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
absolutely convergent for Re(s) > 1 that satisfy four axioms (or assumptions as Selberg calls them):
where Q is real and positive, Γ the gamma function, the ωi real and positive, and the μi complex with non-negative real part, as well as a so-called root number
such that the function
satisfies
with
and, for some θ < 1/2,
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:
satisfies axioms and is also in S. If F is entire function in S, then for is also in S.
From the Ramanujan conjecture, it follows that, for every :
then Dirichlet series defining function is absolutely convergent in the half-plane: .
Despite the unusual version of the Euler product in the axioms, by exponentiation of Dirichlet series, one can deduce that an is a multiplicative sequence and that
From follows that for each factor of Euler product:
is absolutely convergent in . Then is absolutely convergent and 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 in .
From the functional equation follows that every pole of the gamma factor γ(s) in 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, , and by the functional equation, the nontrivial zeros are symmetrical about the critical line, .
The real non-negative number
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:
It can be shown that F = 1 is the only function in S whose degree is . Kaczorowski & Perelli (2011) showed that the only cases of 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 NF(T), [2] Selberg showed that:
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, for is equivalent to
where is a real number and 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 , where . It generalizes the universality property of the Riemann zeta function and Dirichlet L-functions.
A function in S is called primitive if, whenever it is written as , with both of function in Selberg class, then or . As a consequence that degree is additive with respect to multiplication of functions and only function of degree is , 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 Artin L-functions for irreducible representations.
Another example is the L-function of the modular discriminant Δ,
where 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
whose coefficients satisfy . It has the functional equation
and is expected to have all nontrivial zeros on the line .
All known examples are automorphic L-functions, and the reciprocals of Fp(s) are polynomials in p−s of bounded degree. [5]
In ( Selberg 1992 ), Selberg made conjectures concerning the functions in S:
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 Fp(s) of the Euler products are polynomials in p−s of bounded degree.
It is conjectured that, for any F in the Selberg class, is a nonnegative integer. The best particular result due to Kaczorowski & Perelli shows this only for .
The Selberg orthogonality conjecture has numerous consequences for functions in the Selberg class:
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. [10] [11] [12]
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.