2. 湖北工程学院 数学与统计学院, 湖北 孝感 432000
2. School of Mathematics & Statistics, Hubei Engineering University, Xiaogan 432000, Hubei Province, China
设ai∈R+,ti∈[0, 1](i=1, 2, …, n),且
$ \begin{array}{l} {\rm{M}}_n^{\left[ r \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = \\ \left\{ \begin{array}{l} {\left[ {\sum\limits_{i = 1}^n {{t_i}a_i^r} } \right]^{\frac{1}{r}}},\;\;\;r \ne 0,\\ \prod\limits_{i = 1}^n {a_i^{{t_i}}} ,\;\;\;\;r = 0,\\ \max \left\{ {{a_1},{a_2}, \cdots ,{a_n}} \right\},\;\;\;\;r = + \infty ,\\ \min \left\{ {{a_1},{a_2}, \cdots ,{a_n}} \right\},\;\;\;\;r = - \infty , \end{array} \right. \end{array} $ | (1) |
则称Mn[r](t1, t2, …, tn; a1, a2, …, an)为正数a1, a2, …, an的加权r次幂平均,简称为n元加权幂平均.特别地,称
$ {\rm{M}}_n^{\left[ 1 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{A}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ 0 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{G}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ { - 1} \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{H}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ 2 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{SR}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ { - 2} \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{HS}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ 3 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{CR}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right), $ |
$ {\rm{M}}_n^{\left[ { - 3} \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{a_1},{a_2}, \cdots ,{a_n}} \right) = {\rm{HC}}\left( {{a_1},{a_2}, \cdots ,{a_n}} \right) $ |
分别为n元加权算术平均、n元加权几何平均、n元加权调和平均、n元加权平方根平均、n元加权调和平方根平均、n元加权立方根平均、n元加权调和立方根平均.
针对n元加权幂平均,文献[1]进行了深入研究,并给出了许多重要结果,此处不再赘述.
对n元加权幂平均Mn[r](t1, t2, …, tn; a1, a2…, an),由式(1) 易得如下加权幂平均恒等关系式:
(ⅰ)[Mn[r](t1, t2, …, tn; a1, a2, …, an)]r=Mn[1](t1, t2, …, tn; a1r, a2r, …, anr);
(ⅱ)[Mn[1](t1, t2, …, tn; a1, a2, …, an)]
(ⅲ)[Mn[r](t1, t2, …, tn; a1
(ⅳ)expMn[1](t1, t2, …, tn; a1, a2, …, an)=Mn[0](t1, t2, …, tn; exp a1, exp a2, …, exp an);
若ai∈(1, +∞),则
(ⅴ)lnMn[0](t1, t2, …, tn; a1, a2, …, an)=Mn[1](t1, t2, …, tn; ln a1, ln a2, …, ln an).
特别地,∀x1, x2∈R+及∀t1, t2, α∈[0, 1],有
(ⅵ)M2[1](M2[1](α, 1-α; t1, t2), 1-M2[1](α, 1-α; t1, t2); x1, x2)=M2[1](α, 1-α; M2[1](t1, 1-t1; x1, x2), M2[1](t2, 1-t2; x1, x2)).
凸函数[2]、几何凸函数[3]、调和凸函数[4]、平方凸函数[5]、调和平方凸函数[6]、r-平均凸函数[7]等都利用幂平均给出的定义,这些凸函数在许多领域的应用及其重要作用已为人熟知,但缺乏规范、简洁的统一定义,考虑到数学的缜密性和严谨性,各方法尚存在不可忽视的缺失,仅就r-平均凸函数而言,张孔生等[8]在对幂指数做了相应限制的前提下,给出了“P方凸函数”的定义,吴善和[9]充分考虑了幂指数取值的任意性,在定义中不得不用2个公式来确定其定义的“rP-凸函数”,席博彦等[7]通过引入加权平均的概念定义了“r-平均凸函数”,虽然弥补了前2种定义的缺陷,但仍未完全解决凸函数定义的统一和简洁性问题,特别是后续推广应用问题.其他类型的凸函数定义中的一些问题,不再列举.
对区间上的二元幂平均确定的凸函数,本文给出了规范统一的定义,并进行了全面深入研究.
定义1 设I⊆R+, f:I→R+, 若∀x1, x2∈I及∀t∈[0, 1],存在r, p∈R(r,p≠±∞),使得
$ \begin{array}{l} f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) \le \\ \;\;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {{x_1}} \right),f\left( {{x_2}} \right)} \right), \end{array} $ | (2) |
则称f(x)为I上的MrMp-凸函数.如果不等式(2) 中的不等号反向,则称f(x)为I上的MrMp-凹函数.当r=p≠0时,则称f(x)为I上的r次幂平均凸(凹)函数.
