西格尔零点

西格尔零点、西格尔零（Template:Lang-en）、兰道-西格尔零点（Template:Lang-en）、異常零点（Template:Lang-en），是以德国数学家愛德蒙·蘭道和卡爾·西格爾命名的一種对廣義黎曼假設潛在反例的解析數論猜想，是關於與二次域相關的狄利克雷L函數的零點。粗略說，這些可能的零點在可量化的意義上可以非常接近Template:Math。

動機和定義

狄利克雷L函數有與黎曼ζ函數相似的無零點區域。

Template:TransH The way in which Siegel zeros appear in the theory of Dirichlet L-functions is as potential exceptions to the Template:Tsl, which can only occur when the L-function is associated to a real Dirichlet character.

Real primitive Dirichlet characters

For an integer Template:Math, a Dirichlet character modulo Template:Math is an Template:Tsl $\chi :\mathbb{Z}\to ℂ$ satisfying the following properties:

• (Template:Tsl) $\chi (mn)=\chi (m)\chi (n)$ for every Template:Math, Template:Math;
• (Periodic) $\chi (n+q)=\chi (n)$ for every Template:Math;
• (Support) $\chi (n)=0$ if $\mathrm{g}\mathrm{c}\mathrm{d}(n,q)>1$.

That is, Template:Math is the lifting of a Template:Tsl $\tilde{\chi }:(\mathbb{Z}/q\mathbb{Z}{)}^{\times }\to {ℂ}^{*}$.

The trivial character is the character modulo 1, and the principal character modulo Template:Math, denoted ${\chi }_0(\mathrm{m}\mathrm{o}\mathrm{d}q)$, is the lifting of the trivial homomorphism $(\mathbb{Z}/q\mathbb{Z}{)}^{\times }\ni a\mapsto 1\in {ℂ}^{*}$.

A character $\chi (\mathrm{m}\mathrm{o}\mathrm{d}q)$ is called imprimitive if there exists some integer $d\ne q$ with $d\mid q$ such that the induced homomorphism $\tilde{\chi }:(\mathbb{Z}/q\mathbb{Z}{)}^{\times }\to {ℂ}^{*}$ factors as

$(\mathbb{Z}/q\mathbb{Z}{)}^{\times }\twoheadrightarrow (\mathbb{Z}/d\mathbb{Z}{)}^{\times }\stackrel{\widetilde{{\chi }^{\prime }}}{\to }{ℂ}^{*}$

for some character ${\chi }^{\prime }(\mathrm{m}\mathrm{o}\mathrm{d}d)$; otherwise, $\chi (\mathrm{m}\mathrm{o}\mathrm{d}q)$ is called primitive.

A character $\chi$ is real (or quadratic) if it equals its Template:Tsl $\bar{\chi }$ (defined as $\bar{\chi }(n):=\overline{\chi (n)}$), or equivalently if ${\chi }^2={\chi }_0$. The real primitive Dirichlet characters are in one-to-one correspondence with the 克罗内克符号s $(D|\cdot ):\mathbb{Z}\to \{-1,0,1\}$ for $D\in \mathbb{Z}$ a Template:Tsl (i.e., the discriminant of a Template:Tsl). One way to define $(D|\cdot )$ is as the completely multiplicative arithmetic function determined by (for Template:Math prime):

$\left(\frac{D}{p}\right)=\{\begin{array}{cc}1,& (p)\text{ splits in }\mathbb{Q}(\sqrt{D}),\\ -1,& (p)\text{ is inert }\cdots ,\\ 0,& (p)\text{ ramifies }\cdots ,\end{array}\left(\frac{D}{-1}\right)=\text{sign of }D.$

It is thus common to write ${\chi }_D:=(D|\cdot )$, which are real primitive characters modulo $|D|$.

