素数分布的基本定理(一)
目录
- Chapter1 切比雪夫函数\(\psi(x)\)和\(\vartheta(x)\)
- Chapter2 \(\vartheta(x)\)与\(\pi(x)\)的关系
- Chapter3 素数定理的等价形式
Chapter1 切比雪夫函数\(\psi(x)\)和\(\vartheta(x)\)
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Definition:
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对于\(x>1\),\(\psi(x)\)定义为:
\[\psi(x) = \sum_{n\le x}\Lambda(n) = \sum_{m\le \log_2{x}}\sum_{p\le x^{1/m}}\log{p} \] -
对于\(x>0\),\(\vartheta(x)\)定义为:
\[\vartheta(x) = \sum_{p\le x}\log{p} \]
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Theorem:
- Th1:Mangoldt函数\(\Lambda(n)\)的变化,即为什么:\[\sum_{n\le x}\Lambda(n) = \sum_{m\le \log_2{x}}\sum_{p\le x^{1/m}}\log{p} \]
Proof:
根据\(\Lambda(n)\)函数的定于,如果\(n\)不是某个素数的幂,那么\(\Lambda(n)=0\)。
于是我们可以将\(\sum_{n\le x}\Lambda(n)\)表示为
\[\sum_{n\le x}\Lambda(n) =\sum_{m=1}^{\infty}\sum_{p^m\le x}\Lambda(p^m) = \sum_{m=1}^{\infty}\sum_{p\le x^{1/m}}\log p \]关于后面的那个sum,其中的\(p,x,m\)三者形成了一个关系。实际上,p是从2开始的,也就是说\(x^{1/m}<2\)的话,sum是0。
我们将\(x^{1/m}<2\)这个条件进行变换:
\[x^{1/m}<2\\ \dfrac{1}{m}\log x < \log 2\\ m > \dfrac{\log x}{\log 2} = \log_2{x} \]于是我们得到了m和x的一个关系,于是就可以将前面sum的无穷转换为新的形式:
\[\sum_{m=1}^{\infty}\sum_{p\le x^{1/m}}\log p = \sum_{m\le \log_2{x}}\sum_{p\le x^{1/m}}\log p \]有了这样的形式,我们也可以将\(\psi(x)\)与\(\vartheta(x)\)联系起来:
\[\psi(x) = \sum_{m\le \log_2{x}}\vartheta(x^{1/m}) \]- Th2:关于\(\dfrac{\psi(x)}{x}\)与\(\dfrac{\vartheta(x)}{x}\)两者之间的关系:其中一个趋于一个极限,那么另一个也趋于这个极限。也就是说:\[\lim_{x\rightarrow\infty}(\dfrac{\psi(x)}{x}-\dfrac{\vartheta(x)}{x})=0 \]
Proof:
通过\(\psi(x)\)与\(\vartheta(x)\)的关系我们可以得到一个减法:
\[\psi(x) - \vartheta(x) = \sum_{m\le \log_2{x}}\vartheta(x^{1/m}) - \vartheta(x) = \sum_{1 \le m \le \log_2{x}}\vartheta(x^{1/m}) \]现在目光聚集到\(\vartheta\)上,对于\(\vartheta\)的定义,我们可以写出不等式:
\[\vartheta(x) = \sum_{p\le x}\log p \le x \log p \]于是有
\[\begin{align*} 0 \le \psi(x)-\vartheta(x) \le \sum_{2\le m \le \log_2x}x^{1/m}\log x^{1/m}\\ \end{align*}\\ 将其扩大,放缩为:\\ \sum_{2\le m \le \log_2x}\log x^{1/m} \le (\log_2 x)\sqrt{x}\log\sqrt{x} = \dfrac{\log x}{\log 2}\dfrac{\sqrt{x}}{2}\log x = \dfrac{\sqrt{x}(\log x)^2}{2\log 2} \]同时去除x最后得到:
\[0\le \dfrac{\psi(x)}{x} = \dfrac{\vartheta(x)}{x}\le \dfrac{(\log x)^2}{2\sqrt{x}\log2} \]根据夹逼定理:
\[\begin{align*} \lim_{x\rightarrow\infty} \dfrac{(\log x)^2}{2\sqrt{x}\log2} &= \dfrac{1}{2\log2}\lim_{x\rightarrow\infty}\dfrac{(\log x)^2}{\sqrt{x}}\\ 洛必达:\\ &=\dfrac{1}{2\log2}\lim_{x\rightarrow\infty}\dfrac{4\log x}{\sqrt{x}}\\ 洛必达:\\ &=\dfrac{2}{\log2}\lim_{x\rightarrow\infty}\dfrac{2}{\sqrt{x}}\\ &=0 \end{align*} \]可以得出
