq p ,若 \alpha>p ,则存在 r_{1} = r_{1}(\alpha, \beta)\in(0, 1) 使得 J(r) (0, r_{1}) 上严格单调减小,在 (r_{1}, 1) 上严格单调增加;

(2) \beta>(p^2+1)/2 ,则 J(r) (0, 1) 上严格单调增加当且仅当 \alpha\leq L(\beta) ,若 \alpha>L(\beta) ,则 J(r) 是分段单调的,其中 L(\beta) 见引理2.7.

  对数求导得

\begin{equation} \frac{J'(r)}{J(r)} = (1-\beta)\frac{K_{p}-E_{p}}{rE_{p}}+\frac{r^{p-1}E_{p}}{{r'}^p(K_{p}-E_{p})}-\frac{\alpha}{r} = \frac{1}{r}\left[G_{\beta}(r)-\alpha\right], \end{equation}
(2.10)

其中 G_{\beta}(r) 定义见(2.6)式.

下面分四种情形完成引理的证明.

情形Ⅰ   \beta\leq (p^2+1)/2 \alpha\leq p .则由引理2.7(1)和(2.10)式知:对所有 r\in(0, 1) , J'(r)\geq 0 ,因此 J(r) (0, 1) 上严格单调增加.

情形Ⅱ   \beta\leq (p^2+1)/2 \alpha> p .则引理2.7(1)及式(2.10)表明存在 r_{1} = r_{1}(\alpha, \beta)\in(0, 1) ,使得当 r\in(0, r_{1}) 时, J'(r)<0 ,当 r\in(r_{1}, 1) 时, J'(r)>0 .因此, J(r) (0, r_{1}) 上严格单调减小,在 (r_{1}, 1) 上严格单调增加.

情形Ⅲ   \beta>(p^2+1)/2 \alpha\leq L(\beta) .则由引理2.7(1)和(2.10)式知:对所有 r\in(0, 1) , J'(r)\geq 0 ,因此 J(r) (0, 1) 上严格单调增加.

情形Ⅳ   \beta>(p^2+1)/2 \alpha> L(\beta) .则由引理2.7(2), (2.10)式及其相似于情形II的讨论可得 J(r) (0, 1) 上分段单调.

3 定理1.2及其推论1.1的证明

定理1.2的证明  不妨设 0<x\leq y<1 . t = H_{\alpha}(x, y) ,则 \partial t/\partial x = (x/t)^{\alpha-1}/2 ,且当 y>x 时, t>x .下面分 \beta\neq0 \beta = 0 两种情形来完成定理1.2的证明.

情形1   \beta\neq 0 .

\begin{equation} F(x, y) = E_{p}(H_{\alpha}(x, y))^{\beta}-\frac{{E_{p}(x)}^{\beta}+{E_{q}(y)}^{\beta}}{2}. \end{equation}
(3.1)

则对 x 求偏导数得

\begin{eqnarray} \frac{\partial F}{\partial x}& = &\frac{\beta}{2} {E_{p}(t)}^{\beta-1} \frac{E_{p}(t)-K_{p}(t)}{t}\left(\frac{x}{t}\right)^{\alpha-1}-\frac{\beta}{2}{E_{p}(x)}^{\beta-1}\frac{E_{p}(x)-K_{p}(x)}{x}\\ & = &\frac{\beta}{2}x^{\alpha-1}\left[{E_{p}(x)}^{\beta-1}\frac{K_{p}(x)-E_{p}(x)}{x^{\alpha}}-{E_{p}(t)}^{\beta-1}\frac{K_{p}(t)-E_{p}(t)}{t^{\alpha}}\right]. \end{eqnarray}
(3.2)

下面再将情形1分成下面四种子情形.

子情形1.1   \beta\leq (p^2+1)/2 \alpha\leq p .则由引理2.8(1)和(3.2)式知:若 \beta>0 , \partial F/\partial x<0 , F(x, y)>F(y, y) = 0 ;若 \beta<0 , \partial F/\partial x>0 , F(x, y)>F(y, y) = 0 .结合(3.1)式可推得:当 \alpha\leq p \beta\leq (p^2+1)/2 时,始终成立不等式

\begin{equation} E_{p}\left(H_{\alpha}(x, y)\right)\geq H_{\beta}\left({E_{p}}(x), {E_{p}}(y)\right), \end{equation}
(3.3)

而且不难验证当且仅当 x = y 时不等式(3.3)成立等号.

综上可知,当 (\alpha, \beta)\in\{(\alpha, \beta)|\alpha\leq p, \beta\leq (p^2+1)/2, \beta\neq0\} 时, E_{p} (0, 1) 上严格 H_{\alpha, \beta} -凹的.

