Selberg class

In mathematics, the Selberg class is an axiomatic definition of a class of L-functions. The members of the class are Dirichlet series which obey four axioms that seem to capture the essential properties satisfied by most functions that are commonly called L-functions or zeta functions. Although the exact nature of the class is conjectural, the hope is that the definition of the class will lead to a classification of its contents and an elucidation of its properties, including insight into their relationship to automorphic forms and the Riemann hypothesis. The class was defined by Atle Selberg in (Selberg 1992), who preferred not to use the word "axiom" that later authors have employed.¹

The formal definition of the class S is the set of all Dirichlet series

${\displaystyle F(s)=\sum _{n=1}^{\infty }{\frac {a_{n}}{n^{s}}}}$

absolutely convergent for Re(s) > 1 that satisfy four axioms (or assumptions as Selberg calls them):

• Without the condition ${\displaystyle a_{n}=O(n^{\varepsilon })}$, there would be:

${\displaystyle L(s+1/3,\chi _{4})L(s-1/3,\chi _{4})}$

which violates the Riemann hypothesis.

• Without this functional equation we would have Dirichlet L-functions for any imprimitive character. If ${\textstyle \chi }$ is Dirichlet character induced by ${\textstyle \chi ^{\star }}$, then we have:

${\displaystyle L(s,\chi )=L(s,\chi ^{\star })\prod _{p\,|\,q}\left(1-\chi ^{\star }(p)p^{-s}\right)}$

Despite this function satisfies any other axiom, from that additional factors follows that it has infinitely many zeros on ${\displaystyle \operatorname {Re} (s)=0}$. They are not symmetric with respect to ${\displaystyle \operatorname {Re} (s)={\tfrac {1}{2}}}$ and without functional equation proposed by Selberg it is hard to distinguish between trivial and nontrivial zeros of this function.

• If function F satisfies functional equation with different gamma factors, then using Stirling formula for gamma function one can show:

${\displaystyle \gamma _{2}(s)=C\cdot \gamma _{1}(s)\quad {\text{where: }}C\in \mathbb {R} }$.

However, by the multiplication formula the same gamma factor can be expressed in many different ways, involving different number of gamma functions with different constants. Despite this, Selberg proved that the sum ${\textstyle \sum _{i=1}^{k}\omega _{i}}$ is independent of the choice of the gamma factor formula.

• The condition that the real part of μ_i be non-negative is because there are known L-functions that do not satisfy the Riemann hypothesis when μ_i is negative. Specifically, there are Maass forms associated with exceptional eigenvalues, for which the Ramanujan–Petersson conjecture holds, and which have a functional equation, but do not satisfy the Riemann hypothesis.

• Having Euler product is essential, as notable counterexample given by Davenport and Heilbronn:

${\displaystyle F(s)={\frac {1-\alpha i}{2}}L(s,\chi _{5})+{\frac {1+\alpha i}{2}}L(s,{\overline {\chi }}_{5})\quad {\text{for: }}\alpha ={\frac {{\sqrt {10-{\sqrt {5}}}}-2}{{\sqrt {5}}-1}}}$

despite having analytic continuation, having periodic coefficients (thus satisfying Ramanujan conjecture) and satisfying functional equation have zeros lying outside critical line.

The Selberg class is closed under multiplication of functions: product of each two functions belonging to S are also in S. It is also easy to check that if F is in S, then function involved in functional equation:

${\displaystyle {\overline {F({\overline {s}})}}=\sum _{n=1}^{\infty }{\frac {\overline {a_{n}}}{n^{s}}}}$

satisfies axioms and is also in S. If F is entire function in S, then ${\textstyle F(s+it)}$ for ${\textstyle t\in \mathbb {R} }$ is also in S.

From the Ramanujan conjecture, it follows that, for every ${\displaystyle \epsilon >0}$:

${\textstyle \sum _{i=1}^{n}\vert a_{i}\vert =O(n^{1+\epsilon })}$

then Dirichlet series defining function is absolutely convergent in the half-plane: ${\textstyle \operatorname {Re} (s)>1}$.