\[\lim_{x\rightarrow\infty}(\dfrac{\psi(x)}{x}-\dfrac{\vartheta(x)}{x})=0 \] - Th1:Mangoldt函数\(\Lambda(n)\)的变化,即为什么:
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Note:
Chapter2 \(\vartheta(x)\)与\(\pi(x)\)的关系
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Theorem:
- Th1:(阿贝尔等式)对任一数论函数\(a(n)\),令其部分和为\[A(x) = \sum_{n\le x}a(n) \]当\(x<1\)时,\(A(x)=0\)。如果函数\(f\)在区间\([y,x],(0<y<x)\)上有连续导数,那么有:\[\sum_{y<n\le x}a(n)f(n) = A(x)f(x)-A(y)f(y)-\int_{y}^{x}A(t)f'(t)dt \]
Proof:
令\(k = \lfloor x\rfloor,m = \lfloor y\rfloor\)。这样一来\(A(k)=A(x),A(m)=A(y)\)
\[\begin{align*} \sum_{y<n\le x}a(n)f(n)&=\sum_{n=m+1}^ka(n)f(n)\\ &=\sum_{n=m+1}^k\{A(n)-A(n-1)\}f(n)\\ &=\sum_{n=m+1}^kA(n)f(n) -\sum_{n=m}^{k-1}A(n)f(n+1)\\ &=\sum_{n=m+1}^{k-1}A(n)f(n)+A(k)f(k)-\sum_{n=m}^{k-1}A(n)f(n+1)\\ &=\sum_{n=m+1}^{k-1}A(n)\{f(n)-f(n+1)\}+A(k)f(k)-A(m)f(m+1)\\ &=-\sum_{n=m+1}^{k-1}A(n)\int_{n}^{n+1}f'(t)dt+A(k)f(k)-A(m)f(m+1)\\ &=-\sum_{n=m+1}^{k-1}\int_{n}^{n+1}A(t)f'(t)dt+A(k)f(k)-A(m)f(m+1)\\ &=-\int_{m+1}^{k}A(t)f'(t)dt+A(k)f(k)-A(m)f(m+1)\\ 用同样的思想向着\int_y^x出发: &=-\int_{m+1}^{k}A(t)f'(t)dt-\{\int_k^xA(t)f'(t)dt+A(k)f(x)\}-\{\int_y^m+1A(t)f'(t)dt+A(m)f(y)\}\\ &=A(x)f(x)-A(y)f(y)-\int_y^xA(t)f'(t)dt \end{align*} \]- Th2:接下来我们分别用\(\pi(x)\)将\(\vartheta(x)\)表示出来,再用\(\vartheta(x)\)将\(\pi(x)\)表示出来:\[\vartheta(x) = \pi(x)\log x-\int_2^x\dfrac{\pi(t)}{t}dt\\ 和\\ \pi(x) = \dfrac{\vartheta(x)}{\log x}+\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt \]
Proof:
我们知道,\(\pi(x)\)计算的是小于等于x的范围内有多少个素数,如果用求和的形式将其表示出来的话就是\(\pi(x) = \sum_{p\le x}1\),我们可以用一个特征函数表示:
\[a(n) = \begin{cases} 1,若n是素数\\ 0,其他 \end{cases} \]所以\(\pi(x) = \sum_{p\le x}1 = \sum_{1<n\le x}a(n)\)
基于这个特征函数也可以将\(\vartheta(x)\)表示成\(\vartheta(x) = \sum_{p\le x}\log p= \sum_{1<n\le x}a(n)\log n\)
这两个函数在配合上面的阿贝尔等式,$f(n) = \log n $,我们就可以得到:
\[\vartheta(x) = \pi(x)\log{x}-\pi(1)\log{1}-\int_1^x\pi(t)(\log t)'dt \]当t<2的时候,\(\pi(t)=0\),于是最终形式如下:
\[\vartheta(x) = \pi(x)\log{x}-\int_2^x\dfrac{\pi(t)}{t}dt \]接下来对\(\pi(x)\)进行转换。
如果我们想用\(\vartheta(x)\)将\(\pi(x)\)表示出来,可以用\(b(n)=a(n)\log n\)将\(\vartheta(x)\)表示为部分和\(\vartheta(x)=\sum_{n\le x}b(n)\)的形式,然后就可以使用\(f(n)=\dfrac{1}{\log n}\)阿贝尔等式展开了:
\[\pi(x) = \sum_{y<n\le x}b(n)\dfrac{1}{\log n} = b(x)\dfrac{1}{\log x}-b(y)\dfrac{1}{\log y} - \int_y^xb(t)(\dfrac{1}{\log t})'dt \]展开后得到:
\[\pi(x) =\vartheta(x)\dfrac{1}{\log x}-\vartheta(y)\dfrac{1}{\log y} + \int_y^x\dfrac{\vartheta(t)}{t\log^2 t}dt \]当\(0<y<2\)时,\(\vartheta(y)=0\),最终我们得到:
\[\pi(x) =\vartheta(x)\dfrac{1}{\log x} + \int_2^x\dfrac{\vartheta(t)}{t\log^2 t}dt \] - Th1:(阿贝尔等式)对任一数论函数\(a(n)\),令其部分和为
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Note:
Chapter3 素数定理的等价形式
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Theorem:
- Th1:下面这几个式子是等价的\[\begin{equation} \lim_{x\rightarrow\infty}\dfrac{\pi(x)\log x}{x}=1\\ \lim_{x\rightarrow\infty}\dfrac{\vartheta(x)}{x}=1\\ \lim_{x\rightarrow\infty}\dfrac{\psi(x)}{x}=1 \end{equation} \]
Proof:
由C2.Th2得到
\[\dfrac{\vartheta(x)}{x} = \dfrac{\pi(x)\log x}{x}-\dfrac{1}{x}\int_2^x\dfrac{\pi(t)}{t}dt\\ 和\\ \dfrac{\pi(x)\log x}{x} = \dfrac{\vartheta(x)}{x}+\dfrac{\log x}{x}\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt \]证明\(\dfrac{\vartheta(x)}{x}\)和\(\dfrac{\pi(x)\log x}{x}\)的等价关系,只需要分别证明:
(1)\(\lim_{x\rightarrow\infty}\dfrac{1}{x}\int_2^x\dfrac{\pi(t)}{t}dt=0\):
通过第一个式子我们可以得到\(\dfrac{\pi(x)}{x}=\Omicron(\dfrac{1}{\log x})\),于是
\[\dfrac{1}{x}\int_2^x\dfrac{\pi(t)}{t}dt = \Omicron(\dfrac{1}{x}\int_2^x\dfrac{1}{\log t}dt)\\ \int_2^x\dfrac{1}{\log t}dt = \int_2^{\sqrt x}\dfrac{1}{\log t}dt + \int_{\sqrt x}^x\dfrac{1}{\log t}dt \le \dfrac{\sqrt{x}}{\log 2}+\dfrac{x-\sqrt{x}}{\log \sqrt{x}} \]乘起来
\[\dfrac{1}{x}\int_2^x\dfrac{\pi(t)}{t}dt \le \dfrac{1}{x}\{\dfrac{\sqrt{x}}{\log 2}+\dfrac{x-\sqrt{x}}{\log \sqrt{x}}\}=\dfrac{1}{\sqrt{x}\log 2}+\dfrac{1-\dfrac{1}{\sqrt{x}}}{\log \sqrt{x}}\\ \lim_{x\rightarrow\infty}\{\dfrac{1}{\sqrt{x}\log 2}+\dfrac{1-\dfrac{1}{\sqrt{x}}}{\log \sqrt{x}}\} = 0\\ 所以\lim_{x\rightarrow\infty}\dfrac{1}{x}\int_2^x\dfrac{\pi(t)}{t}dt \rightarrow0 \](2)\(\lim_{x\rightarrow\infty}\dfrac{\log x}{x}\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt=0\):
通过第二个式子我们可以得到\(\vartheta(t)=\Omicron(t)\),于是
\[\dfrac{\log x}{x}\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt = \Omicron(\dfrac{\log x}{x}\int_2^x\dfrac{1}{\log^2t}dt)\\ \int_2^x\dfrac{1}{\log^2t}dt = \int_2^{\sqrt{x}}\dfrac{1}{\log^2t}dt + \int_{\sqrt{x}}^x\dfrac{1}{\log^2t}dt\le \dfrac{\sqrt{x}}{\log^22}+\dfrac{x-\sqrt{x}}{\log^2\sqrt{2}} \]乘起来
