Singularities of the Hilbert scheme of non-reduced curves

In this article, we study the Hilbert scheme of generically non-reduced curves in ${\mathbb{P}}^3$. We prove the existence of generically non-reduced curves in ${\mathbb{P}}^3$ for which there exist infinitesimal deformations of the curve that do not induce deformations of the associated reduced scheme. We show that such infinitesimal deformations contribute to the non-reducedness of the corresponding Hilbert scheme. We introduce the notion of extension of curves and prove that such infinitesimal deformations (such that the associated reduced scheme does not deform) are inherited by the extended curve. Finally, we give examples of extension of curves.