凹凸反转:证明指对跨阶不等式的利器

发布时间 2023-08-11 00:25:23作者: 沼中蒻杨

凹凸反转:证明指对跨阶不等式的利器

概述

在证明指对跨阶的不等式时,常可以将指数部分与对数部分分离在不等号两边,并在不等号两边构造凹凸性相反的函数,使得上凹函数的最小值大于上凸函数的最大值来证明原不等式

例题

\[\text{please prove}\quad\forall x>0,\quad xe^x-\ln{x}>\frac{3}{2}\tag{@} \]

凹凸反转思路

\[(@)\Leftrightarrow f(x)=\frac{e^x}{x^a}>\frac{\ln{x}+\frac{3}{2}}{x^{a+1}}=g(x)\\ f'(x)=\frac{e^x(x-a)}{x^{a+1}}\\ g'(x)=\frac{1-(a+1)(\ln{x}+\frac{3}{2})}{x^{a+2}}\\ \because\text{we want}\quad f(x)_{min}>g(x)_{max}\therefore a>0\\ \therefore f(x)_{min}=f(a)=\frac{e^a}{a^a}\\ \therefore g(x)_{max}=g(e^{\frac{1}{a+1}-\frac{3}{2}})=\frac{e^{\frac{3}{2}a+\frac{1}{2}}}{a+1}\\ \text{then we want}\quad \frac{e^a}{a^a}>\frac{e^{\frac{3}{2}a+\frac{1}{2}}}{a+1}\\ \Leftrightarrow \ln{(a+1)}-a\ln{a}-\frac{a+1}{2}=h(a)>0\\ h'(a)=\frac{1}{a+1}-\ln{a}-\frac{3}{2}\quad\big\downarrow\\ h(0)->-\frac{1}{2},h'(0)->+\infty\\ \text{try}\quad a=1,\quad h(1)=\ln{2}-1<0,h'(1)=-1\\ \text{try}\quad a=\frac{1}{2},\quad h(\frac{1}{2})=\ln{3}-\frac{\ln{2}}{2}-\frac{3}{4}=\frac{1}{4}\ln{(\frac{81}{4e^3})}>0\\ \therefore a=\frac{1}{2}\quad \text{is good} \]

注意:本题并非典型的凹凸反转,g(x)只在极值点附近与f(x)凹凸性相反

实际证明

\[(@)\Leftrightarrow\frac{e^x}{\sqrt{x}}=f(x)>g(x)=\frac{\ln{x}+\frac{3}{2}}{x\sqrt{x}}\\ f'(x)=\frac{e^x(x-\frac{1}{2})}{x\sqrt{x}}\\ g'(x)=\frac{-\frac{5}{4}-\frac{3}{2}\ln{x}}{x^2\sqrt{x}}\\ \therefore f(x)_{min}=f(\frac{1}{2})=\sqrt{2e},\quad g(x)_{max}=g(e^{-\frac{5}{6}})=\frac{2}{3}e^{\frac{5}{4}}\\ \therefore f(x)\ge f(x)_{min}>g(x)_{max}\ge g(x) \]