Despite the unusual version of the Euler product in the axioms, by exponentiation of Dirichlet series, one can deduce that a_n is a multiplicative sequence and that

${\displaystyle F_{p}(s)=\sum _{n=0}^{\infty }{\frac {a_{p^{n}}}{p^{ns}}}{\text{ for Re}}(s)>1.}$

From ${\textstyle \theta <{\tfrac {1}{2}}}$ follows that for each factor of Euler product:

${\displaystyle \log F_{p}=\sum _{n=1}^{\infty }{\frac {b_{p^{n}}}{p^{ns}}}}$

is absolutely convergent in ${\textstyle \operatorname {Re} (s)>{\tfrac {1}{2}}}$. Then ${\displaystyle F_{p}(s)}$ is absolutely convergent and ${\displaystyle F_{p}(s)\neq 0}$ in this region. In half-plane of absolute convergence of original Dirichlet series function is absolutely convergent product of non-vanishing factors, then for functions in Selberg class ${\textstyle F(s)\neq 0}$ in ${\textstyle \operatorname {Re} (s)>1}$.

From the functional equation follows that every pole of the gamma factor γ(s) in ${\textstyle {\text{Re}}(s)<0}$ must be cancelled by a zero of F. Such zeros are called trivial zeros; the other zeros of F are called non-trivial zeros. All nontrivial zeros are located in the critical strip, ${\textstyle 0<{\text{Re}}(s)<1}$, and by the functional equation, the nontrivial zeros are symmetrical about the critical line, ${\textstyle {\text{Re}}(s)={\frac {1}{2}}}$.

The real non-negative number

${\displaystyle d_{F}=2\sum _{i=1}^{k}\omega _{i}}$

is called the degree (or dimension) of F. Since this sum is independent of the choice of functional equation, it is well-defined for any function F. If F and G are in the Selberg class, then degree of their product is:

${\displaystyle d_{FG}=d_{F}+d_{G}.}$

It can be shown that F = 1 is the only function in S whose degree is ${\textstyle d_{F}<1}$. Kaczorowski & Perelli (2011) showed that the only cases of ${\textstyle d_{F}<2}$ are the Dirichlet L-functions for primitive Dirichlet characters (including the Riemann zeta-function). Denoting the number of non-trivial zeros of F with 0 ≤ Im(s) ≤ T by N_F(T),² Selberg showed that:

${\displaystyle N_{F}(T)=d_{F}{\frac {T\log(T+C)}{2\pi }}+O(\log T).}$

An explicit version of the result was proven by Palojärvi (2019).

It was proven by Kaczorowski & Perelli (2003) that for F in the Selberg class, ${\textstyle F(1+it)\neq 0}$ for ${\displaystyle t\in \mathbb {R} }$ is equivalent to

${\displaystyle \lim _{x\rightarrow \infty }{\frac {\sum _{p\leq x}\vert a_{p}\vert ^{2}}{\pi (x)}}=\kappa _{F},}$

where ${\displaystyle \kappa _{F}>0}$ is a real number and ${\textstyle \pi }$ is the prime-counting function. This result can be thought of as a generalization of the prime number theorem. Nagoshi & Steuding (2010) showed that functions satisfying the prime-number theorem condition have a universality property for the strip ${\textstyle \sigma <Re(s)<1}$, where ${\textstyle \sigma =\max \lbrace {\frac {1}{2}},1-{\frac {1}{d_{F}}}\rbrace }$. It generalizes the universality property of the Riemann zeta function and Dirichlet L-functions.

A function ${\displaystyle F\neq 1}$ in S is called primitive if, whenever it is written as ${\displaystyle F=F_{1}\cdot F_{2}}$, with both of function in Selberg class, then ${\displaystyle F=F_{1}}$ or ${\displaystyle F=F_{2}}$. As a consequence that degree is additive with respect to multiplication of functions and only function of degree ${\displaystyle d_{F}<1}$ is ${\displaystyle F=1}$, every function F ≠ 1 can be written as a product of primitive functions. However, uniqueness of this factorization is still unproven.

The prototypical example of an element in S is the Riemann zeta function.³ Also, most of generalizations of the zeta function, like Dirichlet L-functions or Dedekind zeta functions, belong to the Selberg class.

Examples of primitive functions include the Riemann zeta function and Dirichlet L-functions of primitive Dirichlet characters or some Artin L-functions for irreducible representations of Galois group of Galois extension of rational numbers (in general case of Artin L-function non-existence of poles violating analictity axiom is subject of Artin conjecture).

