Determinantal Ideals by Rosa M. Miró-Roig

By Rosa M. Miró-Roig

Determinantal beliefs are beliefs generated via minors of a homogeneous polynomial matrix. a few classical beliefs that may be generated during this approach are the fitting of the Veronese kinds, of the Segre kinds, and of the rational basic scrolls.

Determinantal beliefs are a imperative subject in either commutative algebra and algebraic geometry, and so they have a number of connections with invariant concept, illustration concept, and combinatorics. because of their very important function, their examine has attracted many researchers and has acquired huge cognizance within the literature. during this publication 3 the most important difficulties are addressed: CI-liaison type and G-liaison classification of ordinary determinantal beliefs; the multiplicity conjecture for normal determinantal beliefs; and unobstructedness and size of households of ordinary determinantal ideals.

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Roughly speaking, liaison is an equivalence relation among subschemes of a given dimension d in a projective space Pn (or ideals in K[x0 , x1 , . . , xn ]) and it involves the study of the properties that are shared by two schemes whose union is well understood. 1 for the precise definition). , G-links). Notice that as complete intersections are particular cases of AG schemes, any two subschemes which are in the same CI-liaison class are also in the same G-liaison class. Most of the work on liaison theory has been done for subschemes of codimension 2 in projective spaces where complete intersection and AG schemes coincide.

2. We consider a rational normal curve X ⊂ Pn and we will prove that X is glicci but not licci. It is well known that after a change of coordinates, we may assume that the homogeneous ideal of I(X) can be generated by the maximal minors of the matrix x0 x1 . . xn−1 . x1 x2 . . 1. On the other hand, the curve X has a linear free R-resolution, 0 −→ R(−n)bn−1 −→ · · · −→ R(−3)b2 −→ R(−2)b1 −→ I(X) −→ 0, n where bi = i i+1 . , X is not in the CI-liaison class of a complete intersection. 3. The way that Gaeta’s theorem is usually stated is that, if X ⊂ Pn is a codimension 2 subscheme, then X is ACM if and only if X is licci.

The proof is rather technical and the main idea is the following one. We denote by B the matrix obtained by deleting a “suitable” column of A and we call X the subscheme defined by the maximal minors of B. (“Suitable” means that codim(X) = c − 1. ) We denote by A the matrix obtained by deleting a “suitable” row of B and we call V the subscheme defined by the maximal minors of A . (“Suitable” means that codim(V ) = c. ) ⎛ ⎜ ⎜ A=⎜ ⎜ ⎝ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ • • • • • ⎞ ⎟ ⎟ ⎟ ⎟ ⎠ I(A) = I(V ), codim(V ) = c, 42 Chapter 2.

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