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The Plucker image is a closed projective algebraic set
Statement
The Plucker image of is a closed projective algebraic set in , cut out by the quadratic Plucker relations.
Facts & Assumptions
Given: Coordinates indexed by increasing -subsets of a basis of . Extend the notation to any ordered -tuple by declaring for repeated indices and otherwise using the sign of the permutation that sorts the tuple.
Proof
For or , the Grassmannian and are both one point, so the assertion is immediate. Suppose henceforth that . For ordered tuples and , alternating multilinearity gives the signed Plucker relation The ordered-coordinate convention supplies every insertion sign and makes repeated indices contribute zero.
On a chart with , rescale so that and define the entries of by the coordinates with exactly one index outside , using their alternating signs. The maximal minors of agree with these coordinates when zero or one index is outside the base set. Induct on the number of outside indices in an ordered -tuple . For an outside index , apply the relation of step 1.1 to the -tuple and the -tuple . The term omitting is, up to sign, ; every other term is a product of a coordinate with fewer outside indices and a one-outside coordinate. The minors of satisfy the same relation, so the induction identifies with its corresponding minor. Thus the row span has precisely the given projective coordinates.
The analogous charts cover every nonzero coordinate point satisfying the relations. Thus every such point is decomposable, while step 1.1 proved the reverse inclusion; homogeneous quadratic equations make this locus closed, and the injective Plucker map realizes it as the required projective algebraic set.
Depends on
Used by
- The Grassmannian is smooth, irreducible, and has dimension r(n-r) Corollary
- Lines in P3 and the Klein quadric Gr(2,4) Example
- Quiver full-flag incidence has a smooth projective compactification Lemma
- Semistable loci of finite-quiver representations are open Lemma
- Standard affine charts on the Grassmannian Lemma
- The quiver exact-sequence correspondence is smooth and proper Lemma
- The standard incidence locus is closed Lemma
- The variety of complete flags of a finite-dimensional vector space is smooth projective Lemma
Dependency tree · two levels
12 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- J. S. Milne, Algebraic Geometry, Proposition 6.29 and its second proof (standard reference, not scraped)
- MIT 18.725 Algebraic Geometry, Lecture 4, Theorem 4.1 (standard reference, not scraped)