Dimension counts for singular rational curves via semigroups

We study singular rational curves in projective space, deducing conditions on their parametrizations from the value semigroups $\text{sss}$ of their singularities. In particular, we prove that a natural heuristic for the codimension of the space of nondegenerate rational curves of arithmetic genus $g>0$ and degree $d$ in $\text{mb}{P}^n$, viewed as a subspace of all degree-$d$ rational curves in $\text{mb}{P}^n$, holds whenever $g$ is small. On the other hand, we show that this heuristic fails in general, by exhibiting an infinite family of examples of Severi-type varieties of rational curves containing “excess” components of dimension strictly larger than the space of $g$-nodal rational curves.