显然,当r=p=1, 0, -1, 2, -2或r=p∈R时,MrMp-凸函数为凸函数、几何凸函数、调和凸函数、平方凸函数、调和平方凸函数和r次幂平均凸函数;当r=0且p=1, -1或p∈R(p≠0) 时,MrMp-凸函数为GA-凸函数[10]、GH-凸函数[11]和GMp-凸函数[12];当r=1且p=0, -1, 2或p∈R时,MrMp-凸函数为AG-凸函数(对数凸函数)[13]、AH-凸函数[14]、AR-凸函数[15]和AMp-凸函数[16];当r=-1且p=0, 1或p∈R时,MrMp-凸函数为HG-凸函数[17]、HA-凸函数[18]和HMp-凸函数[19];当r∈R且p=1, 0, -1时,MrMp-凸函数为MrA-凸函数(P-凸函数)[20]、MrG-凸函数[21]、MrH-凸函数[22].
为简洁和统一,将凸函数的记号MrMp及AMp、GMp、HMp和MrA、MrG、MrH分别记为MM及AM、GM、HM和MA、MG、MH.由于所有“M”均表示n元加权幂平均,本文规定:凸函数记号“MM”中,第1个M为n元加权r次幂平均,第2个M为n元加权p次幂平均.
1 MM-凸函数的判定考虑到区间I⊆R+上的幂函数τ(x)=xr(r≠0)、对数函数ω(x)=ln x和幂指复合函数ρ(x)=exp xr(r≠0) 是单调的, 记τ(I)=Ir,ω(I)=ln I,ρ(I)=exp Ir.∀r, p∈R,由于r=0,p=0时,MM-凸函数分别为几何凸函数、GA-凸函数、GH-凸函数、GM-凸函数、AG-凸函数、HG-凸函数、MG-凸函数,相关讨论参见文献[3, 10-13, 17, 21].本文约定:除r=p=0时MM-凸函数为几何凸函数外,其他都只讨论r≠0,p≠0的情形.
定理1 设I⊆R+,f:I→R+,则
(ⅰ)当p>0时,f(x)为I上的MM-凸(凹)函数的充要条件是(f(x
(ⅱ)当p<0时,f(x)为I上的MM-凸(凹)函数的充要条件是(f(x
证明 这里仅证(ⅰ),用类似方法可证明(ⅱ).
令g(x)=(f(x
充分性:设∀x1, x2∈I, 则x1r, x2r∈Ir, 所以,∀t∈[0, 1],由2个正数幂平均的性质[1]知,M2[r](t, 1-t; x1, x2)∈[min{x1, x2}, max{x1, x2}]⊆I,M2[1](t, 1-t; x1r, x2r)∈[min{x1r, x2r}, max{x1r, x2r}]⊆Ir,且[M2[1](t, 1-t; x1r, x2r)]
$ \begin{array}{l} f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) = f\left( {\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;x_1^r,} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;\left. {{{\left. {\left. {x_2^r} \right)} \right)}^{\frac{1}{r}}}} \right) = {\left[ {g\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;x_1^r,x_2^r} \right)} \right)} \right]^{\frac{1}{p}}} \le \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;g\left( {x_1^r} \right),g\left( {x_2^r} \right)} \right)} \right]^{\frac{1}{p}}} = \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{{\left( {f\left( {{x_1}} \right)} \right)}^p},{{\left( {f\left( {{x_2}} \right)} \right)}^p}} \right)} \right]^{\frac{1}{p}}} = \\ \;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {{x_1}} \right),f\left( {{x_2}} \right)} \right), \end{array} $ |
故f(x)为I上的MM-凸函数.
必要性:设∀x1, x2∈Ir,则x1
$ \begin{array}{l} {\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{x_1},{x_2}} \right) \in \left[ {\min \left\{ {{x_1},{x_2}} \right\},\max \left\{ {{x_1},{x_2}} \right\}} \right] \subseteq \\ {{\bf{I}}^r},{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;x_1^{\frac{1}{r}},x_2^{\frac{1}{r}}} \right) \in \left[ {\min \left\{ {x_1^{\frac{1}{r}},x_2^{\frac{1}{r}}} \right\},\max \left\{ {x_1^{\frac{1}{r}},} \right.} \right.\\ \left. {\left. {x_2^{\frac{1}{r}}} \right\}} \right] \subseteq {\bf{I}},{\rm{且}}{\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right]^{\frac{1}{r}}} \in {\bf{I}}. \end{array} $ |
若f(x)是I上的MM-凸函数,且p>0,则由加权幂平均恒等式和MM-凸函数的定义,有
$ \begin{array}{l} g\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) = \left[ {f\left( {\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{x_1},} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {{{\left. {\left. {{x_2}} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = {\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;x_1^{\frac{1}{r}},x_2^{\frac{1}{r}}} \right)} \right)} \right]^p} \le \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {x_1^{\frac{1}{r}}} \right),f\left( {x_2^{\frac{1}{r}}} \right)} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;{{\left( {g\left( {{x_1}} \right)} \right)}^{\frac{1}{p}}},{{\left( {g\left( {{x_2}} \right)} \right)}^{\frac{1}{p}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;g\left( {{x_1}} \right),g\left( {{x_2}} \right)} \right), \end{array} $ |
故g(x)=(f(x
若f(x)为I上的MM-凹函数,则以上证明中的不等号反向,因此定理1的后半部分成立.
