Processing math: 0%
Jun 4

证明
lim

(tian_275461)

证明

\begin{align*}  I&= \frac{\ln{n}}{n}\left(\frac{\sum_{k=1}^{n-1}{\csc{\left(\frac{k\pi}{n}\right)}}}{\ln{n}}-\frac{2}{\pi}n\right)\\  &=\frac{\pi}{n}\sum_{k=1}^{n-1}\csc{\left(\frac{k\pi}{n}\right)}-2\ln{n} \end{align*}
只要证
\lim_{n\rightarrow\infty}{I}=2\gamma-2\ln{\pi}+\ln{4}
也就是
\lim_{n\rightarrow\infty}{(2\gamma-I)}=2\ln{\pi}-\ln{4}
S=2\gamma-I,我们有
\gamma=1+\frac{1}{2}+\frac{1}{3}+\cdots+\frac{1}{n-1}-\ln{n}+c_{n} \quad \text{其中} c_{n}\rightarrow 0\quad  (n\rightarrow\infty)
\begin{align*}  S&=2\sum_{k=1}^{n-1}-\frac{\pi}{n}\sum_{k=1}^{n-1}{\csc{\left(\frac{k\pi}{n}\right)}}+2c_{n}\\  &=\sum_{k=1}^{n-1}{\left(\frac{1}{k}+\frac{1}{n-k}\right)}-\frac{\pi}{n}\sum_{k=1}^{n-1}{\csc{\left(\frac{k\pi}{n}\right)}}+2c_{n}\\  &=\frac{\pi}{n}\sum_{k=1}^{n-1}{\left(\dfrac{1}{\frac{k\pi}{n}}+\dfrac{1}{\pi-\frac{k\pi}{n}}\right)}-\frac{\pi}{n}\sum_{k=1}^{n-1}{\csc{\left(\frac{k\pi}{n}\right)}}+2c_{n} \end{align*}
所以有
  \lim_{n\rightarrow\infty}{S}=\int_{0}^{\pi}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}
故只要证
\int_{0}^{\pi}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}=2\ln{\pi}-\ln{4}

\int_{0}^{\pi}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}=\int_{0}^{\frac{\pi}{2}}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}+\int_{\frac{\pi}{2}}^{\pi}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}
 第二部分用替换 y=\pi-x
\Rightarrow \lim_{n\rightarrow\infty}{S}=2\int_{0}^{\frac{\pi}{2}}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}
注意到
\frac{1}{\sin{x}}=\frac{1}{x}+\sum_{n=1}^{\infty}{(-1)^{n}\left(\frac{1}{x+n\pi}+\frac{1}{x-n\pi}\right)}
\begin{align*}  \int_{0}^{\frac{\pi}{2}}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}&=\sum_{n=1}^{\infty}{(-1)^{n+1}\int_{0}^{\frac{\pi}{2}}{\left(\frac{1}{x+n\pi}+\frac{1}{x-n\pi}   \right)dx}}\\  &=\sum_{n=1}^{\infty}{(-1)^{n+1}\cdot\ln{\left(1-\frac{1}{4n^2}\right)}} \end{align*}
n分奇偶性讨论
(1) n=2m-1\ (m=1,2,\cdots)
(-1)^{n+1}\cdot\ln{\left(1-\frac{1}{4n^2}\right)}=\ln{\left(\frac{(4m-1)(4m-3)}{(4m-2)^2}\right)}
(2) n=2m\ (m=1,2,\cdots)
(-1)^{n+1}\cdot\ln{\left(1-\frac{1}{4n^2}\right)}=\ln{\left(\frac{(4m)^2}{(4m+1)(4m-1)}\right)}

\sum_{m=1}^{k}{(-1)^{n+1}\cdot\ln{\left(1-\frac{1}{4n^2}\right)}}=\ln{\left[\frac{1}{4k+1}\left(\frac{(2k)!!}{(2k-1)!!}\right)^{2}\right]}\rightarrow \ln{\frac{\pi}{4}} \quad \text{(用Wallis公式)}
马上得到
\int_{0}^{\pi}{\left(\frac{1}{x}+\frac{1}{\pi-x}-\frac{1}{\sin{x}}\right)dx}=2\ln{\pi}-\ln{4}

