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The Weil Conjectures
open
Posed by André Weil · 1949 · algebraic geometry / number theory
The problem
For a variety X over a finite field: (1) its zeta function is rational; (2) it satisfies a functional equation; (3) (the Riemann Hypothesis analogue) eigenvalues of Frobenius on étale cohomology have absolute value \(q^{i/2}\) in degree i; (4) Betti numbers match topological intuition when X reduces mod p.
History & significance
Weil conjectured them from wartime cell-counting experiments (in prison, by legend), imagining a yet-unborn cohomology theory. Grothendieck built étale cohomology largely to attack them, delivering rationality (1964) and the functional equation; the Riemann hypothesis resisted until Pierre Deligne's 1974 proof via the monodromy-weight machinery — earning the 1978 Fields Medal. Payoff example: Ramanujan's 1916 observation that |τ(p)| ≤ \(2p^{11/2}\) for his tau function followed as a corollary, settling a 58-year-old mystery about a single arithmetic function via the geometry of higher-dimensional spaces.
Still open.
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