\[\dfrac{\log x}{x}\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt \le \dfrac{\log x}{x}\{\dfrac{\sqrt{x}}{\log^22}+\dfrac{x-\sqrt{x}}{\log^2\sqrt{x}}\} = \dfrac{\log x}{\sqrt{x}\log^22}+\dfrac{2(1-\dfrac{1}{\sqrt{x}})}{\log\sqrt{x}}\\ lim_{x\rightarrow\infty}\{\dfrac{\log x}{\sqrt{x}\log^22}+\dfrac{2(1-\dfrac{1}{\sqrt{x}})}{\log\sqrt{x}}\} =0\\ 所以:lim_{x\rightarrow\infty}\dfrac{\log x}{x}\int_2^x\dfrac{\vartheta(t)}{t\log^2t}dt \rightarrow0 \]- Th2:接下来我们将素数定理和第n个素数的渐进值联系起来:令\(p_n\)是第n个素数,下面几个渐进式是逻辑等价的。\[\begin{gather} \lim_{x\rightarrow\infty}\dfrac{\pi(x)\log x}{x} = 1\\ \lim_{x\rightarrow\infty}\dfrac{\pi(x)\log{\pi(x)}}{x} = 1\\ \lim_{n\rightarrow\infty}\dfrac{p_n}{n\log n } = 1 \end{gather} \]
Proof:
第一个式子推第二个:
对第一个式子两边取对数得
\[\lim_{x\rightarrow\infty}[\log{\pi(x)}+\log{\log x}-\log{x}]=0\\ \lim_{x\rightarrow\infty}\log x[{\dfrac{{\log \pi(x)}}{\log{x}}+\dfrac{\log{\log x}}{\log{x}}-1}]=0 \]当\(x\rightarrow\infty\)时,前面的\(\log x\rightarrow\infty\),于是后面的\(\lim_{x\rightarrow\infty}[{\dfrac{{\log\pi(x)}}{\log{x}}+\dfrac{\log{\log x}}{\log{x}}-1}]=0\)。然而中间的又是0
即\(\lim_{x\rightarrow\infty}{\dfrac{{\log\pi(x)}}{\log{x}}}=1\)
于是\(\lim_{x\rightarrow\infty}{\dfrac{{\log\pi(x)}}{\log{x}}\dfrac{\pi(x)\log x}{x}}=\lim_{x\rightarrow\infty}\dfrac{\pi(x)\log x}{x}=1\)
第二个式子推第三个:
\(p_n\)是第n个素数,我们用x表示\(p_n\)。那么\(\pi(x)=n,\pi(x)\log\pi(x) = n\log n\)
于是\(\lim_{x\rightarrow\infty}\dfrac{\pi(x)\log{\pi(x)}}{x} = \lim_{x\rightarrow\infty}\dfrac{n\log n}{p_n}=1\)
第三个式子推第二个:
式3成立,给定x,由不等式\(p_n\le x < p_{n+1}\)确定n,\(n=\pi(x)\),于是有不等式:
\[\dfrac{p_n}{n\log n} \le \dfrac{x}{n\log n } < \dfrac{p_{n+1}}{n\log n}=\dfrac{p_{n+1}}{(n+1)\log{(n+1)}}\dfrac{(n+1)\log{(n+1)}}{n\log n} \]如此一来就构成了满足式3的\(p_n,p_{n+1}\),根据夹逼可以知道\(\lim_{x\rightarrow\infty}\dfrac{x}{n\log n}=1\)
将n替换为\(\pi(x)\)就得到了式2
第二个式子推第一个:
对式2取对数得
\[\lim_{x\rightarrow\infty}[\log\pi(x) + \log \log{\pi(x)} - \log x] = 0 \\ \lim_{x\rightarrow\infty}\log\pi(x)[1+\dfrac{\log\log\pi(x)}{\log\pi(x)}-\dfrac{\log x}{\log\pi(x)}] = 0 \\ \]外面的\(\log\pi(x)\rightarrow\infty\),那么里面的就有$\lim_{x\rightarrow\infty}1+\dfrac{\log\log\pi(x)}{\log\pi(x)}-\dfrac{\log x}{\log\pi(x)} = 0 \(,中间的\)\lim_{x\rightarrow\infty}\dfrac{\log\log\pi(x)}{\log x}=0$
于是剩下的\(\lim_{x\rightarrow\infty}\dfrac{\log x}{\log\pi(x)}=1\)
结合式2得到\(\lim_{x\rightarrow\infty}\dfrac{\log x}{\log\pi(x)}\dfrac{\pi(x)\log\pi(x)}{x}=\dfrac{\pi(x)\log x}{x}=1\)
于是证出第一个式子。
- Th1:下面这几个式子是等价的
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Note: