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Non-reduced Components of the Hilbert Scheme of Curves Using Triple Covers

  • Youngook Choi,
  • Hristo Iliev,
  • Seonja Kim

摘要

In this paper, we consider curves on a cone that pass through the vertex and are also triple covers of the base of the cone, which is a general smooth curve of genus \(\gamma \) γ and degree e in \({\mathbb {P}}^{e-\gamma }\) P e - γ . Using the free resolution of the ideal of such a curve found by Catalisano and Gimigliano, and a technique concerning deformations of curves introduced by Ciliberto, we show that the deformations of such curves remain on cones over a deformation of the base curve. This allows us to prove that for \(\gamma \ge 3\) γ 3 and \(e \ge 4\gamma + 5\) e 4 γ + 5 , there exists a non-reduced component \({\mathcal {H}}\) H of the Hilbert scheme of smooth curves of genus \(3e + 3\gamma \) 3 e + 3 γ and degree \(3e+1\) 3 e + 1 in \({\mathbb {P}}^{e-\gamma +1}\) P e - γ + 1 . We show that \(\dim T_{[X]} {\mathcal {H}} = \dim {\mathcal {H}} + 1 = (e - \gamma + 1)^2 + 7e + 5\) dim T [ X ] H = dim H + 1 = ( e - γ + 1 ) 2 + 7 e + 5 for a general point \([X] \in {\mathcal {H}}\) [ X ] H .