\square
 

Jun 4

计算

\int_0^1 \int_0^1 \frac{\ln(2-xy)}{1-xy}dx \ dy

(tian_275461)

\begin{align*} \int_0^1 \int_0^1 \frac{\ln(2-xy)}{1-xy}dx \ dy &= \int_0^1 \int_0^1 \frac{\ln \left(1+(1-xy) \right)}{1-xy}dx \ dy \\  &= \sum_{n=1}^\infty \frac{(-1)^{n+1}}{n}\int_0^1 \int_0^1 (1-xy)^{n-1}dx \ dy \\ &=\sum_{n=1}^\infty \frac{(-1)^{n+1}}{n^2}\int_0^1 \left(\frac{1-(1-y)^n}{y} \right)dy \\  &=\sum_{n=1}^\infty \frac{(-1)^{n+1}}{n^2}H_n\\ &=\frac{(-1)^{n}}{n}\int_{0}^{1}{(1-x)^{n-1}\ln{x}dx}\\ &=-\int_{0}^{1}{\sum_{n=1}^{\infty}{\frac{(x-1)^{n-1}}{n}\ln{x}}dx}\\ &=\int_{0}^{1}{\frac{\Li_{1}(x-1)\ln{x}}{1-x}dx}\\ &=-\int_{0}^{1}{\frac{\ln{(1+t)}\ln{(1-t)}}{t}dt} \end{align*}
为了计算上面的式子,我们先算
\int_{0}^{1}{\frac{\ln^{2}{(1+x)}}{x}dx}
为此考虑一般的\begin{align*} I(t) &= \int_0^t \frac{\ln^2(1+x)}{x}dx \\ &= \ln t\ln^2(1+t)-2\int_0^t \frac{\ln(x)\ln(1+x)}{1+x}dx \\ &= \ln^2{t}\ln(1+t)-2\int_1^{1+t}\frac{\ln(y) }{y} \left( \ln y+\ln \left( 1-\frac{1}{y}\right)\right)dy \quad (y=x+1)\\ &= \ln t\ln^2(1+t)-\frac{2}{3}\ln^3(1+t)-2\int_{1}^{1+t}{\frac{\ln{y}}{y}\ln{\left(1-\frac{1}{y}\right)}dy} \quad (z=\frac{1}{y})\\ &=\ln t\ln^2(1+t)-\frac{2}{3}\ln^3(1+t)+2\int_{\frac{1}{1+x}}^{1}{\frac{\ln{z}\ln{(1-z)}}{z}dz}\\ &=\ln{t}\ln^2(1+t)-\frac{2}{3}\ln^3(1+t)-2\int_{\frac{1}{1+x}}^{1}{\ln{z}d \Li_{2}(z)}\\ &=\ln{t}\ln^2(1+t)-\frac{2}{3}\ln^3(1+t)-2\left(\ln{z}\Li_{2}(z)\bigg|_{\frac{1}{1+x}}^{1}-\int_{\frac{1}{1+x}}^{1}{\frac{\Li_{2}(z)}{z}dz}\right)\\ &=\ln{t}\ln^2(1+t)-\frac{2}{3}\ln^3(1+t)-2\ln{(1+t)}\Li_{2}\left(\frac{1}{1+t}\right)-2\Li_{3}\left( \frac{1}{1+t} \right)+2\Li_{3}(1)  \end{align*}

I(1)=-\frac{2}{3}\ln^3{2}-2\ln{2}\Li_{2}\left(\frac{1}{2}\right)-2\Li_{3}\left(\frac{1}{2}\right)+2\Li_{3}(1)

