Nodal surfaces with obstructed deformations

Abstract

In this text we show that the deformation space of a nodal surface $X$ of degree $d$ is smooth and of the expected dimension if $d\leq 7$ or $d\geq 8$ and $X$ has at most $4d-5$ nodes. (The case $d\leq 7$ was previously covered by Alexandru Dimca by using different techniques.) For $d\geq 8$ we give explicit examples of nodal surfaces with $4d-4$ nodes, for which the tangent space to the deformation space has larger dimension than expected. We give a short discussion on the shape of the deformation space of surfaces of the form $f_1f_2+f_3^2f_4$, where $f_1$ is a linear form.

Theorems & Definitions (17)

• Theorem 1.1
• Theorem 1.2
• Proposition 2.1
• proof
• Theorem 2.2: Macaulay-Gotzmann
• Remark 2.3
• Proposition 2.4
• proof
• Theorem 2.5: Dimca, Dim
• Proposition 4.1