Classical zero-free regions

The Dirichlet L-function associated to a character $\chi (\mathrm{m}\mathrm{o}\mathrm{d}q)$ is defined as the Template:Tsl of the 狄利克雷级数 $L(s,\chi )=\sum _{n\ge 1}\chi (n)n^{-s}$ defined for $\mathrm{R}\mathrm{e}(s)>1$, where s is a Template:Tsl. For $\chi$ non-principal, this continuation is Template:Tsl; otherwise it has a Template:Tsl of Template:Tsl $\prod _{p\mid q}(1-p^{-1})$ at Template:Math as its only singularity. For $\mathrm{R}\mathrm{e}(s)>1$, Dirichlet L-functions can be expanded into an 欧拉乘积 $L(s,\chi )=\prod _p(1-\chi (p)p^{-s}{)}^{-1}$, from where it follows that $L(s,\chi )$ has no zeros in this region. The Template:Tsl is equivalent (in a certain sense) to $L(1+it,\chi )\ne 0$ ($\mathrm{\forall }t\in ℝ$). Moreover, via the Template:Tsl, we can reflect these regions through $s\mapsto 1-s$ to conclude that, with the exception of negative integers of same parity as Template:Math, all the other zeros of $L(s,\chi )$ must lie inside $\{0<\mathrm{R}\mathrm{e}(s)<1\}$. This region is called the critical strip, and zeros in this region are called non-trivial zeros.

The classical theorem on zero-free regions (Grönwall, Landau, Titchmarsh) states that there exists a(n) (effectively computable) real number $A>0$ such that, writing $s=\sigma +it$ for the complex variable, the function $L(s,\chi )$ has no zeros in the region

$\sigma >1-\frac{A}{(\log q(|t|+2))}$

if $\chi (\mathrm{m}\mathrm{o}\mathrm{d}q)$ is non-real. If $\chi$ is real, then there is at most one zero in this region, which must necessarily be real and simple. This possible zero is the so-called Siegel zero.

The Template:Tsl (GRH) claims that for every $\chi (\mathrm{m}\mathrm{o}\mathrm{d}q)$, all the non-trivial zeros of $L(s,\chi )$ lie on the line $\mathrm{R}\mathrm{e}(s)=\frac{1}{2}$.

定義「西格爾零點」

The definition of Siegel zeros as presented ties it to the constant Template:Math in the zero-free region. This often makes it tricky to deal with these objects, since in many situations the particular value of the constant Template:Math is of little concern. Hence, it is usual to work with more definite statements, either asserting or denying, the existence of an infinite family of such zeros, such as in:

• Conjecture ("no Siegel zeros"): If ${\beta }_D$ denotes the largest real zero of $L(s,{\chi }_D)$, then $1-{\beta }_D\gg \frac{1}{\log |D|}.$

The possibility of existence or non-existence of Siegel zeros has a large impact in closely related subjects of number theory, with the "no Siegel zeros" conjecture serving as a weaker (although powerful, and sometimes fully sufficient) substitute for GRH (see below for an example involving Siegel–Tatuzawa's Theorem and the Template:Tsl problem). An equivalent formulation of "no Siegel zeros" that does not reference zeros explicitly is the statement:

$\frac{L^{\prime }}{L}(1,{\chi }_D)=O(\log |D|).$

The equivalence can be deduced for example by using the zero-free regions and classical estimates for the number of non-trivial zeros of $L(s,\chi )$ up to a certain height.

Landau–Siegel estimates

The first breakthrough in dealing with these zeros came from Landau, who showed that there exists an effectively computable constant Template:Math such that, for any ${\chi }_D$ and ${\chi }_{D^{\prime }}$ real primitive characters to distinct moduli, if $\beta ,{\beta }^{\prime }$ are real zeros of $L(s,{\chi }_D),L(s,{\chi }_{D^{\prime }})$ respectively, then

$\min \{\beta ,{\beta }^{\prime }\}<1-\frac{B}{\log |D{D}^{\prime }|}.$

This is saying that, if Siegel zeros exist, then they cannot be too numerous. The way this is proved is via a 'twisting' argument, which lifts the problem to the Template:Tsl of the Template:Tsl $\mathbb{Q}(\sqrt{D},\sqrt{D^{\prime }})$. This technique is still largely applied in modern works.

This 'repelling effect' (see Template:Tsl), after more careful analysis, led Landau to his 1936 theorem, which states that for every $\epsilon >0$, there is $C(\epsilon )\in {ℝ}_{+}$ such that, if $\beta$ is a real zero of $L(s,{\chi }_D)$, then $\beta <1-C(\epsilon )|D{|}^{-\frac{3}{8}-\epsilon }$. However, in the same year, in the same issue of the same journal, Siegel directly improved this estimate to

$\beta <1-C(\epsilon )|D{|}^{-\epsilon }.$

Both Landau's and Siegel's proofs provide no explicit way to calculate $C(\epsilon )\in {ℝ}_{+}$, thus being instances of an Template:Tsl.

Siegel–Tatuzawa 定理

In 1951, Template:Tsl proved an 'almost' effective version of Siegel's theorem, showing that for any fixed $0<\epsilon <\frac{1}{11.2}$, if $|D|>e^{1/\epsilon }$ then

$L(1,{\chi }_D)>0.655|D{|}^{-\epsilon },$

with the possible exception of at most one fundamental discriminant. Using the 'almost effectivity' of this result, Template:Tsl (1973) showed that Euler's list of 65 Template:Tsl is complete except for at most one element.

Relation to quadratic fields

Siegel zeros often appear as more than an artificial issue in the argument for deducing zero-free regions, since zero-free region estimates enjoy deep connections to the arithmetic of quadratic fields. For instance, the identity ${\zeta }_{\mathbb{Q}(\sqrt{D})}(s)=\zeta (s)L(s,{\chi }_D)$ can be interpreted as an analytic formulation of Template:Tsl (see Template:Tsl). The precise relation between the distribution of zeros near Template:Math and arithmetic comes from Template:Tsl:

$L(1,{\chi }_D)=\{\begin{array}{cc}\frac{2\pi }{w_D\sqrt{|D|}}h(D),& \text{if }D<0\\ \frac{\log {\epsilon }_D}{\sqrt{D}}h(D),& \text{if }D>0,\end{array}$

where:

• $h(D)$ is the Template:Tsl of $\mathbb{Q}(\sqrt{D})$;
• $w_D$ is the number of Template:Tsl in $\mathbb{Q}(\sqrt{D})$ (Template:Math);
• ${\epsilon }_D$ is the Template:Tsl of $\mathbb{Q}(\sqrt{D})$ (Template:Math).

This way, estimates for the largest real zero of $L(s,{\chi }_D)$ can be translated into estimates for $L(1,{\chi }_D)$ (via, for example, the fact that $|L^{\prime }(\sigma ,\chi )|=O({\log }^{2}q)$ for $1-\frac{1}{\log q}\le \sigma \le 1$), which in turn become estimates for $h(D)$. Classical works in the subject treat these three quantities essentially interchangeably, although the case Template:Math brings additional complications related to the fundamental unit.

Siegel zeros as 'quadratic phenomena'

There is a sense in which the difficulty associated to the phenomenon of Siegel zeros in general is entirely restricted to quadratic extensions. It is a consequence of the Template:Tsl, for example, that the Template:Tsl ${\zeta }_K(s)=\sum _{I\subseteq {𝔒}_K}[{𝔒}_K:I{]}^{-s}$ of an Template:Tsl $K/\mathbb{Q}$ can be written as a product of Dirichlet L-functions. Thus, if ${\zeta }_K(s)$ has a Siegel zero, there must be some subfield $F\subseteq K$ with $[F:\mathbb{Q}]=2$ such that ${\zeta }_F(s)$ has a Siegel zero.

While for the non-abelian case ${\zeta }_K(s)$ can only be factored into more complicated Template:Tsls, the same is true:

• Theorem (Template:Tsl, 1974). Let $K/\mathbb{Q}$ be a number field of degree Template:Math. There is a constant $c(n)$ ($=4$ if $K/\mathbb{Q}$ is normal, $=4n!$ otherwise) such that, if there is a real $\beta$ in the range

$1-\frac{c(n)}{\log |{\mathrm{\Delta }}_K|}\le \beta <1$

with ${\zeta }_K(\beta )=0$, then there is a quadratic subfield $F\subseteq K$ such that ${\zeta }_F(\beta )=0$. Here, ${\mathrm{\Delta }}_K$ is the Template:Tsl of the extension $K/\mathbb{Q}$.

"No Siegel zeros" for D < 0

When dealing with quadratic fields, the case $D>0$ tends to be elusive due to the behaviour of the fundamental unit. Thus, it is common to treat the cases $D<0$ and $D>0$ separately. Much more is known for the negative discriminant case:

Lower bounds for h(D)

In 1918, Template:Tsl showed that "no Siegel zeros" for $D<0$ implies that $h(D)\gg \sqrt{|D|}(\log |D|{)}^{-1}$ (see Template:Tsl for comparison). This can be extended to an equivalence, as it is a consequence of Theorem 3 in Template:Tsl–Template:Tsl (2000):

$\text{``No Siegel zeros'' for }D<0⟺h(D)\gg \frac{\sqrt{|D|}}{\log |D|}\sum _{(a,b,c)}\frac{1}{a},$

where the summation runs over the Template:Tsl Template:Tsl $a{x}^2+bxy+c{y}^2$ of discriminant $D$. Using this, Granville and Stark showed that a certain uniform formulation of the Template:Tsl for number fields implies "no Siegel zeros" for negative discriminants.

