Interpolation for Curves in Projective Space with Bounded Error

Abstract: Given n general points p_1, p_2,..., p_n \in P^r, it is natural to ask whether there is a curve of given degree d and genus g passing through them; by counting dimensions a natural conjecture is that such a curve exists if and only if [n \leq \left\lfloor \frac{(r + 1)d - (r - 3)(g - 1)}{r - 1}\right\rfloor.] In the case of curves with nonspecial hyperplane section, the above conjecture was recently shown to hold with exactly three exceptions. In this paper, we prove a "bounded-error analog" for special linear series on general curves; more precisely we show that existance of such a curve subject to the stronger inequality [n \leq \left\lfloor \frac{(r + 1)d - (r - 3)(g - 1)}{r - 1}\right\rfloor - 3.] Note that the -3 cannot be replaced with -2 without introducing exceptions (as a canonical curve in P^3 can only pass through 9 general points, while a naive dimension count predicts 12). We also use the same technique to prove that the twist of the normal bundle N_C(-1) satisfies interpolation for curves whose degree is sufficiently large relative to their genus, and deduce from this that the number of general points contained in the hyperplane section of a general curve is at least [\min\left(d, \frac{(r - 1)^2 d - (r - 2)^2 g - (2r^2 - 5r + 12)}{(r - 2)^2}\right).] As explained in arXiv:1809.05980, these results play a key role in the author's proof of the Maximal Rank Conjecture.