No abstract

Abstract. The space of smooth rational cubic curves in projective space P r (r ≥ 3) is a smooth quasi-projective variety, which gives us an open subset of the corresponding Hilbert scheme, the moduli space of stable maps, or the moduli space of stable sheaves. By taking its closure, we obtain three compactifications H, M, and S respectively. In this paper, we compare these compactifications. First, we prove that H is the blow-up of S along a smooth subvariety which is the locus of stable sheaves which are planar (i.e. support is contained in a plane). Next we prove that S is obtained from M by three blow-ups followed by three blow-downs and the centers are described explicitly. Using this, we calculate the cohomology of S.

The space of smooth rational curves of degree d in a projective variety X has compactifications by taking closures in the Hilbert scheme, the moduli space of stable sheaves or the moduli space of stable maps respectively. In this paper we compare these compactifications by explicit blow-ups and -downs when X is a projective homogeneous variety and d ≤ 3. Using the comparison result, we calculate the Betti numbers of the compactifications when X is a Grassmannian variety.