An alternating matrix and a vector, with application to Aluffi algebras

Abstract: Let $\mathbf X$ be a generic alternating matrix, $\mathbf t$ be a generic row vector, and $J$ be the ideal $\operatorname{Pf}_4({\mathbf X})+I_1({\mathbf {t X}})$. We prove that $J$ is a perfect Gorenstein ideal of grade equal to the grade of $\operatorname{Pf}_4({\mathbf X})$ plus two. This result is used by Ramos and Simis in their calculation of the Aluffi algebra of the module of derivations of the homogeneous coordinate ring of a smooth projective hypersurface. We also prove that $J$ defines a domain, or a normal ring, or a unique factorization domain if and only if the base ring has the same property. The main object of study in the present paper is the module $\mathcal N$ which is equal to the column space of $\mathbf X$, calculated mod $\operatorname{Pf}_4({\mathbf X})$. The module $\mathcal N$ is a self-dual maximal Cohen-Macaulay module of rank two; furthermore, $J$ is a Bourbaki ideal for $\mathcal N$. The ideals which define the homogeneous coordinate rings of the Pl\"ucker embeddings of the Schubert subvarieties of the Grassmannian of planes are used in the study of the module $\mathcal N$.