\Li_{2}\left(\frac{1}{2}\right)=\frac{1}{12}(\pi^{2}-6\ln^2{2})
\Li_{3}\left(\frac{1}{2}\right)=\frac{1}{24}[4\ln^3{2}-2\pi^{2}\ln{2}+21\zeta(3)]
I(1)=\frac{\zeta(3)}{4}
对已知结论
\int_{0}^{1}{\frac{\ln^2{(1-x)}}{x}dx}=2\zeta(3)
x=t^{2}
\Rightarrow \int_{0}^{1}{\frac{\ln^2{(1-t^2)}}{t}dx}=\zeta(3)
\Leftrightarrow \int_{0}^{1}{\frac{\ln^2{(1-t)}}{t}dx}+\int_{0}^{1}{\frac{\ln^2{(1+t)}}{t}dx}+2\int_{0}^{1}{\frac{\ln{(1-t)}\ln{(1+t)}}{t}dx}=\zeta(3)
\Rightarrow \int_{0}^{1}{\frac{\ln{(1-t)}\ln{(1+t)}}{t}dx}=-\frac{5}{8}\zeta(3)
故有
\int_0^1 \int_0^1 \frac{\ln(2-xy)}{1-xy}dx \ dy=\frac{5}{8}\zeta(3)

__________________________________________________________________________________________________

根据上面的办法,我们可以得到

\int_{0}^{1}{\frac{\ln^{2}(1-t)}{t}dt}=2\zeta(3)

\int_{0}^{1}{\frac{\ln^{2}(1+t)}{t}dt}=\frac{1}{4}\zeta(3)

\sum_{n=1}^{\infty}{\frac{H_{n}}{n^2}}=2\zeta(3)

\int_{0}^{1}\int_{0}^{1}\frac{\ln(1-xy)}{1-xy}dxdy =-\zeta(3)

\sum_{n=1}^{\infty}{\frac{H_{n}}{n^3}}=\frac{\pi^4}{72}
 

May 21

记得这个问题曾经在大约1年前和网友讨论过,那时候真心不会。前几天在AMM上找到了一样的。

May 16

f(x)[0,1]\rightarrow R上的连续函数,且记\displaystyle F(x)=\int_{0}^{x}{f(t)dt},并有
\int_{0}^{1}{x^2f(x)dx}=-2\int_{\frac{1}{2}}^{1}{F(t)dt}
求证
\int_{0}^{1}{f^2(x)dx}\geq 80\left( \int_{0}^{1}{f(x)dx}\right)^2
(tian_275461)

 首先由条件,我们看到
   \int_{0}^{1}{x^2F'(x)dx}=x^2F(x)\bigg|_{0}^{1}-2\int_{0}^{1}{xF(x)dx}=F(1)-2\int_{0}^{1}{xF(x)dx}
  \Rightarrow\qquad F(1)=2\int_{0}^{1}{xF(x)dx}-2\int_{\frac{1}{2}}^{1}{F(t)dt}=2\left(\int_{0}^{\frac{1}{2}}{xF(x)dx}+\int_{\frac{1}{2}}^{1}{(x-1)F(x)dx}  \right)
 而要证的不等式等价于
  \int_{0}^{1}{(F'(x))^{2}dx}\geq 24(F(1))^2=80\left[2\left(\int_{0}^{\frac{1}{2}}{xF(x)dx}+\int_{\frac{1}{2}}^{1}{(x-1)F(x)dx}  \right)\right]^{2}
 注意到由Cauchy-Schwarz不等式,我们有
 \begin{align*}  \int_{0}^{\frac{1}{2}}{x^4} \int_{0}^{\frac{1}{2}}{(F'(x))^{2}dx}&\geq \left(\int_{0}^{\frac{1}{2}}{x^2F'(x)dx}\right)^{2}\\  &=\left[\frac{1}{4}F\left(\frac{1}{2}\right)-2\int_{0}^{\frac{1}{2}}{xF(x)dx}\right]^{2}\\  &=A^2  \end{align*}
 \begin{align*}  \int_{\frac{1}{2}}^{1}{(x-1)^4}\int_{\frac{1}{2}}^{1}{(F'(x))^{2}dx}&\geq  \left(\int_{\frac{1}{2}}^{1}{(x-1)^2F'(x)dx}\right)^{2}\\  &= \left[-\frac{1}{4}F\left(\frac{1}{2}\right)-2\int_{\frac{1}{2}}^{1}{(x-1)F(x)dx}\right]^{2}\\  &=B^2  \end{align*}
 而由Cauchy-Schwarz不等式,我们有
 A^2+B^2\geq \frac{1}{2} (A+B)^2=\frac{1}{2} F^{2}(1)
 而
  \int_{0}^{\frac{1}{2}}{x^4dx}=\int_{\frac{1}{2}}^{1}{(1-x)^4dx}=\frac{1}{160}
 故
  \int_{0}^{1}{f^2(x)dx}\geq 80\left( \int_{0}^{1}{f(x)dx}\right)^2
 