子情形1.2   \beta\leq (p^2+1)/2 \alpha>p .则由(3.1)和(3.2)式,引理2.8(1)及其类似子情形1.1的讨论可知:当 \alpha>p \beta\leq (p^2+1)/2 时,存在 x_{0}, y_{0}\in(0, r_{1}) x_{1}, y_{1}\in(r_{1}, 1) 使得 E_{p}\left(H_{\alpha}(x_{0}, y_{0})\right)\leq H_{\beta}\left({E_{p}}(x_{0}), {E_{p}}(y_{0})\right) , E_{p}\left(H_{\alpha}(x_{1}, y_{1})\right)\geq H_{\beta}\left({E_{p}}(x_{1}), {E_{p}}(y_{1})\right) .因此, E_{p} 在整个 (0, 1) 区间上既不是 H_{\alpha, \beta} -凹的也不是 H_{\alpha, \beta} -凸的.

子情形1.3   \beta>(p^2+1)/2 \alpha\leq L(\beta) .则(3.1)和(3.2)式,引理2.8(2)及其类似子情形1.1的讨论表明:当 (\alpha, \beta)\in\{(\alpha, \beta)|\alpha\leq L(\beta), \beta>(p^2+1)/2, \beta\neq0\} 时, E_{p} (0, 1) 上严格 H_{\alpha, \beta} -凹的.

子情形1.4   \beta>(p^2+1)/2 \alpha>L(\beta) .则从(3.1)和(3.2)式,引理2.8(2)及其上述子情形1.2的类似讨论可知: E_{p} 在整个 (0, 1) 区间上既不是 H_{\alpha, \beta} -凹的也不是 H_{\alpha, \beta} -凸的.

情形2   \beta = 0 .

\begin{equation} G(x, y) = \frac{\left[E_{p}(H_{\alpha}(x, y))\right]^2}{E_{p}(x)E_{p}(y)}, \end{equation}
(3.4)

则对 x 求偏导数得

\begin{equation} \frac{1}{G(x, y)}\frac{\partial G}{\partial x} = x^{\alpha-1}\left[\frac{K_{p}(x)-E_{p}(x)}{x^\alpha E_{p}(x)}-\frac{K_{p}(t)-E_{p}(t)}{t^\alpha E_{p}(t)}\right]. \end{equation}
(3.5)

下面将情形2的证明分如下两种情形.

子情形2.1   \alpha\leq p .则(3.5)式和引理2.8(1)表明 \partial G/\partial x<0 ,故 G(x, y)>G(y, y) = 1 .于是从(3.4)式便知

\begin{eqnarray*} E_{p}\left(H_{\alpha}(x, y)\right)\geq H_{\beta}\left(E_{p}(x), E_{p}(y)\right), \end{eqnarray*}

且上述不等式成立等式当且仅当 x = y .

因此,当 (\alpha, \beta)\in\{(\alpha, \beta)|\alpha\leq p, \beta = 0\} 时, E_{p} (0, 1) 上严格 H_{\alpha, \beta} -凹的.

子情形2.2   \alpha>p .则利用(3.4)和(3.5)式及其引理2.8(1)推断得: E_{p} (0, 1) 上既不是 H_{\alpha, \beta} -凹的也不是 H_{\alpha, \beta} -凸的.

推论1.1的证明  假设 \alpha, \beta\in{{\Bbb R}} , x, y\in(0, 1) . a\in(0, 1) ,令 p = 1/(1-a) ,则从(1.8)式可推断不等式

{\cal E}_{a}(H_{\alpha}(x, y))\geq (\leq) H_{\beta}({\cal E}_{a}(x), {\cal E}_{a}(y))

等价于

{\cal E}_{a}\left(H_{\alpha}(x^{\frac{p}{2}}, y^{\frac{p}{2}})\right) = {\cal E}_{a}\left(\left[H_{\frac{p\alpha}{2}}(x, y)\right]^{\frac{p}{2}}\right)\geq (\leq) H_{\beta}({{\cal E}}_{a}(x^{\frac{p}{2}}), {{\cal E}}_{a}(y^{\frac{p}{2}})),

再对不等式两边同时乘以 \pi_{p}/\pi ,便得

E_{p}\left(\left[H_{\frac{p\alpha}{2}}(x, y)\right]\right)\geq (\leq) H_{\beta}(E_{p}(x), E_{p}(y)).

因此,由定理1.2便得推论1.1.

注3.1  当 p\in(1, \infty) 时, p<(p^2+1)/2 ,故由定理1.2可知:第二类完全 p -椭圆积分 E_{p}(r) (0, 1) H_{\alpha, \alpha} -凹的,即不等式

E_{p}(H_{\alpha}(x, y))\geq H_{\alpha}(E_{p}(x), E_{p}(y))

对所有 x, y\in (0, 1) 恒成立当且仅当 \alpha\leq p ,且不存在 \alpha\in {{\Bbb R}} 使得 {\cal E}_{a}(r) (0, 1) 上是 H_{\alpha, \alpha} -凸的.

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