In mathematics, the Nagata conjecture on curves, named after Masayoshi Nagata, governs the minimal degree required for a plane algebraic curve to pass through a collection of very general points with prescribed multiplicities.

Nagata arrived at the conjecture via work on the 14th problem of Hilbert, which asks whether the invariant ring of a linear group action on the polynomial ring k[x₁, ..., x_n] over some field k is finitely generated. Nagata published the conjecture in a 1959 paper in the American Journal of Mathematics, in which he presented a counterexample to Hilbert's 14th problem.

Nagata Conjecture. Suppose p₁, ..., p_r are very general points in P² and that m₁, ..., m_r are given positive integers. Then for r > 9 any curve C in P² that passes through each of the points p_i with multiplicity m_i must satisfy

${\displaystyle \deg C>{\frac {1}{\sqrt {r}}}\sum _{i=1}^{r}m_{i}.}$

The condition r > 9 is necessary: The cases r > 9 and r ≤ 9 are distinguished by whether or not the anti-canonical bundle on the blowup of P² at a collection of r points is nef. In the case where r ≤ 9, the cone theorem essentially gives a complete description of the cone of curves of the blow-up of the plane.

The only case when this is known to hold is when r is a perfect square, which was proved by Nagata. Despite much interest, the other cases remain open. A more modern formulation of this conjecture is often given in terms of Seshadri constants and has been generalised to other surfaces under the name of the Nagata–Biran conjecture.

• Harbourne, Brian (2001), "On Nagata's conjecture", Journal of Algebra, 236 (2): 692–702, arXiv:math/9909093, doi:10.1006/jabr.2000.8515, MR 1813496.
• Nagata, Masayoshi (1959), "On the 14-th problem of Hilbert", American Journal of Mathematics, 81 (3): 766–772, doi:10.2307/2372927, JSTOR 2372927, MR 0105409.
• Strycharz-Szemberg, Beata; Szemberg, Tomasz (2004), "Remarks on the Nagata conjecture", Serdica Mathematical Journal, 30 (2–3): 405–430, hdl:10525/1746, MR 2098342.