定理2 设I⊆R+,f:I→R+,则
(ⅰ)当p>0时,f(x)为I上的MM-凸(凹)函数的充要条件是exp[f((ln x)
(ⅱ)当p<0时,f(x)为I上的MM-凸(凹)函数的充要条件是exp[f((ln x)
证明 仅证(ⅰ),用相同的方法可证明(ⅱ).
令g(x)=exp[f((ln x)
充分性:设∀x1, x2∈I, 则x1r, x2r∈Ir,exp x1r, exp x2r∈exp Ir,所以,∀t∈[0, 1],由2个正数幂平均的性质知,M2[r](t, 1-t; x1, x2)∈I,M2[1](t, 1-t; x1r, x2r)∈Ir,M2[0](t, 1-t; exp x1r, exp x2r)∈exp Ir,且exp M2[1](t, 1-t; x1r, x2r)∈exp Ir.若g(x)=exp(f((ln x)
$ \begin{array}{l} {\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right)} \right]^p} = \\ \;\;\;\;\;\;\;{\left[ {f\left( {{{\left( {\ln \left( {\exp {\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;x_1^r,x_2^r} \right)} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\ln g\left( {\exp {\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;x_1^r,x_2^r} \right)} \right) = \\ \;\;\;\;\;\;\;\ln g\left( {{\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;\exp x_1^r,\exp x_2^r} \right)} \right) \le \\ \;\;\;\;\;\;\;\ln {\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;g\left( {\exp x_1^r} \right),g\left( {\exp x_2^r} \right)} \right) = \\ \;\;\;\;\;\;\;\ln {\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;\exp{{\left( {f\left( {{x_1}} \right)} \right)}^p},\exp{{\left( {f\left( {{x_2}} \right)} \right)}^p}} \right) = \\ \;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;\ln \left( {\exp{{\left( {f\left( {{x_1}} \right)} \right)}^p}} \right),\ln \left( {\exp{{\left( {f\left( {{x_2}} \right)} \right)}^p}} \right)} \right) = \\ \;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{{\left( {f\left( {{x_1}} \right)} \right)}^p},{{\left( {f\left( {{x_2}} \right)} \right)}^p}} \right), \end{array} $ |
即f(M2[r](t, 1-t; x1, x2))≤[M2[1](t, 1-t; (f(x1))p, (f(x2))p)]
故f(x)为I上的MM-凸函数.
必要性:设∀x1, x2∈exp Ir,则ln x1, ln x2∈Ir,(ln x1)
$ \begin{array}{l} g\left( {{\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) = \\ \;\;\;\;\;\;\;\exp {\left[ {f\left( {{{\left( {\ln {\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\exp {\left[ {f\left( {{{\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;ln{x_1},ln{x_2}} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\exp {\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{{\left( {\ln {x_1}} \right)}^{\frac{1}{r}}},{{\left( {\ln {x_2}} \right)}^{\frac{1}{r}}}} \right)} \right)} \right]^p} \le \\ \;\;\;\;\;\;\;\exp {\left[ {{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {{{\left( {\ln {x_1}} \right)}^{\frac{1}{r}}}} \right),f\left. {\left( {{{\left( {\ln {x_2}} \right)}^{\frac{1}{r}}}} \right)} \right)} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\exp {\left[ {{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;{{\left( {\ln g\left( {{x_1}} \right)} \right)}^{\frac{1}{p}}},{{\left( {\ln g\left( {{x_2}} \right)} \right)}^{\frac{1}{p}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\exp \left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;\ln g\left( {{x_1}} \right),\ln g\left( {{x_2}} \right)} \right)} \right] = \\ \;\;\;\;\;\;\;\exp \left[ {\ln {\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;g\left( {{x_1}} \right),g\left( {{x_2}} \right)} \right)} \right] = \\ \;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 0 \right]}\left( {t,1 - t;g\left( {{x_1}} \right),g\left( {{x_2}} \right)} \right), \end{array} $ |
所以g(x)=exp[f((ln x)
由以上证明可知,定理2(ⅰ)的后半部分亦成立.
类似地可证明:
定理3 设I⊆R+,f:I→R+,则
(ⅰ)当p>0时,f(x)为I上的MM-凸(凹)函数的充要条件是(f(x))p为I上的MA-凸(凹)函数;
(ⅱ)当p<0时,f(x)为I上的MM-凸(凹)函数的充要条件是(f(x))p为I上的MA-凹(凸)函数.
