Cohomological dimension and arithmetical rank of some determinantal ideals

Abstract

Let $M$ be a $(2 \times n)$ non-generic matrix of linear forms in a polynomial ring. For large classes of such matrices, we compute the cohomological dimension (cd) and the arithmetical rank (ara) of the ideal $I_2(M)$ generated by the $2$-minors of $M$. Over an algebraically closed field, any $(2 \times n)$-matrix of linear forms can be written in the Kronecker-Weierstrass normal form, as a concatenation of scroll, Jordan and nilpotent blocks. B\u{a}descu and Valla computed $\mathrm{ara}(I_2(M))$ when $M$ is a concatenation of scroll blocks. In this case we compute $\mathrm{cd}(I_2(M))$ and extend these results to concatenations of Jordan blocks. Eventually we compute $\mathrm{ara}(I_2(M))$ and $\mathrm{cd}(I_2(M))$ in an interesting mixed case, when $M$ contains both Jordan and scroll blocks. In all cases we show that $\mathrm{ara}(I_2(M))$ is less than the arithmetical rank of the determinantal ideal of a generic matrix.