Another example is the L-function of the modular discriminant Δ,

${\displaystyle L(s,\Delta )=\sum _{n=1}^{\infty }{\frac {\tau (n)/n^{11/2}}{n^{s}}},}$

where ${\textstyle \tau (n)}$ is the Ramanujan tau function.⁴ This example can be considered a "normalized" or "shifted" L-function for the original Ramanujan L-function, defined as

${\displaystyle L(s)=\sum _{n=1}^{\infty }{\frac {\tau (n)}{n^{s}}},}$

whose coefficients satisfy ${\textstyle \vert \tau (n)\vert \leq n^{\frac {11}{2}}}$. It has the functional equation

${\displaystyle ({\frac {1}{2\pi }})^{s}\Gamma (s)L(s)=({\frac {1}{2\pi }})^{12-s}\Gamma (12-s)L(12-s)}$

and is expected to have all nontrivial zeros on the line ${\textstyle Re(s)=6}$.

All known examples are automorphic L-functions, and the reciprocals of F_p(s) are polynomials in p^−s of bounded degree.⁵

In (Selberg 1992), Selberg made conjectures concerning the functions in S:

• Conjecture 1: For all F in S, there is an integer n_F such that $${\displaystyle \sum _{p\leq x}{\frac {|a_{p}|^{2}}{p}}=n_{F}\log \log x+O(1)}$$ and n_F = 1 whenever F is primitive.
• Conjecture 2: For distinct primitive F, F′ ∈ S, $${\displaystyle \sum _{p\leq x}{\frac {a_{p}{\overline {a_{p}^{\prime }}}}{p}}=O(1).}$$
• Conjecture 3: If F is in S with primitive factorization $${\displaystyle F=\prod _{i=1}^{m}F_{i},}$$ χ is a primitive Dirichlet character, and the function $${\displaystyle F^{\chi }(s)=\sum _{n=1}^{\infty }{\frac {\chi (n)a_{n}}{n^{s}}}}$$ is also in S, then the functions F_i^χ are primitive elements of S (and consequently, they form the primitive factorization of F^χ).
• Generalized Riemann hypothesis for S: For all F in S, the non-trivial zeros of F all lie on the line Re(s) = 1/2.

The first two Selberg conjectures are often collectively called the Selberg orthogonality conjecture.

It is conjectured that Selberg class is equal to class of automorphic L-functions. Primitive functions are expected to be associated with irreducible automorphic representations.

It is conjectured that all reciprocals of factors F_p(s) of the Euler products are polynomials in p^−s of bounded degree.

It is conjectured that, for any F in the Selberg class, ${\textstyle d_{F}}$ is a nonnegative integer. The best particular result due to Kaczorowski & Perelli shows this only for ${\textstyle d_{F}<2}$.

The Selberg orthogonality conjecture has numerous consequences for functions in the Selberg class:

• The factorization of function F in S into primitive functions would be unique.
• If ${\textstyle F=F_{1}^{e_{1}}\dots F_{n}^{e_{n}}}$ is a factorization of F in S into primitive functions, then ${\textstyle n_{F}=e_{1}^{2}+\dots +e_{n}^{2}}$. In particular, this implies that ${\textstyle n_{F}=1}$ if and only if F is a primitive function.⁶
• The functions in S have no zeros on ${\textstyle Re(s)=1}$. This implies that they satisfy a generalization of the prime number theorem and have a universality property.
• If F has a pole of order m at s = 1, then F(s)/ζ(s)^m is entire. In particular, they imply Dedekind's conjecture.⁷
• M. Ram Murty showed in (Murty 1994) that the orthogonality conjecture implies the Artin conjecture.⁸
• In the same article Murty proven that ortogonality conjecture implies Langlands reciprocity for Artin L-functions of solvable extensions.
• L-functions of irreducible cuspidal automorphic representations that satisfy the Ramanujan conjecture are primitive.⁹

The Generalized Riemann Hypothesis for S implies many different generalizations of the original Riemann Hypothesis, the most notable being the generalized Riemann hypothesis for Dirichlet L-functions and extended Riemann Hypothesis for Dedekind zeta functions, with multiple consequences in analytic number theory, algebraic number theory, class field theory, and numerous branches of mathematics.

Combined with the Generalized Riemann hypothesis, different versions of orthogonality conjecture imply certain growth rates for the function and its logarithmic derivative.¹⁰¹¹¹²

If the Selberg class equals the class of automorphic L-functions, then the Riemann hypothesis for S would be equivalent to the Grand Riemann hypothesis.