定理4 设I⊆R+,f:I→R+,则f(x)为I上的MM-凸(凹)函数的充要条件是f(x
定理5 设I⊆R+,f:I→R+,则
(ⅰ)当p>0时,f(x)为I上的MM-凸(凹)函数的充要条件是exp(f(x))p为I上的MG-凸(凹)函数;
(ⅱ)当p<0时,f(x)为I上的MM-凸(凹)函数的充要条件是exp(f(x))p为I上的MG-凹(凸)函数.
定理6 设I⊆R+,f:I→R+,则f(x)为I上的MM-凸(凹)函数的充要条件是f((ln x)
定理7 设I⊆R+,f:I→R+,且f在区间I上连续,则
(ⅰ)当p>0时,函数f(x)是I上的MM-凸(凹)函数的充分必要条件是:∀x1, x2∈I,函数φ(t)=[f(M2[r](t, 1-t; x1, x2))]p为[0, 1]上的凸(凹)函数;
(ⅱ)当p<0时,函数f(x)是I上的MM-凸(凹)函数的充分必要条件是:∀x1, x2∈I,函数φ(t)=[f(M2[r](t, 1-t; x1, x2))]p为[0, 1]上的凹(凸)函数.
证明 只证(ⅰ),同理可证(ⅱ).
由φ(t)=[f(M2[r](t, 1-t; x1, x2))]p(t∈[0, 1]),知φ(0)=(f(x2))p,φ(1)=(f(x1))p.
充分性:若φ(t)为[0, 1]上的凸函数,且p>0,则∀x1, x2∈I及∀t∈[0, 1],有
$ \begin{array}{l} f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) = {\left[ {\varphi \left( t \right)} \right]^{\frac{1}{p}}} = \\ \;\;\;\;\;\;\;\;{\left[ {\varphi \left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;1,0} \right)} \right)} \right]^{\frac{1}{p}}} \le \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;\varphi \left( 1 \right),\varphi \left( 0 \right)} \right)} \right]^{\frac{1}{p}}} = \\ \;\;\;\;\;\;\;\;{\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;{{\left( {f\left( {{x_1}} \right)} \right)}^p},{{\left( {f\left( {{x_2}} \right)} \right)}^p}} \right)} \right]^{\frac{1}{p}}} = \\ \;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {{x_1}} \right),f\left( {{x_2}} \right)} \right), \end{array} $ |
故函数f(x)是区间I上的MM-凸函数.
必要性:∀x1, x2∈I及∀t1, t2∈[0, 1],由2正数幂平均的性质知,M2[r](t1, 1-t1; x1, x2), M2[r](t2, 1-t2; x1, x2)∈I, 所以,∀α∈[0, 1]亦有M2[r](α, 1-α; M2[r](t1, 1-t1; x1, x2), M2[r](t2, 1-t2; x1, x2))∈I,且M2[1](α, 1-α; t1, t2)∈[min{t1, t2}, max{t1, t2}]⊆[0, 1],因此,M2[r](M2[1](α, 1-α; t1, t2), 1-M2[1](α, 1-α; t1, t2); x1, x2)∈I.若f(x)是I上的MM-凸函数,且p>0,则∀t1, t2∈[0, 1]及∀α∈[0, 1],由加权幂平均的恒等关系式和MM-凸函数的定义,有
$ \begin{array}{l} \varphi \left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;{t_1},{t_2}} \right)} \right) = \left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;{t_1},} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {\left. {\left. {{t_2}} \right),1 - {\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;{t_1},{t_2}} \right);{x_1},{x_2}} \right)} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\;\left[ {f\left( {\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;{t_1},{t_2}} \right),1 - {\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,} \right.} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {{{\left. {\left. {\left. {1 - \alpha ;{t_1},{t_2}} \right);x_1^r,x_2^r} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = \left[ {f\left( {\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;1 - \alpha ;{\rm{M}}_2^{\left[ 1 \right]}\left( {{t_1},1 - {t_1};x_1^r,x_2^r} \right),{\rm{M}}_2^{\left[ 1 \right]}\left( {{t_2},1 - {t_2};} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {{{\left. {\left. {\left. {x_1^r,x_2^r} \right)} \right)} \right)}^{\frac{1}{r}}}} \right)} \right]^p} = \left[ {f\left( {\left[ {{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_1},} \right.} \right.} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {{{\left. {\left. {{{\left. {\left. {1 - {t_1};x_1^r,x_2^r} \right)} \right)}^r},{{\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_2},1 - {t_2};{x_1},{x_2}} \right)} \right)}^r}} \right)} \right]}^{\frac{1}{r}}}} \right)} \right]^p} = \\ \;\;\;\;\;\;\;\;\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {\alpha ,1 - \alpha ;{\rm{M}}_2^{\left[ r \right]}\left( {{t_1},1 - {t_1};{x_1},{x_2}} \right),} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {\left. {{\rm{M}}_2^{\left[ r \right]}\left( {{t_2},1 - {t_2};{x_1},{x_2}} \right)} \right)} \right)} \right]^p} \le \left[ {{\rm{M}}_2^{\left[ p \right]}\left( {\alpha ,1 - \alpha ;} \right.} \right.\\ \;\;\;\;\;\;\;\;f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_1},1 - {t_1};{x_1},{x_2}} \right)} \right),f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_2},1 - {t_2};} \right.} \right.\\ \;\;\;\;\;\;\;\;{\left. {\left. {\left. {\left. {{x_1},{x_2}} \right)} \right)} \right)} \right]^p} = {\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_1},1 - {t_1};} \right.} \right.} \right.} \right.\\ \;\;\;\;\;\;\;\;\left. {{{\left. {\left. {\left. {{x_1},{x_2}} \right)} \right)} \right]}^p},{{\left[ {f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {{t_2},1 - {t_2};{x_1},{x_2}} \right)} \right)} \right]}^p}} \right) = \\ \;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 1 \right]}\left( {\alpha ,1 - \alpha ;\varphi \left( {{t_1}} \right),\varphi \left( {{t_2}} \right)} \right), \end{array} $ |
故φ(t)=[f(M2[r](t, 1-t; x1, x2))]p是[0, 1]上的凸函数.
若f(x)为I上的MM-凹函数,则以上证明中不等号反向,故定理7(ⅰ)的后半部分成立.
定理8 设I⊆R+,f:I→R+,
(ⅰ)若p>0,则f(x)为I上的MM-凸(凹)函数的充要条件是:∀x1, x2, x3∈I且x1<x2<x3,当r>0时,有
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \ge \left( \le \right)0; \end{array} $ | (3) |
当r<0时,有
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \le \left( \ge \right)0. \end{array} $ | (4) |
(ⅱ)若p<0,则f(x)为I上的MM-凸(凹)函数的充要条件是:∀x1, x2, x3∈I且x1<x2<x3, 当r>0时,有
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \le \left( \ge \right)0; \end{array} $ | (5) |
当r<0时,有
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \ge \left( \le \right)0. \end{array} $ | (6) |
证明 只证(ⅰ),同理可证(ⅱ).
必要性:∀x1, x2, x3∈I且x1<x2<x3,令
若f(x)为I上的MM-凸函数,则
$ \begin{array}{l} f\left( {{x_2}} \right) = f\left( {M_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_3}} \right)} \right) \le M_2^{\left[ p \right]}\left( {t,1 - t;} \right.\\ \left. {f\left( {{x_1}} \right),f\left( {{x_3}} \right)} \right) = {\left[ {t{{\left( {f\left( {{x_1}} \right)} \right)}^p} + \left( {1 - t} \right){{\left( {f\left( {{x_3}} \right)} \right)}^p}} \right]^{\frac{1}{p}}} = \\ {\left[ {\frac{{x_3^r - x_2^r}}{{x_3^r - x_1^r}}{{\left( {f\left( {{x_1}} \right)} \right)}^p} + \frac{{x_2^r - x_1^r}}{{x_3^r - x_1^r}}{{\left( {f\left( {{x_3}} \right)} \right)}^p}} \right]^{\frac{1}{p}}}. \end{array} $ |
因为∀x∈I, f(x)>0,且p>0,注意到x1<x2<x3,则当r>0时,将上式整理即得
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \ge 0. \end{array} $ |
当r<0时,类似地,有
$ \begin{array}{l} \left( {x_3^r - x_2^r} \right){\left( {f\left( {{x_1}} \right)} \right)^p} + \left( {x_1^r - x_3^r} \right){\left( {f\left( {{x_2}} \right)} \right)^p} + \\ \;\;\;\;\;\;\left( {x_2^r - x_1^r} \right){\left( {f\left( {{x_3}} \right)} \right)^p} \le 0. \end{array} $ |
由于p>0,且当r>0或r<0时,以上证明步步可逆,所以充分性成立.
若f(x)在I上是MM-凹的,则以上证明中的不等号反向,故定理8(ⅰ)的后半部分成立.
定理9 设I⊆R+,f:I→R+,且f在I上二阶可导,则f为I上MM-凸(凹)函数的充分必要条件是
$ \begin{array}{l} \left( {p - 1} \right){\left( {f'\left( x \right)} \right)^2}x + f\left( x \right)f''\left( x \right)x + \\ \;\;\;\;\;\;\;\left( {1 - r} \right)f\left( x \right)f'\left( x \right) \ge \left( \le \right)0\left( {p \ne 0} \right). \end{array} $ | (7) |
证明 只证f为I上MM-凸函数的情形,同理可证f为I上MM-凹函数的情形.