In 1976, Template:Tsl proved the following unconditional, effective lower bound for $h(D)$:

$h(D)\gg \prod _{p\mid D}(1-\frac{2\sqrt{p}}{p+1})\log |D|.$

Complex multiplication

Another equivalence for "no Siegel zeros" for $D<0$ can be given in terms of Template:Tsl for Template:Tsl of Template:Tsl:

$h(j({\tau }_D))\ll \log |D|,$

where:

• $h$ is the absolute Template:Tsl for number fields;
• $j$ is the Template:Tsl;
• ${\tau }_D:=(D+\sqrt{D})/2$.

The number $j({\tau }_D)$ generates the Template:Tsl of $\mathbb{Q}(\sqrt{D})$, which is its maximal unramified abelian extension. This equivalence is a direct consequence of the results in Granville–Stark (2000), and can be seen in C. Táfula (2019).

A precise relation between heights and values of L-functions was obtained by Template:Tsl (1993, 1998), who showed that, for an elliptic curve $E_D/ℂ$ with Template:Tsl by $\mathbb{Z}[{\tau }_D]$, we have

$-2h_{\mathrm{F}\mathrm{a}\mathrm{l}}(E_D)-\frac{1}{2}\log |D|=\frac{L^{\prime }}{L}(0,{\chi }_D)+\log 2\pi ,$

where $h_{\mathrm{F}\mathrm{a}\mathrm{l}}$ denotes the Template:Tsl. Using the identities $h_{\mathrm{F}\mathrm{a}\mathrm{l}}(E_D)=\frac{1}{12}h(j({\tau }_D))+O(\log h(j({\tau }_D)))$ and $\frac{L^{\prime }}{L}(1,{\chi }_D)=-\frac{L^{\prime }}{L}(0,{\chi }_D)-\log |D|+\log 2\pi +\gamma$, Colmez' theorem also provides a proof for the equivalence above. Template:TransF

西格爾零點存在所造成的結果

盡管一般預期廣義黎曼猜想是對的，但由於「西格爾零點不存在」的猜想依舊開放之故，因此研究「假如廣義黎曼猜想如此的反例存在的話，會有什麼結果」，也是一個令人感興趣的題目。

另一個研究如此可能性的理由，是迄今為止，部分的無條件證明要分成兩部分：第一部分是假定西格爾零點不存在，第二部分是假定西格爾零點存在，並證明說想要的定理在這兩種狀況下都成立。一個如此為之的經典案例是關於算數數列中最小的質數的林尼克定理。

以下是在西格爾零點存在的狀況下，所會造成的結果。

存在無限多個孿生質數

羅傑·希斯-布朗在1983年做出的一個令人驚訝的結果，用陶哲軒的話，可如下陳述：

• 定理（Heath-Brown, 1983）：以下兩个命题至少有一為真：(1)不存在西格爾零點；(2)存在有無限多的孿生質數。

換句話說，如果(1)不成立，也就是西格爾零點存在的話，那(2)就必須成立；反之若(1)成立，也就是西格爾零點不存在的話，那(2)是否成立依舊是未知數。

篩法的奇偶性問題

篩法的奇偶性問題指的是篩法無法顯示出篩選出的整數有奇數個或偶數個質因數這樣的問題。

這使得很多運用篩法的估計，像是使用線性篩（linear sieve）做出的估計，會以一個2的因子，與預期值產生誤差。

在2020年，Template:Link-en證明說假若西格爾零點存在，那麼篩法篩選區間的一般上界就是最佳的，換句話說，在這種狀況下，奇偶性多出來的這個2的因子，就不會是篩法的人為限制。

另見

• 广义黎曼猜想
• Template:Tsl
• Template:Tsl
• Template:Tsl
• Template:Tsl

參考

• Template:Cite book

• Template:Citation

• Template:Cite book

• Template:Cite book