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Hierarchical structure of the family of curves with maximal genus verifying flag conditions
Authors:Vincenzo Di Gennaro
Affiliation:Dipartimento di Matematica, Università di Roma Tor Vergata, Via della Ricerca Scientifica, 00133 Roma, Italia
Abstract:Fix integers $ r,s_{1},dots ,s_{l}$ such that $ 1leq lleq r-1$ and $ s_{l}geq r-l+1$, and let $ mathcal{C}(r;s_{1},dots ,s_{l})$ be the set of all integral, projective and nondegenerate curves $ C$ of degree $ s_{1}$ in the projective space $ mathbf{P}^{r}$, such that, for all $ i=2,dots ,l$, $ C$ does not lie on any integral, projective and nondegenerate variety of dimension $ i$ and degree $ <s_{i}$. We say that a curve $ C$ satisfies the flag condition $ (r;s_{1},dots ,s_{l})$ if $ C$ belongs to $ mathcal{C}(r;s_{1},dots ,s_{l})$. Define $ G(r;s_{1},dots ,s_{l})=operatorname{max}left {p_{a}(C):,Cin mathcal{C}(r;s_{1},dots ,s_{l})right }, $ where $ p_{a}(C)$ denotes the arithmetic genus of $ C$. In the present paper, under the hypothesis $ s_{1}gg dots gg s_{l}$, we prove that a curve $ C$ satisfying the flag condition $ (r;s_{1},dots ,s_{l})$ and of maximal arithmetic genus $ p_{a}(C)=G(r;s_{1},dots ,s_{l})$ must lie on a unique flag such as $ C=V_{s_{1}}^{1}subset V_{s_{2}}^{2}subset dots subset V_{s_{l}}^{l}subset {mathbf{P}^{r}}$, where, for any $ i=1,dots ,l$, $ V_{s_{i}}^{i}$ denotes an integral projective subvariety of $ {mathbf{P}^{r}}$ of degree $ s_{i}$ and dimension $ i$, such that its general linear curve section satisfies the flag condition $ (r-i+1;s_{i},dots ,s_{l})$ and has maximal arithmetic genus $ G(r-i+1;s_{i},dots ,s_{l})$. This proves the existence of a sort of a hierarchical structure of the family of curves with maximal genus verifying flag conditions.

Keywords:Complex projective curve   Castelnuovo-Halphen theory   arithmetically Cohen-Macaulay curve   arithmetic genus   flag condition   adjunction formula
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