The problem, three ways
Curious
An elliptic curve is an equation such as y² = x³ − x. Some have infinitely many rational solutions and some only finitely many. Bryan Birch and Peter Swinnerton-Dyer, computing on one of the first computers, guessed that a single analytic function predicts which, and how many.
Undergraduate
For an elliptic curve E over ℚ, Mordell's theorem gives E(ℚ) ≅ ℤ^r × T with T finite. The conjecture says r equals the order of vanishing of L(E, s) at s = 1, and gives the leading coefficient in terms of the regulator, the Tate–Shafarevich group, Tamagawa numbers and torsion.
Specialist
Known when the analytic rank is 0 or 1, by Gross–Zagier and Kolyvagin together with modularity (Wiles; Breuil, Conrad, Diamond and Taylor). Bhargava, Skinner and Zhang showed that more than 66% of elliptic curves over ℚ satisfy the rank part. Finiteness of the Tate–Shafarevich group is open in general.