∀x1, x2∈I,不失一般性,设x1<x2,令x=[tx1r+(1-t)x2r]
对函数φ(t)=[f(M2[r](t, 1-t; x1, x2))]p=[f([tx1r+(1-t)x2r]
$ \begin{array}{l} \varphi '\left( t \right) = \frac{p}{r}{\left( {f\left( x \right)} \right)^{p - 1}}f'\left( x \right){x^{1 - r}}\left( {x_1^r - x_2^r} \right),\\ \varphi ''\left( t \right) = \frac{p}{{{r^2}}}{\left( {f\left( x \right)} \right)^{p - 1}}{x^{1 - 2r}}{\left( {x_1^r - x_2^r} \right)^2}\left[ {\left( {p - 1} \right) \times } \right.\\ \;\;\;\;\;\;\;\;\;{\left( {f'\left( x \right)} \right)^2}x + f\left( x \right)f''\left( x \right)x + \\ \;\;\;\;\;\;\;\;\;\left. {\left( {1 - r} \right)f\left( x \right)f'\left( x \right)} \right], \end{array} $ |
注意到x∈I⊆R+,f(x)>0(∀x∈I),所以
$ \begin{array}{l} \varphi ''\left( t \right) \ge \left( \le \right)0 \Leftrightarrow p\left[ {\left( {p - 1} \right){{\left( {f'\left( x \right)} \right)}^2}x + } \right.\\ \;\;\;\;\;\;\;\;\;f\left( x \right)f''\left( x \right)x + \left. {\left( {1 - r} \right)f\left( x \right)f'\left( x \right)} \right] \ge \left( \le \right)0. \end{array} $ |
由定理7知,当p>0时,f(x)是I上的MM-凸函数⇔∀x1, x2∈I, 函数φ(t)=[f(M2[r](t, 1-t; x1, x2))]p是[0, 1]上的凸函数⇔φ″(t)≥0⇔p((p-1)(f′(x))2x+f(x)f″(x)x+(1-r)f(x)f′(x))≥0⇔(p-1)(f′(x))2x+f(x)f″(x)x+(1-r)f(x)f′(x)≥0.
所以,当p>0时,f(x)为I上的MM-凸函数的充要条件是∀x∈I,有
$ \begin{array}{l} \left( {p - 1} \right){\left( {f'\left( x \right)} \right)^2}x + f\left( x \right)f''\left( x \right)x + \\ \;\;\;\;\;\;\left( {1 - r} \right)f\left( x \right)f'\left( x \right) \ge 0. \end{array} $ |
当p<0时,f(x)是I上的MM-凸函数⇔∀x1, x2∈I, 函数φ(t)=[f(M2[r](t, 1-t; x1, x2))]p是[0, 1]上的凹函数⇔φ″(t)≤0⇔p[(p-1)(f′(x))2x+f(x)f″(x)x+(1-r)f(x)f′(x)]≤0⇔(p-1)(f′(x))2x+f(x)f″(x)x+(1-r)f(x)f′(x)≥0.
所以,当p<0时,f(x)为I上的MM-凸函数的充要条件仍然是∀x∈I不等式(7) 成立.
故当p≠0时,f(x)为I上的MM-凸函数的充要条件是∀x∈I不等式(7) 成立.
2 MM-凸函数的复合运算性质定理10 设A, I⊆R+,B⊆I,f:I→R+, μ:A→B,则
(ⅰ)若y=f(u)为I上严格递增的MM-凸函数,u=μ(x)为A上的r次幂平均凸函数(r-平均凸函数[7]),则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递减的MM-凸函数,u=μ(x)为A上的r次幂平均凹函数(r-平均凹函数),则y=f(μ(x))为A上的MM-凸函数;
(ⅲ)若y=f(u)为I上严格递增的MM-凹函数,u=μ(x)为A上的r次幂平均凹函数(r-平均凹函数),则y=f(μ(x))为A上的MM-凹函数;
(ⅳ)若y=f(u)为I上严格递减的MM-凹函数,u=μ(x)为A上的r次幂平均凸函数(r-平均凸函数),则y=f(μ(x))为A上的MM-凹函数.
证明 只证(ⅰ),同理可证(ⅱ)、(ⅲ)、(ⅳ).
∀x1, x2∈A⊆R+及∀t∈[0, 1],显然有M2[r](t, 1-t; x1, x2)∈[min{x1, x2}, max{x1, x2}]⊆A,因为μ:A→B⊆I,所以,μ(x1), μ(x2), μ(M2[r](t, 1-t; x1, x2))∈B⊆I, 且
$ \begin{array}{l} {\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right) \in \left[ {\min \left\{ {\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right\},} \right.\\ \;\;\;\;\;\;\;\;\;\;\;\;\;\left. {\max \left\{ {\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right\}} \right] \subseteq {\bf{B}} \subseteq {\bf{I}}, \end{array} $ |
又u=μ(x)是A上的r次幂平均凸函数,所以
$ \begin{array}{l} \mu \left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) \le \\ \;\;\;\;\;\;\;{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right), \end{array} $ |
且y=f(u)为I上严格递增的MM-凸函数,所以
$ \begin{array}{l} f\left( {\mu \left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right)} \right) \le \\ \;\;\;\;\;\;\;\;f\left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right)} \right) \le \\ \;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {\mu \left( {{x_1}} \right)} \right),f\left( {\mu \left( {{x_2}} \right)} \right)} \right), \end{array} $ |
即y=f(μ(x))是A上的MM-凸函数.
