Hasse–Weil zeta function
In mathematics, the Hasse–Weil zeta function attached to an algebraic variety "V" defined over an algebraic number field "K" is a meromorphic function on the complex plane defined in terms of the number of points on the variety after reducing modulo each prime number "p". It is a global "L"-function defined as an Euler product of local zeta functions. Hasse–Weil "L"-functions form one of the two major classes of global "L"-functions, alongside the "L"-functions associated to automorphic representations. Conjecturally, these two types of global "L"-functions are actually two descriptions of the same type of  global "L"-function; this would be a vast generalisation of the Taniyama-Weil conjecture, itself an important result in number theory. For an elliptic curve over a number field "K", the Hasse–Weil zeta function is conjecturally related to the group of rational points of the elliptic curve over "K" by the Birch and Swinnerton-Dyer conjecture. Definition. The description of the Hasse–Weil zeta function "up to finitely many factors of its Euler product" is relatively simple. This follows the initial suggestions of Helmut Hasse and André Weil, motivated by the case in which "V" is a single point, and  the Riemann zeta function results. Taking the case of "K" the rational number field Q, and "V" a non-singular projective variety, we can for almost all prime numbers "p" consider the reduction of "V" modulo "p", an algebraic variety "V" over the finite field F with "p" elements, just by reducing equations for "V". Scheme-theoretically, this reduction is just the pullback of "V" along the canonical map Spec F → Spec Z. Again for almost all "p" it will be non-singular. We define to be the Dirichlet series of the complex variable "s", which is the infinite  product of the local zeta functions Then "Z"("s"), according to our definition, is well-defined only up to multiplication by rational functions in a finite number of formula_3. Since the indeterminacy is relatively harmless, and has meromorphic continuation everywhere, there is a sense in which the properties of "Z(s)" do not essentially depend on it. In particular, while the exact form of the functional equation for "Z"("s"), reflecting in a vertical line in the complex plane, will definitely depend on the 'missing' factors, the existence of some such functional equation does not. A more refined definition became possible with the  development of étale cohomology; this neatly explains what to do about the missing, 'bad reduction' factors. According to general principles visible in ramification theory, 'bad' primes carry good information (theory of the "conductor"). This manifests itself in the étale theory in the Ogg–Néron–Shafarevich criterion for good reduction; namely that there is good reduction, in a definite sense, at all primes "p" for which the Galois representation ρ on the étale cohomology groups of "V" is "unramified". For those, the definition of local zeta function can be recovered in terms of the characteristic polynomial of Frob("p") being a Frobenius  element for "p". What happens at the ramified "p" is that ρ is non-trivial on the inertia group "I"("p") for "p". At those primes the definition must be 'corrected', taking the largest quotient of the representation ρ on which the inertia group acts by the trivial representation. With this refinement, the definition of "Z"("s") can be upgraded successfully from 'almost all' "p" to "all" "p" participating in the Euler product. The consequences for the functional equation were worked out by Serre and Deligne in the later 1960s; the functional equation itself has not been proved in general. Example: elliptic  curve over Q. Let "E" be an elliptic curve over Q of conductor "N". Then, "E" has good reduction at all primes "p" not dividing "N", it has multiplicative reduction at the primes "p" that "exactly" divide "N" (i.e. such that "p" divides "N", but "p" does not; this is written "p" || "N"), and it has additive reduction elsewhere (i.e. at the primes where "p" divides "N"). The Hasse–Weil zeta function of "E" then takes the form Here, ζ("s") is the usual Riemann zeta function and "L"("s", "E") is called the "L"-function of "E"/Q, which takes the form  where, for a given prime "p", where, in the case of good reduction "a" is "p" + 1 − (number of points of "E" mod "p"), and in the case of multiplicative reduction "a" is ±1 depending on whether "E" has split or non-split multiplicative reduction at "p". Hasse–Weil conjecture. The Hasse–Weil conjecture states that the Hasse–Weil zeta function should extend to a meromorphic function for all complex "s", and should satisfy a functional equation similar to that of the Riemann zeta function. For elliptic curves over the rational numbers, the Hasse–Weil conjecture follows from  the modularity theorem. Birch and Swinnerton-Dyer conjecture. The Birch and Swinnerton-Dyer conjecture states that the rank of the abelian group "E"("K") of points of an elliptic curve "E" is the order of the zero of the Hasse–Weil "L"-function "L"("E", "s") at "s" = 1, and that the first non-zero coefficient in the Taylor expansion of "L"("E", "s") at "s" = 1 is given by more refined arithmetic data attached to "E" over "K". The conjecture is one of the seven Millennium Prize Problems listed by the Clay Mathematics Institute, which has offered a $1,000,000 prize  for the first correct proof.