Source: Duke Math. J. Volume 107, Number 3
(2001), 427-467.
We show that a Gorenstein subcanonical codimension 3 subscheme
Z⊂X=ℙN, N≥4, can be realized as the locus
along which two Lagrangian subbundles of a twisted orthogonal bundle
meet degenerately and conversely. We extend this result to singular
Z and all quasi-projective ambient schemes X under the necessary
hypothesis that Z is strongly subcanonical in a sense defined
below. A central point is that a pair of Lagrangian subbundles can be
transformed locally into an alternating map. In the local case our
structure theorem reduces to that of D. Buchsbaum and D. Eisenbud [6]
and says that Z is Pfaffian.
We also prove codimension 1 symmetric and skew-symmetric analogues of our structure theorems.
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