One-dimensional Schubert problems with respect to osculating flags

Abstract

We consider Schubert problems with respect to flags osculating the rational normal curve. These problems are of special interest when the osculation points are all real -- in this case, for zero-dimensional Schubert problems, the solutions are "as real as possible". Recent work by Speyer has extended the theory to the moduli space $\overline{M_{0,r}}$, allowing the points to collide. These give rise to smooth covers of $\overline{M_{0,r}}(\mathbb{R})$, with structure and monodromy described by Young tableaux and jeu de taquin. In this paper, we give analogous results on one-dimensional Schubert problems over $\overline{M_{0,r}}$. Their (real) geometry turns out to be described by orbits of Sch\"{u}tzenberger promotion and a related operation involving tableau evacuation. Over $M_{0,r}$, our results show that the real points of the solution curves are smooth. We also find a new identity involving `first-order' K-theoretic Littlewood-Richardson coefficients, for which there does not appear to be a known combinatorial proof.