A Brill–Noether theorem for (toric) surfaces

Abstract The classical Brill–Noether theorem states that a map from a general curve to a projective space deforms in a family of the expected dimension if its image does not lie in any hyperplane. In this note, we observe, as a direct consequence of standard results on Severi varieties, an analogous statement for maps from a general curve to any smooth projective surface. Namely, a non-constant map deforms in a family of the expected dimension if its image has anti-canonical degree at least 4. In the case of toric surfaces, curves of anti-canonical degree at most 3 admit a particularly elegant description in terms of certain toric contractions. We raise the question of whether a Brill–Noether theorem could hold for toric varieties of higher dimensions.