May 16

\forall m\neq 0,若一个连续函数f:\mathbb{R}\rightarrow\mathbb{R}满足函数方程
f\left(2x-\frac{f(x)}{m}\right)=mx
则有f(x)=m(x-c).
证明:
我们设g(x)=2x-\frac{f(x)}{m},显然g(x)是连续函数且有
g(g(x))=2g(x)-x \qquad \forall x\in\mathbb{R}
g(x_1)=g(x_2)则有g(g(x_{1}))=g(g(x_{2})),我们得到
x_{1}=x_{2}
g(x)是一个单射,而我们知道,若g(x)是一个连续的单射,则g(x)严格单调。(关于这一点可以用反证法证明),因此,g(x)有2种情况,严格递增或者严格递减。下证明g(x)只能严格递增。
(反证)设g(x)严格递减,则对于x_{1}<x_{2},我们有g(x_{1})>g(x_{2}),接着又有g(g(x_{1}))<g(g(x_{2})).而这等价于
2g(x_{1})-x_{1}<2g(x_{2})-x_{2}
\Leftrightarrow 2[g(x_{1})-g(x_{2})]<x_{1}-x_{2}
上面不可能成立,因为左边大于0而右边小于0。故g(x)只能严格递增。
改写g(g(x))=2g(x)-x
g(g(x))-g(x)=g(x)-x
递推后得到
g^{n}(x)=ng(x)-(n-1)x  \qquad (n\geq 1)
这里g^(n)(x)表示n次符合。
则有
g^{n}(x)-g^{n}(0)=n[g(x)-x-g(0)]+x
\Leftrightarrow \frac{ g^{n}(x)-g^{n}(0)}{n}=g(x)-x-g(0)+\frac{x}{n}
g(x)严格递增,g^{n}(x)也严格递增,故对上式令n\rightarrow\infty,由g(x)的单调性,我们得到

g(x)\leq x+g(0),\qquad x<0
g(x)\geq x+g(0),\qquad x>0

这样,我们得到g(x)的值域也是\mathbb{R},故g(x)是一个一一映射。且g^{-1}存在。现在,用x=g^{-1}(g^{-1}(y))带入原来的方程,则有
g^{-1}(g^{-1}(y))=2g^{-1}(y)-y
g^{-1}(y)同样满足这个方程,则用相同的手段,我们得到

g^{-1}(y)\leq y+g(0),\qquad y<0
g^{-1}(y)\geq y+g(0),\qquad y>0

现在,用x=g^{-1}(y)带入
g(g(x))-g(x)=g(x)-x
得到
g(y)-y=y-g^{-1}(y)
y=0得到g^{-1}(0)=-g(0)
假设g(0)\geq 0,则对x>0g(x)\geq x+g(0)>0,则对y=g(x)>0x>g(x)+g^{-1}(0)=g(x)-g(0).故得到
g(x)=x+g(0) \qquad (x>0)
同理可得
g(x)=x+g(0) \qquad (x<0)
这样我们得到f(x)=m(x-g(0))x\in \mathbb{R}成立。