类似地可以证明:
定理11 设A, I⊆R+,B⊆I,f:I→R+, μ:A→B,则
(ⅰ)若y=f(u)为I上严格递增的r次幂平均凸函数(r-平均凸函数),u=μ(x)为A上的MM-凸函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递减的r次幂平均凸函数(r-平均凸函数),u=μ(x)为A上的MM-凹函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅲ)若y=f(u)为I上严格递减的r次幂平均凹函数(r-平均凹函数),u=μ(x)为A上的MM-凸函数,则y=f(μ(x))为A上的MM-凹函数;
(ⅳ)若y=f(u)为I上严格递增的r次幂平均凹函数(r-平均凹函数),u=μ(x)为A上的MM-凹函数,则y=f(μ(x))为A上的MM-凹函数.
定理12 设A, I⊆R+,B⊆I,f:I→R+, μ:A→B,则
(ⅰ)若y=f(u)为I上严格递增的AM-凸函数,u=μ(x)为A上的MA-凸函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递减的AM-凸函数,u=μ(x)为A上的MA-凹函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅲ)若y=f(u)为I上严格递增的AM-凹函数,u=μ(x)为A上的MA-凹函数,则y=f(μ(x))为A上的MM-凹函数;
(ⅳ)若y=f(u)为I上严格递减的AM-凹函数,u=μ(x)为A上的MA-凸函数,则y=f(μ(x))为A上的MM-凹函数.
证明 只证(ⅰ),同理可证(ⅱ)、(ⅲ)、(ⅳ).
∀x1, x2∈A及∀t∈[0, 1], 有M2[r](t, 1-t; x1, x2)∈A,因为μ:A→B⊆I,所以,μ(x1), μ(x2),μ(M2[r](t, 1-t; x1, x2))∈B⊆I,且M2[r](t, 1-t; μ(x1), μ(x2))∈[min{μ(x1), μ(x2)}, max{μ(x1), μ(x2)}]⊆B⊆I.
又u=μ(x)是A上的MA-凸函数,所以
$ \begin{array}{l} \mu \left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right) \le \\ \;\;\;\;\;\;\;\;\;{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right), \end{array} $ |
且y=f(u)为I上严格递增的AM-凸函数,所以
$ \begin{array}{l} f\left( {\mu \left( {{\rm{M}}_2^{\left[ r \right]}\left( {t,1 - t;{x_1},{x_2}} \right)} \right)} \right) \le f\left( {{\rm{M}}_2^{\left[ 1 \right]}\left( {t,1 - t;} \right.} \right.\\ \;\;\;\;\;\;\;\left. {\left. {\mu \left( {{x_1}} \right),\mu \left( {{x_2}} \right)} \right)} \right) \le {\rm{M}}_2^{\left[ p \right]}\left( {t,1 - t;f\left( {\mu \left( {{x_1}} \right)} \right),} \right.\\ \;\;\;\;\;\;\;\left. {f\left( {\mu \left( {{x_2}} \right)} \right)} \right), \end{array} $ |
即y=f(μ(x))是A上的MM-凸函数.
类似地有:
定理13 设A, I⊆R+,B⊆I,f:I→R+, μ:A→B,则
(ⅰ)若y=f(u)为I上严格递增的GM-凸函数,u=μ(x)为A上的MG-凸函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递减的GM-凸函数,u=μ(x)为A上的MG-凹函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递增的GM-凹函数,u=μ(x)为A上的MG-凹函数,则y=f(μ(x))为A上的MM-凹函数;
(ⅳ)若y=f(u)为I上严格递减的GM-凹函数,u=μ(x)为A上的MG-凸函数,则y=f(μ(x))为A上的MM-凹函数.
定理14 设A, I⊆R+,B⊆I,f:I→R+, μ:A→B,则
(ⅰ)若y=f(u)为I上严格递增的HM-凸函数,u=μ(x)为A上的MH-凸函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅱ)若y=f(u)为I上严格递减的HM-凸函数,u=μ(x)为A上的MH-凹函数,则y=f(μ(x))为A上的MM-凸函数;
(ⅲ)若y=f(u)为I上严格递增的HM-凹函数,u=μ(x)为A上的MH-凹函数,则y=f(μ(x))为A上的MM-凹函数;
(ⅳ)若y=f(u)为I上严格递减的HM-凹函数,u=μ(x)为A上的MH-凸函数,则y=f(μ(x))为A上的MM-凹函数.
3 MM-凸函数的Jensen型不等式定理15 设I⊆R+,若f(x)是区间I上的MM-凸(凹)函数,则∀xi∈I和∀ti∈[0, 1](i=1, 2, …, n)且
$ \begin{array}{l} f\left( {{\rm{M}}_n^{\left[ r \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{x_1},{x_2}, \cdots ,{x_n}} \right)} \right) \le \left( \ge \right)\\ \;\;\;\;\;\;\;\;{\rm{M}}_n^{\left[ p \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};f\left( {{x_1}} \right),f\left( {{x_2}} \right), \cdots ,f\left( {{x_n}} \right)} \right). \end{array} $ |
证明 令g(x)=[f(x)]p(x∈I),因为f(x)是I上的MM-凸函数,所以,当p>0时,由定理3(ⅰ)知,g(x)是I上的MA-凸函数,因此有g(x)在I上的Jensen型不等式[17]:
$ \begin{array}{l} g\left( {{\rm{M}}_n^{\left[ r \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{x_1},{x_2}, \cdots ,{x_n}} \right)} \right) \le \\ {\rm{M}}_n^{\left[ 1 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};g\left( {{x_1}} \right),g\left( {{x_2}} \right), \cdots ,g\left( {{x_n}} \right)} \right)\\ \Rightarrow {\left[ {f\left( {{\rm{M}}_n^{\left[ r \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{x_1},{x_2}, \cdots ,{x_n}} \right)} \right)} \right]^p} \le \\ {\rm{M}}_n^{\left[ 1 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{{\left( {f\left( {{x_1}} \right)} \right)}^p},{{\left( {f\left( {{x_2}} \right)} \right)}^p}, \cdots ,{{\left( {f\left( {{x_n}} \right)} \right)}^p}} \right)\\ \Rightarrow f\left( {{\rm{M}}_n^{\left[ r \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{x_1},{x_2}, \cdots ,{x_n}} \right)} \right) \le \\ {\left[ {{\rm{M}}_n^{\left[ 1 \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};{{\left( {f\left( {{x_1}} \right)} \right)}^p},{{\left( {f\left( {{x_2}} \right)} \right)}^p}, \cdots ,{{\left( {f\left( {{x_n}} \right)} \right)}^p}} \right)} \right]^{\frac{1}{p}}} = \\ {\rm{M}}_n^{\left[ p \right]}\left( {{t_1},{t_2}, \cdots ,{t_n};f\left( {{x_1}} \right),f\left( {{x_2}} \right), \cdots ,f\left( {{x_n}} \right)} \right). \end{array} $ |
当p<0时,类似可证结论成立.
f(x)是区间I上MM-凹函数的情形同理可证.
定理15的一个等价形式:
定理16 设I⊆R+, f(x)是I上的MM-凸(凹)函数,则∀xi∈I及∀qi∈R+(i=1, 2, …, n),有
$ \begin{array}{l} f\left[ {{\rm{M}}_n^{\left[ r \right]}\left[ {\frac{{{q_1}}}{{{q_1} + {q_2} + \cdots + {q_n}}},\frac{{{q_2}}}{{{q_1} + {q_2} + \cdots + {q_n}}}, \cdots ,} \right.} \right.\\ \left. {\left. {\;\;\;\;\;\;\;\;\frac{{{q_n}}}{{{q_1} + {q_2} + \cdots + {q_n}}};{x_1},{x_2}, \cdots ,{x_n}} \right]} \right] \le \left( \ge \right)\\ \;\;\;\;\;\;\;\;{\rm{M}}_n^{\left[ p \right]}\left[ {\frac{{{q_1}}}{{{q_1} + {q_2} + \cdots + {q_n}}},\frac{{{q_2}}}{{{q_1} + {q_2} + \cdots + {q_n}}}, \cdots ,} \right.\\ \left. {\;\;\;\;\;\;\;\;\frac{{{q_n}}}{{{q_1} + {q_2} + \cdots + {q_n}}};f\left( {{x_1}} \right),f\left( {{x_2}} \right), \cdots ,f\left( {{x_n}} \right)} \right]. \end{array} $ |
进一步,当q1=q2=…=qn时,有
推论 设I⊆R+,f(x)是I⊆R+上的MM-凸(凹)函数,对∀xi∈I(i=1, 2, …, n),有
$ \begin{array}{l} f\left( {{\rm{M}}_n^{\left[ r \right]}\left( {\frac{1}{n},\frac{1}{n}, \cdots ,\frac{1}{n};{x_1},{x_2}, \cdots ,{x_n}} \right)} \right) \le \left( \ge \right)\\ \;\;\;\;\;\;\;\;{\rm{M}}_n^{\left[ p \right]}\left( {\frac{1}{n},\frac{1}{n}, \cdots ,\frac{1}{n};f\left( {{x_1}} \right),f\left( {{x_2}} \right), \cdots ,f\left( {{x_n}} \right)} \right). \end{array} $ |
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